Introduction

For the next 365 days, I will post a dead bird lizard (colloquially known as “dinosaurs”) every day. These will not be in any particular order and do not represent every known genus or species. Megalosaurus will be the first, however, since it was the first to be scientifically described.

Some things to remember: Late Triassic (237 million years ago - 201 million years ago) / Early Jurassic (201 million years ago - 174 million years ago) / Middle Jurassic (174 million years ago - 161 million years ago) / Late Jurassic (161 million years ago - 143 million years ago) / Early Cretaceous (143 million years ago - 100 million years ago) / Late Cretaceous (100 million years ago - 66 million years ago)

• “Basal” = primitive animal or trait in comparison to derived animals or traits. More similar to shared ancestors.
• “Derived” = advanced animal or trait in comparison to basal animals or traits. More removed from shared ancestors.
• “Theropod” = bipedal and mostly, though not always, carnivorous dead bird lizards. Live bird non-lizards of today belong to this group.
• “Sauropod” = quadrupedal, long-necked herbivorous dead bird lizards.
• “Ornithopod” = ancestrally bipedal, later both quadrupedal and bipedal herbivorous dead bird lizards. “duck-billed” hadrosaurs belong to them.
• “Thyreophoran” = quadrupedal, heavily armored dead bird lizards (ankylosaurs and stegosaurs).
• “Ceratopsian” = originally bipedal, later mostly quadrupedal, large-headed and horned dead bird lizards (Triceratops and kin).

Despite calling them “dead bird lizards”, dead bird lizards aren’t true lizards. True lizards belong to the order Squamata, which they share with snakes. Dinosaurs belong to a group called the Archosauria, which they share with modern crocodiles. The pigeon who shit on my head when I was a child is closer phylogenetically to crocodiles than crocodiles are to my pet iguana Devin and I tried to exhume after its death.

Megalosaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Megalosauroidea (?) > Megalosauridae > Megalosaurinae

Overview: Fossil remains later attributed to Megalosaurus had been known to English academics since the 1600’s, but were scant and mistaken for remains of Roman war elephants or giant humans. More fossils would be uncovered, leading to its 1824 description, published by the theologian and naturalist William Buckland. Its name, meaning “great” or “large lizard” of course refers to its size, though we know today there were many far larger theropod dinosaurs. Megalosaurus is considered to be the first validly described non-avian dinosaur genus. Indeed, it was named before the concept of the Dinosauria even existed. In 1842, the famed naturalist Richard Owen used Megalosaurus (along with Iguanodon and Hylaeosaurus) to establish dinosaurs as a taxonomic clade, distinguishing them from other large prehistoric reptile groups like the mosasaurs or plesiosaurs.

Early restorations of Megalosaurus depicted it as a bulky, quadrupedal creature. Its remains were incomplete and scientists had yet to learn that theropod dinosaurs were bipeds. Modern depictions restore it as a largely “stereotypical” large-bodied theropod. It had a large head with fairly long jaws. The arms weren’t overly long, but were probably quite powerful, each hand equipped with three large claws. Megalosaurus appears to have been the largest predator in its local environment. Potential food sources included long-necked sauropods like Cetiosaurus. This genus acts as the namesake of the larger family Megalosauridae, which also contained notable dinosaurs like Torvosaurus and Eustreptospondylus. These animals saw their greatest success during the Middle to Late Jurassic. Known fossils include parts of the jaw and skull, the pelvis, limb bones and some vertebrae.

Iguanodon

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Ornithopoda > Iguanodontia > Ankylopollexia > Styracosterna > Hadrosauriformes > Iguanodontidae

Overview: In the 1820’s, an English physician by the name of Gideon Mantell came to possess a set of scattered fossils, clearly belonging to an enormous reptile. These consisted of teeth and some other bones. One of Mantell’s associates pointed out similarities between the creature’s teeth and those of modern iguanas, so when Mantell officially described the animal in 1825, it was named for this fact. In 1842, along with Megalosaurus and Hylaeosaurus, Iguanodon was one of the taxa used to establish the Dinosauria as a distinct clade. Early restorations depicted Iguanodon as a massively scaled-up iguana or, in some later depictions, as an elephant-like reptile walking on all fours. One bony spike associated with the find was interpreted as a large nasal horn. Decades later, in the 1870’s, far more complete fossils were found deep within a Belgian coal mine, consisting of many complete skeletons. These were used as the basis for a new species - Iguanodon bernissartensis. One of these specimens was so complete that it was designated as the new type specimen for the genus Iguanodon.

The Belgian specimens revealed Iguanodon to be quite different than previously imagined. It was not an entirely quadrupedal animal, as it retained the ability to walk up on its hind limbs (bipedal locomotion being the ancestral state of ornithopods), though much of its time would’ve been spent down on all fours as it grazed. Like other iguanodonts, it possessed both a beak and a set of grinding teeth. Its supposed horn was actually a set of thumb-spikes, likely used for both foraging and defense against predators. Iguanodon possessed long and quite muscular arms, which in addition to its overall bulk, would’ve made it dangerous prey for most of the predators in its environment. Changing the type specimen of the genus to that of I. bernissartensis has caused a bit of an issue in recent years. It turns out that the genus described by Mantell and that from Belgium may not have been the same taxon, but as the type specimen was changed, the generic name of Iguanodon is now fixed to the latter. Mantell’s specimens have since been referred to a new related genus called Mantellisaurus. Mantellisaurus may belong to the same iguanodontid family, though this is debated. Both animals were relatives of the famous “duck-billed” dinosaurs, though a fair bit more basal in comparison, retaining traits of their ancestors.

Hylaeosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Thyreophora > Thyreophoroidea > Eurypoda > Ankylosauria > Euankylosauria > Nodosauridae (?)

Overview: The renowned English naturalist Sir Richard Owen used three genera to establish the Dinosauria as a taxonomic clade - Megalosaurus, Iguanodon and Hylaeosaurus. The former two are relatively well known, but Hylaeosaurus is notably more obscure. Fossils of the animal, which make up a portion of its body and dermal armor, were first discovered in the 1830’s. They originated from a quarry in the southeast of England, in what is now Tilgate Forest. Said remains were obtained by Gideon Mantell, who had previously named and described Iguanodon. Mantell would do the same for Hylaeosaurus in 1833, naming in reference to where it was found. Along with Megalosaurus, Iguanodon and many other prehistoric animals, a statue of Hylaeosaurus would be erected in London’s Crystal Palace Park in the 1850’s. There, it was depicted as a large, lizard-like creature with a line of spines running down its back.

Nowadays, we know that the spines on Hylaeosaurus were arranged somewhat differently. It was an ankylosaur, meaning it had dermal armor formed out of many different rows of osteoderms - bony lumps growing within its skin. These protected its back, sides, neck, tail and head. Some were flatter, smaller and oval-shaped, others were long and spiked. Ankylosaurs were, in general, low-slung herbivores that fed upon ground-level vegetation like ferns. Size-wise, Hylaeosaurus was on the low to average end. Ankylosaurs tend to be split into two major families - the typically clubless Nodosauridae and the usually club-tailed Ankylosauridae. Traditionally, Hylaeosaurus has been seen as a nodosaurid, possibly closely related to a genus called Polacanthus. Recent studies have instead suggested it could’ve been a basal ankylosaurid. Others suggest Hylaeosaurus could’ve been an ankylosaur belonging outside either family.

Crystal Palace Park

As a bonus I’ll include the Crystal Palace Park dinosaur statues mentioned here, from the 1850’s.

Megalosaurus

Here is the Crystal Palace statue of Megalosaurus. Note that scientists at that time had yet to find good remains of theropods, so they didn’t know they were bipedal. The hump over its shoulder is probably elongated back spines from another, separate dinosaur.

Iguanodon

This is the Crystal Palace statue of Iguanodon. It too was interpreted as strictly quadrupedal and kind of elephant or rhino-like. Note the small nasal horn, which was actually affixed to the hands, being a modified thumb bone.

Hylaeosaurus

Lastly, the statue of Hylaeosaurus at Crystal Palace. Its dermal armor was interpreted as a row of spines running down the center of its back. Most of its spinier armor would’ve been situated along its sides or neck like in the main photo I posted of it. Ankylosaurs as a grew weren’t yet known.

Staurikosaurus

Phylogeny: Dinosauria > Saurischia > Herrerasauria > Herrerasauridae

Overview: Staurikosaurus is one of the oldest known dinosaurs, having lived up to two hundred and thirty-three million years ago, in the Late Triassic. It wasn’t a very large animal, measuring maybe a little over two meters long. It was a carnivore that probably went after mostly smaller prey, though it probably fed upon the kills of other animals in addition to what it could catch itself. Dinosaurs were only just beginning to find their footing during its time, with most of them being similarly small. The largest predator in its local environment would’ve been an animal like Prestosuchus - a member of the clade Pseudosuchia, making it more closely related to modern crocodiles than to dinosaurs. Prestosuchus could grow to be over five meters in length. Predatory dinosaurs wouldn’t fully overtake such animals until after the Triassic-Jurassic extinction. Described in 1970, the generic name of Staurikosaurus refers to the Southern Cross constellation, which is featured on the flag of Brazil, where it was first discovered back in the 1930’s.

Most studies classify Staurikosaurus as a member of the family Herrerasauridae. This made it closely related to dinosaurs like Herrerasaurus and Gnathovorax. Similar to Staurikosaurus, these were mostly smaller predators, though some Herrerasaurus specimens appear to have grown to be decently large (at least by Triassic standards). These creatures had long, rectangular skulls and jaws lined with blade-like teeth. Many of their anatomical traits were very primitive, retaining features later lost among more derived dinosaurs. Originally, herrerasaurids were classified as theropod dinosaurs, but later studies would question this. Herrerasaurids may’ve been closer to the sauropodomorphs (the lineage leading to the long-necked Brontosaurus and its kin), or more likely, were very primitive saurischian dinosaurs outside either the Theropoda or Sauropodomorpha. Some even speculate that Staurikosaurus and its relatives may’ve belonged outside the Dinosauria proper, but this is contested by other researchers.

Montanoceratops

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Marginocephalia > Ceratopsia > Neoceratopsia > Euceratopsia > Leptoceratopsidae

Overview: Montanoceratops was a relative of the famous horned ceratopsid dinosaurs - the family that included Triceratops and its ilk. It even coexisted with a few of such dinosaurs, including genera like Pachyrhinosaurus and Regaliceratops. However, Montanoceratops itself belonged to a different family, known to science as the Leptoceratopsidae. Leptoceratopsids were generally on the smaller side. Montanoceratops was average-sized for a leptoceratopsid, or possibly on the larger end. These creatures lacked the large ornamental horns seen on ceratopsids, but shared an expanded crest of bone at the back of the skull, which may’ve been an anchor point for jaw muscles. The jaws of Montanoceratops were notably deep and robust. It was a largely or entirely herbivorous animal, its beak useful for both shearing plants and for defense against predators. It probably walked down on all fours, unlike some of its earlier relatives.

Described as a genus in 1951, Montanoceratops takes its name from the American state where it was first discovered, back in the mid-1910’s. Originally, the bones were assigned to Leptoceratops, a close relative known from somewhat younger rock layers. Montanoceratops itself is known definitively from Montana’s St. Mary River Formation, so it probably lived around seventy million years ago, in the Late Cretaceous. This formation has also yielded the fossils of Albertosaurus - a large-bodied tyrannosaur that was probably this animal’s main threat. Fossils possibly referable to Montanoceratops may also be known from the similarly-aged Horseshoe Canyon Formation in Canada, but in some studies, their classification as such as been questioned. Leptoceratopsids are notable for living towards the end of the Cretaceous while retaining a number of rather “primitive” traits and smaller body sizes. Ceratopsians in general were still a very diverse clade up until that point.

Paralititan

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropoda > Gravisauria > Eusauropoda > Neosauropoda > Macronaria > Somphospondyli > Titanosauria > Lithostrotia > Saltasauridae

Overview: Fossils of large-bodied dinosaurs are far from unheard of in Egypt’s Bahariya Formation, but Paralititan was particularly enormous. Length estimates put it at around eighty-five feet or twenty-six meters long, with a weight of between thirty to fifty tons. It lived during the earliest part of the Late Cretaceous, or about ninety-five million years ago. Sauropods had long since reached such sizes, but Paralititan belonged to a specific group that would give rise to some of the largest terrestrial animals ever recorded - the Titanosauria. Some titanosaurs, including Argentinosaurus and Patagotitan, grew to be even larger than Paralititan. Not all titanosaurs were huge, however. Some of the smallest known sauropods also belonged to this lineage. Within the larger clade Titanosauria, most studies place Paralititan within the family Saltasauridae, making it a somewhat more derived titanosaur. Titanosaurs first appear in the fossil record during the earliest days of the Cretaceous, eventually replacing most other sauropod groups like the diplodocids.

Formally described as a genus in 2001, Paralititan is based on relatively limited fossils, mainly made up of a few vertebrae and a large limb bone. These fossils preserve enough features to reveal how it should be classified, but also enough to get some decent size estimates. Paralititan was probably the largest animal in its local environment, which would’ve primarily been along the northern coast of what is now North Africa. This region would’ve been far wetter than it is today. Paralititan had a wide variety of plants to feed upon growing along tidal flats, lagoons and mangrove swamps. Its generic name, meaning “tidal giant”, refers to this habitat. The specific name, P. stromeri, honors the German paleontologist Ernst Stromer, who first described fossils from the Bahariya Formation. Paralititan notably lived in a region with a high diversity of large predatory theropods. This included the famous Spinosaurus, though it primarily hunted aquatic animals. Other genera like Carcharodontosaurus and the related Tameryraptor were another matter, however.

Pelecanimimus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Ornithomimosauria

Overview: Ornithomimosaurs were one of the most successful theropod lineages to emerge in the Cretaceous Period. They were coelurosaurs, so they were more closely related to modern birds (as well as to dinosaurs like Tyrannosaurus) than to the likes of Megalosaurus and Allosaurus. This can be seen to a degree in their physical appearances, though their resemblance to modern emus and ostriches is probably the result of convergent evolution. Most of the well known and “advanced” ornithomimosaurs, such as Ornithomimus and Gallimimus, lived towards the end of the period. For this reason, the discovery of Pelecanimimus is significant, as it is one of the earliest confirmed genera within this group. It was a basal member, lying outside either of the two major ornithomimosaur families, the Deinocheiridae and Ornithomimidae.

Pelecanimimus was a relatively small dinosaur, growing to be maybe two or three meters long, but this was fairly average for at least earlier ornithomimosaurs (some later species grew to be quite large). Its neck was fairly long and the skull low and slender. Later ornithomimosaurs were entirely toothless, but Pelecanimimus still possessed some small teeth along its jaws. Ornithomimosaurs in general are seen as omnivores, which is likely the case with Pelecanimimus too, though traces of a pelican-like throat pouch suggest it could’ve preyed mainly on small fish or other aquatic prey. It is for this feature that the genus was named. In addition to this pouch, traces of a small, keratinous crest were also found atop its head. This was likely a display feature. This genus is known from Spain’s La Huérguina Formation, so it was probably preyed upon by larger theropods like Concavenator - an early carcharodontosaur.

Moabosaurus

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda > Gravisauria > Eusauropoda > Turiasauria

Overview: Length estimates for Moabosaurus usually put it at around eleven or twelve meters long, which by sauropod standards isn’t overly impressive, though still enormous by today’s. It possibly grew to be a bit larger, as many of the specimens used to describe the genus appear to come from sub-adults. Regardless, Moabosaurus would’ve been safe from most predators when fully grown, excluding the largest of theropods. Large-bodied sauropods required a lot of food while growing, which in the case of this dinosaur probably took the form of conifers, ferns or even cycads. Moabosaurus had fairly robust teeth, which implies a diet of mostly tougher plants, though we can’t be sure. Its neck, while elongated, was not as long as those of some other sauropods, so it may’ve specialized at browsing mid-level vegetation. Some sauropods are known to have lived in groups for at least part of their lives, which may’ve been the case with this animal.

Described as a genus in 2017, Moabosaurus takes its generic name from the town of Moab, today located in the state of Utah. All known fossils of the animal come from the sediments of the Cedar Mountain Formation, which covers a wide span of time from the Early Cretaceous to just after the start of the Late Cretaceous. Moabosaurus was originally classified as a macronarian sauropod, or in other words as a relative of Camarasaurus and more distantly of Brachiosaurus. Many skeletal reconstructions of it draw heavily from the appearance of the former. Most now agree that it was a member of the more basal clade Turiasauria, which saw their greatest success during the Late Jurassic and into the early part of the Cretaceous. They’re known for their heart-shaped teeth and rather unusual vertebrae, as well as many superficial traits they shared in common with various macronarians (though they weren’t macronarians themselves). Mierasaurus, another turiasaur, is also known from Cedar Mountain, though it was unearthed from somewhat older rock layers.

Dryptosaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Tyrannosauroidea > Pantyrannosauria > Eutyrannosauria > Dryptosauridae

Overview: Dryptosaurus today is a somewhat obscure dinosaur, but back when its fossils were first discovered, it was quite the significant find. Said bones were unearthed in the 1860’s, back when it had yet to be fully established that theropods, such as Megalosaurus, were bipeds. Dryptosaurus, like Megalosaurus, was known from very limited material. This included some jaw bones, parts of its limbs and at least one enormous claw. However, enough of the limbs were found to prove it was bipedal. These fossils were first described in 1866 by the American paleontologist Edward D. Cope, who gave it the binomial label of Laelaps aquilunguis - the generic name referring to a hunting dog from Greek myth and meaning “hurricane” or “whirlwind”. Its specific name means “eagle-clawed”, referring to the aforementioned claw, which Cope placed on its feet. Still, Cope would oversee a famous painting of the animal, created by the renowned early paleoartist Charles R. Knight, which depicted it as unusually active and dynamic for a dinosaur reconstruction from the 1890’s. Knight, perhaps at Cope’s insistence, would name the painting after his original generic name, naming it Leaping Laelaps.

Laelaps would turn out to be a name already in use for another animal, namely a genus of mite, so it had to be changed. To Cope’s consternation, his hated rival Othniel C. Marsh would officially rename the genus in 1877, coining the name Dryptosaurus, or “ripping lizard”. This name, like the specific name, refers to its claws. Back then, Dryptosaurus was usually classified as a relative of Megalosaurus, in the family Megalosauridae. We know today that it was actually a lot closer to Tyrannosaurus, belonging to the same tyrannosauroid superfamily. Dryptosaurus tends to be classified just outside the family Tyrannosauridae itself, as a basal eutyrannosaur, within its own family - the Dryptosauridae. The claw that Cope believed was a toe talon was actually on its hands. Some reconstructions have depicted it with three fingers, but most now believe it had only two digits, similar to Tyrannosaurus. Unlike Tyrannosaurus, Dryptosaurus still possessed fairly long and quite powerful arms. Its skull was more slender, suggesting a different hunting strategy. The two lived at around the same time, but Dryptosaurus lived across a narrow seaway to the east, in the prehistoric and mysterious Appalachian landmass. Dryptosaurids dominated this region while the more derived tyrannosaurids did so in the western landmass of Laramidia.

Leaping Laelaps

Leaping Laelaps – Charles R. Knight, 1897

Here is the above mentioned painting of Dryptosaurus by Knight. Cope and some other researchers of his time picked upon the fact that dinosaur anatomy suggested they were likely far more active animals than your typical reptile. This view would fall out of favor by the 1920’s, partly because the idea of an evolutionary link between non-avian dinosaurs and birds fell out of favor, though at the end of the 1960’s it would see a resurgence.

Spiclypeus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Marginocephalia > Ceratopsia > Neoceratopsia > Coronosauria > Ceratopsoidea > Ceratopsidae > Chasmosaurinae

Overview: With its enormous head, bony frill and numerous horns, Spiclypeus is a good example of a dinosaur in the family Ceratopsidae. Ceratopsids were the most derived members of the larger clade Ceratopsia. Members of said lineage started out as mostly small and bipedal creatures, in most cases lacking such large frills or horns, though all had parrot-like beaks. Spiclypeus and its kin, due to having such gigantic skulls, had evolved quadrupedal movement, though this trait appeared fairly late into the evolution of ceratopsians. Ceratopsids are most easily distinguished from each other by their cranial ornamentation, which was often unique to each species or genus. Spiclypeus possessed a frill with a notable forward bend towards its apex, fringed by triangular hornlets (epiparietals). On its snout was a fairly small horn, though above its brow was a pair of slightly longer ones. These features acted as both defensive and display structures, to ward off predators or rivals among its own kind.

Spiclypeus belonged to the ceratopsid subfamily Chasmosaurinae. Chasmosaurines tended to have longer, narrower snouts and more elongated frills. These frills usually had large, paired openings which were covered by tissue and skin in life, which made them not overly useful for defense (these fills possibly had striking patterns or other such display features). Being a chasmosaurine made Spiclypeus a close relative of the famous Triceratops, which also belonged to this subfamily, though it was a lot more derived than this genus. Spiclypeus was most closely related to ceratopsids like Kosmoceratops and Pentaceratops. Described as a genus in 2016, Spiclypeus takes its name from Latin, meaning “spiked shield”, in reference to its spined frill. Fossils of the animal were first unearthed in what is now Montana, in the mid-2000’s, from rock layers of the Judith River Formation. It coexisted with other herbivores including fellow ceratopsids, hadrosaurs and pachycephalosaurs, as well as carnivores like Gorgosaurus and Daspletosaurus (two large tyrannosaurid genera).

Changyuraptor

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Maniraptora > Pennaraptora > Paraves > Dromaeosauridae > Microraptorinae

Overview: Fossil preservation varies wildly depending on the conditions present - both before and during fossilization itself. Depending on these conditions, some fossils may be scant, preserving little detail, but the opposite can also be true. In the northeast of China, the famous Yixian Formation, formed during the Early Cretaceous, is particularly famous for its beautifully preserved fossils. Some theropod dinosaurs are so well preserved that traces or impressions of feathers or proto-feathers can be found surrounding their skeletons, preserved in the fine volcanic sediment that entombed them. Changyuraptor is one such dinosaur. Described in 2014, its generic name is a combination of Mandarin Chinese and Latin, meaning “long-feathered thief”, referring to its plumage. This took the form of not just fur-like proto-feathers, but of long-veined, pennaceous feathers like those on the wings of modern birds. Unsurprisingly, Changyuraptor is classified close to the lineage from which birds descend.

Pennaceous feathers appear to have been the norm for dinosaurs in the family Dromaeosauridae, like Changyuraptor. Dromaeosaurids are commonly known as “raptor” dinosaurs to the public, with famous members including Deinonychus and Velociraptor. Changyuraptor was, of course, related to both, but was somewhat more basal. It belonged to the subgroup Microraptorinae (also known as the Microraptoria), which included mostly smaller-sized dromaeosaurids from the Early Cretaceous, largely from Asia. Microraptorines possessed long feathers on their arms, but also on their hind limbs, giving them a “four-winged” appearance. Changyuraptor and its kin were possibly capable of some degree of flight, though they were probably not overly graceful in the air. Gliding was another possibility. Even dromaeosaurids incapable of flight had many potential uses for their “wings”. They could be used to insulate clutches of eggs (evidence suggests they brooded in a fashion similar to birds), for display or to give themselves a boost up steep terrain.

Heterodontosaurus

Phylogeny: Dinosauria > Ornithischia > Saphornithischia > Heterodontosauridae > Heterodontosaurinae

Overview: As the name “different-toothed lizard” would imply, Heterodontosaurus possessed a few different types of teeth throughout its jaws, all situated behind a beak - beaks being common to all ornithischian dinosaurs. Toward the tip of the upper jaw were a few pointed teeth, situated just in front of a pair of enlarged, tusk-like canines on the bottom jaw. Behind them were situated rows of tightly-packed, chisel-shaped teeth that were well suited for tearing apart vegetation. These rear teeth imply Heterodontosaurus was mostly herbivorous, though it’s not unreasonable to assume it supplemented this diet with insects or small vertebrates. The beak, which in life was coated by a sheath of keratin, was useful for snipping stems or branches. Heterodontosaurus may’ve employed its “tusks” to break apart tougher plants, to dig up roots or possibly even for sexual display purposes. Some speculate only males had these canines, though we can’t be certain as determining sex from bones alone can be extremely difficult for most dinosaurs.

Heterodontosaurus only grew to be a little over a meter in length, so it wasn’t an overly large dinosaur, even for the Early Jurassic. Like all basal ornithischians, it was a biped. Evasion would’ve been one of its main means of defense against predators, though its teeth could probably inflict painful bites when caught. This dinosaur is the namesake of the family Heterodontosauridae, which represents some of the earliest confirmed ornithischian dinosaurs. Earlier potential ornithischians are of debated phylogenetic status or may not even be true dinosaurs at all. Heterodontosaurids were, on the whole, smaller-sized herbivores that mainly lived during the Jurassic, though at least a few of them persisted into the Early Cretaceous. Some of them, like Pegomastax, also possessed enlarged canines. China’s Tianyulong has even been found with preserved bristle-like integument or proto-feathers, implying Heterodontosaurus had them too. Many of these dinosaurs, including Heterodontosaurus itself, are known from what is now South Africa. Described as a genus in 1962, Heterodontosaurus is mainly known from the upper layers of the famous Elliot Formation.

Irritator

Phylogeny: Dinosauria > Saurischia > Neotheropoda > Tetanurae > Megalosauroidea (?) > Spinosauridae > Spinosaurinae

Overview: The holotype specimen of this carnivorous dinosaur consisted of a partial skull and portions of the lower jaws, originally unearthed in northeastern Brazil. These bones weren’t found by paleontologists, however, but by fossil poachers intending to sell to private collectors. Sometime after, the remains wound up in Germany, where researchers were able to analyze them. At first, the skull was reported as that of a giant pterosaur (flying reptiles like Pteranodon), though its dinosaurian affinities would soon be recognized. How it fit in amongst other dinosaurs, however, wasn’t fully clear at that time. We know now that this animal belonged to the family Spinosauridae, fossils of which had been known since at least the 1910’s, but even as late as the 1990’s, spinosaurids were poorly understood. Spinosaurids are often classified in the megalosauroid superfamily, which if accurate, made them relatives of dinosaurs like Megalosaurus, in the Megalosauridae. However, in recent studies, spinosaurids have been found to potentially belong to the clade Carnosauria

Paleontologists would formally describe Irritator as a genus in 1996. Irritator takes its name from the irritation these scientists felt upon realizing its discoverers had artificially lengthened and altered the skull with plaster prior to selling, hoping a larger skull would fetch a higher price. Due to this, preparation of the specimen took significantly longer than expected. Other spinosaurid fossils have also been unearthed in Brazil, including those referred to a genus called Angaturama, which was also formally named in 1996. Later studies have found that the two were probably the same animal. Irritator was described first, so its name has priority. This would increase the known material of Irritator to include other parts of the body. It appears to have been an average-sized spinosaurid of maybe seven or eight meters in length. Irritator, like other spinosaurids, had a narrow, elongated snout. The jaws were lined with conical teeth, similar to those of a crocodile, implying a diet of fish and other aquatic prey, though they ate other animals as well. One specimen was found to have consumed parts of a pterosaur.

Alaskacephale

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Marginocephalia > Pachycephalosauria > Pachycephalosauridae > Pachycephalosaurinae

Overview: Dinosaurs in the clade Marginocephalia are split into two major lineages - the Ceratopsia and the Pachycephalosauria. Alaskacephale belonged to the latter group. While later ceratopsians evolved to be quadrupedal and grew quite enormous, pachycephalosaurs generally stayed relatively small and were entirely bipedal. Both groups are famous for their unusual skull anatomy, however. Alaskacephale is a good example of a pachycephalosaur. These animals, with few exceptions, tended to possess thickened domes of bone atop their skulls, the exact purpose of which is hard to determine. One idea, often depicted in popular media, is that these dinosaurs used their domes for direct head-butting behavior, either as a defense against predators or for intraspecific competition (competing males for instance). Some evidence suggests that side-to-side strikes between individuals were more common, based on their neck anatomy. The thickened bone could also store minerals or be used in some way for heat exchange.

Alaskacephale and its relatives are generally thought to have been herbivorous, though some do speculate that pachycephalosaurs were omnivores. They usually had fairly narrow, beaked snouts with small teeth lining the jaws. Food sources included ferns, cycads, young conifers, tubers and even fruit, possibly supplemented by insects or grubs. Pachycephalosaurs tended to be low-browsers due to their size. The exact size of Alaskacephale itself is hard to determine as it’s only known from parts of the skull, which is fairly common for pachycephalosaurs. Often, their skull domes were the most solid and easily fossilized part of the body. This genus may’ve been about two or so meters long, so average-sized for its kin. Scientists place Alaskacephale within the family Pachycephalosauridae, so Pachycephalosaurus itself was a close relative. Alaskacephale is known from much farther north, in what is now the northern part of Alaska, within the rocks of the Prince Creek Formation. Earth in the Late Cretaceous was warmer than today, but Alaskacephale still had to contend with colder periods of extended darkness.

Lythronax

Phylogeny: Dinosauria > Saurischia > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Tyrannosauroidea > Pantyrannosauria > Eutyrannosauria > Tyrannosauridae > Tyrannosaurinae > Teratophoneini

Overview: Tyrannosaurs as a lineage first appear in the fossil record during the Middle Jurassic, at a time when other theropod groups were dominant. For much of their existence, with a small handful of exceptions, tyrannosaurs were smaller-sized predators living in the shadow of the megalosaurids and later the allosauroids. However, as those lineages began their decline, some tyrannosaurs evolved to fill those niches themselves, obtaining ever larger sizes. The largest were in the family Tyrannosauridae, to which the famous Tyrannosaurus rex belonged. Lythronax, having lived over eighty million years ago, was one of the earliest confirmed members of the family. It was a lot smaller than Tyrannosaurus, at maybe seven meters long, but easily reigned as the top predator of its own local environment. Its fossils are known from Utah’s Wahweap Formation, so it would’ve lived alongside dinosaurs like Diabloceratops – a ceratopsid and potential food source.

In its general appearance, Lythronax was similar to most other tyrannosaurids. It was a strongly built biped with a large skull, equipped with powerful jaws. Lythronax had a notably short, but deep snout in comparison to most other tyrannosaurids. The arms were short, but muscular, bearing only two digits on each hand, each bearing a curved claw. For the most part, Lythronax and its kin relied mainly upon their jaws to kill. Researchers usually divide the Tyrannosauridae into two major subfamilies called the Albertosaurinae and the Tyrannosaurinae. Lythronax belonged to the latter, so it was closer to Tyrannosaurus than to Albertosaurus or Gorgosaurus. Its closest relatives were dinosaurs like Teratophoneus and Dynamoterror, which along with Lythronax are classified within a tribe called the Teratophoneini. Members of this group mainly lived in what is now the southwest of the United States. Described in 2013, the generic name of Lythronax means “king of gore” in Latin, which fits a trend of giving imposing names to tyrannosaurs.

Valdosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Ornithopoda > Iguanodontia > Dryosauridae

Overview: Valdosaurus was a medium-sized, bipedal herbivore that lived in what is now England some one hundred and thirty million years ago. Fossils attributed to the genus are known from the Wessex Formation and other formations of similar age. Valdosaurus probably lived alongside a wide array of other dinosaurs including the sauropod Ornithopsis, the armored Polacanthus, various ornithopods like Iguanodon or Mantellisaurus, and predatory dinosaurs like Neovenator (a major threat to Valdosaurus itself). These dinosaurs shared a seasonal environment with landscapes of rivers and lake-dotted forests. Potential sources of food for Valdosaurus included cycads, ferns, conifers and other such plants. Lacking armor and being of a moderate size, this dinosaur had little means of defense beyond evasion, camouflage or living in groups.

The generic name of Valdosaurus, coined in 1977, refers to the Wealden Group – a series of geological formations in the south of England, which includes the Wessex Formation. The first known fossils of Valdosaurus, consisting of leg bones, were found in the 1880’s, on the Isle of Wight, located off England’s southern coast. They weren’t recognized as a distinct genus at that time, being referred to Hypsilophodon and soon after to the genus Camptosaurus as a supposed new species. Both dinosaurs, like Valdosaurus, belonged to the clade Ornithopoda, but this animal wasn’t especially closely related to either of them. By the 1970’s, it was recognized to have been closer to North America’s Dryosaurus and was even referred to said genus for a time, before being recognized as a distinct taxon. Valdosaurus and Dryosaurus are both classified in the family Dryosauridae.

Kritosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Ornithopoda > Iguanodontia > Ankylopollexia > Styracosterna > Hadrosauriformes > Hadrosauroidea > Hadrosauridae > Saurolophinae > Kritosaurini

Overview: Hadrosaurids were the most derived of the “duck-billed” dinosaurs. Most of the more popular and widely known “duck-bills” belonged to the family, which was itself split into two main subfamilies – the Lambeosaurinae and the Saurolophinae. Kritosaurus belonged to the latter group. Saurolophines usually lacked the bony head crests seen among lambeosaurines, though this wasn’t always the case. Both lineages were mostly the same in terms of their overall body plan, being sturdily built herbivores that could walk either down on all fours or on their hind legs. Most of their diet was made up of low to mid-level vegetation. Kritosaurus, like all hadrosaurs, possessed both a keratinous beak and rows of tightly-packed, grinding teeth. Hadrosaurs would constantly replace their teeth throughout their lives, growing them rapidly, even as elders. They were able to chew and process their food far more efficiently than any other group of herbivorous dinosaurs.

Saurolophine hadrosaurids are further split into a few major tribes. Kritosaurus is classified within the tribe Kritosaurini, for which it is obviously the namesake. Other notable members of this group includes genera like Gryposaurus and Anasazisaurus. These dinosaurs tended to have prominent, arched nasal bones and large nares (nasal openings). In life, fleshy sacs may’ve encased this structure to act as resonating chambers to make loud calls, or sheathes of keratin formed display structures atop it. Kritosaurus was first described in 1910 by the American paleontologist Barnum Brown (the same man who discovered the holotype specimen of Tyrannosaurus rex). Its name, meaning “separated lizard”, refers to how most of its facial bones were found disarticulated. The earliest reconstruction attempts failed to recognize its nasal arch. Confirmed fossils are known from the southwestern United States and possibly in Mexico. Canadian specimens once referred to it probably belong to the related Gryposaurus, which for a time was seen as the same genus as Kritosaurus.

Anchisaurus

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Anchisauridae

Overview: Anchisaurus is a good example of a basal sauropodomorph, though it’s by no means the most “primitive” of the sauropodomorphs known. More derived sauropodomorph lineages, such as the true sauropods, had already emerged by its time. They were far larger, quadrupedal animals. In the case of Anchisaurus, however, it was much smaller and retained the bipedal posture of its earlier ancestors. Being smaller had its advantages. Anchisaurus likely wasn’t especially agile when compared to some other dinosaurs of its size, but it certainly was in comparison to its larger kin. Its arms being freed up also allowed for their use in defense and foraging. Basal sauropodomorphs of its ilk tended to have large hand claws, useful for manipulating branches, digging up roots and for swatting at an attacker. Food sources for this animal included horsetails, ferns, cycads and conifers. Like other basal sauropodomorphs, Anchisaurus had fairly simple teeth.

The holotype specimen of Anchisaurus, consisting of a partial skeleton, was accidentally unearthed in the mid-1850’s, in the state of Massachusetts. Later, a researcher by the name of Edward Hitchcock Jr. would study the bones. He would make a connection between these fossils and a set of fossilized tracks his own father had described in the area decades before. Hitchcock would at first describe it under the name Megadactylus. Another paleontologist, Othniel C. Marsh, pointed out that said name was already in use for another taxon, so its current generic name was officially chosen by him in 1885. Its name, meaning “near lizard”, refers to how Marsh saw it as a transitional form between later and earlier dinosaurs. Anchisaurus is today classified as a basal sauropodomorph in the clade Sauropodiformes. This made it more closely related to true sauropods than it was to dinosaurs like Plateosaurus, despite it retaining so many basal traits.

Nigersaurus

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda > Gravisauria > Eusauropoda > Neosauropoda > Diplodocoidea > Rebbachisauridae > Rebbachisaurinae

Overview: Nigersaurus was a highly unusual sauropod dinosaur. For starters, it doesn’t fit with the general stereotype of sauropods being enormous, only having been maybe nine meters long and a few tons in weight. This actually isn’t too unusual, as even derived sauropods tended to vary widely in size. What was really unusual was its skull and jaws. The creature’s snout was wide and squared-off at the front, lined by a large number of tiny, tightly-packed teeth. Jaw shape, of course, tends to differ among animals in accordance with their diets and lifestyles, so a wide muzzle in and of itself wasn’t too strange. What sets Nigersaurus apart was how its jaws were modified as it evolved toward this form. Instead of the front of its snout widening, the sides of its jaws actually rotated forward. Its teeth, which could number in the hundreds, formed a straight cutting surface and appear to have constantly replaced themselves throughout its life. Overall, Nigersaurus had a very lightly constructed and fragile skull, even for a sauropod.

There is some debate as to why Nigersaurus evolved such unique jaws, but most see it as a highly specialized low-browser. Most sauropods were mid to high-browsing herbivores, using their long necks to feed from tall conifers and other trees, but Nigersaurus went in the opposite direction. In some studies, it has been found that Nigersaurus may’ve habitually held its head low towards the ground, though to what extent this is true is debated. Nigersaurus belonged to a family called the Rebbachisauridae, which mainly contained other odd, low-browsing and mostly small to medium-sized sauropods. These animals belonged to the larger superfamily Diplodocoidea, so they were distantly related to famous diplodocid sauropods like Diplodocus and Apatosaurus. Described in 1999, Nigersaurus takes its name from the nation of Niger, where it was found, in what is today a desert but was once a far more humid environment. It’s known from Niger’s Elrhaz Formation, so it lived during the Early Cretaceous, alongside dinosaurs like Ouranosaurus and Suchomimus.

Dilophosaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Dilophosauridae

Overview: Dilophosaurus fossils were first discovered in the early 1940’s, on what is today Navajo land in the state of Arizona. They consisted of a few partial skeletons, which were first described in 1954 by paleontologist Samuel P. Welles. Welles would refer these specimens to the famous genus Megalosaurus, as a new species. More complete fossils would come to light in the 1960’s, some of which included the remnants of prominent head crests which revealed to Welles the actual truth – it was its own, separate genus. Welles formally established Dilophosaurus as such in 1970. The genus takes its name from Greek root words, together meaning “two-crested lizard”, inspired by the paired crests that ran along the length of its snout and over the eyes. All confirmed fossils of this dinosaur come from the Kayenta Formation, dated to the Early Jurassic. Some remains found in China were once referred to Dilophosaurus as an additional species, but they have since been re-classified as belonging to a separate genus called Sinosaurus. Dilophosaurus may’ve coexisted with the basal sauropodomorph Sarahsaurus and the small armored dinosaur Scutellosaurus, both of which were potential prey.

Contrary to some famous film portrayals, Dilophosaurus wasn’t a small theropod. Indeed, growing to be six or seven meters long, it was actually one of the largest land predators of the Early Jurassic, rivaled only by dinosaurs like Cryolophosaurus. Both animals, however, were lightly built, as was typical for such early theropods. Dilophosaurus was at first classified as a megalosauroid and later as a relative of either Coelophysis or Ceratosaurus. Currently, paleontologists treat it as a basal member of the clade Neotheropoda. It was “primitive”, but more derived than Coelophysis and its family. Dilophosaurus may belong to its own family, the Dilophosauridae, but there is some debate over what if any other dinosaurs really belonged to the clade. There is also no fossil evidence of either venom spitting or an extendable neck frill. This animal was easily capable of killing most prey with its jaws and claws alone and its sheer size, at least as an adult, would’ve warded off other dangerous predators. Its crests, the exact shape and full size of which have yet to be determined, were probably visual display structures.

Tarchia

Phylogeny: Dinosauria > Ornithischia > Genasauria > Thyreophora > Thyreophoroidea > Eurypoda > Ankylosauria > Euankylosauria > Ankylosauridae > Ankylosaurinae

Overview: Like all ankylosaurs, Tarchia was a sturdily built, heavily armored animal. Its body was slung low to the ground, supported by four short, but strong legs. Ankylosaur armor was formed by an array of bony masses, called osteoderms, growing within its skin. Modern crocodiles and alligators have similar dermal armor on their backs today, though not to the same extent. Tarchia possessed armor over its whole back, along its sides, down its tail, across its neck and even on its head. Its osteoderms took different forms depending on where they were located, including oval-shaped masses across the back, spinier ones along the sides or closely packed, tile-shaped masses over the skull. Tarchia belonged to the ankylosaur family Ankylosauridae, which made it more closely related to the famous Ankylosaurus than it was to nodosaurid ankylosaurs like Hylaeosaurus or Edmontonia. Ankylosaurids usually possessed bony clubs on the ends of their tails, affixed to rows of fused tail vertebrae. This was its more active means of defense.

Tarchia needed extensive armor, considering it had to contend with powerful predatory dinosaurs like Tarbosaurus – a close relative of North America’s Tyrannosaurus. Other herbivores in the region included titanosaurian sauropods like Nemegtosaurus and a wide array of ceratopsians and ornithopods. The first known specimens of Tarchia were found in by Polish and Mongolian fossil hunters in the 1970’s, in what is now the Gobi Desert of Mongolia. Described in 1977, referred fossil material included parts of its braincase, which inspired its generic name, meaning “brainy one” in Mongolian. The creature’s brain itself wasn’t very large. These fossils were found within Mongolia’s Barun Goyot Formation, but more fossils would also be attributed to Tarchia from the geologically younger Nemegt Formation. Mongolia at this time was transitioning between an arid, desert-like environment and a somewhat wetter one, which is represented by the rock deposits within the formations themselves.

Kelumapusaura

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Ornithopoda > Iguanodontia > Ankylopollexia > Styracosterna > Hadrosauriformes > Hadrosauroidea > Hadrosauridae > Saurolophinae > Austrokritosauria

Overview: Hadrosaurid dinosaurs, the true “duck-bills”, were once thought to be a lineage unique to the northern hemisphere, primarily to North America and Asia. Discoveries in recent decades have proven this to be a false assumption. Kelumapusaura is an example of a southern hadrosaur, its fossils being known from the Allen Formation of Argentina. It would’ve lived around seventy million years ago, possibly alongside other dinosaurs like Austroraptor and a myriad of other prehistoric animals. Described as a genus in 2022, Kelumapusaura takes part of its name from the language of the local Mapuche people, combined with Greek, meaning “red earth lizard”. This was chosen in reference to the reddish sediments from which its remains were excavated. Fossils of some other hadrosaurs are also known from the Allen Formation.

Kelumapusaura was a decently large hadrosaur, measuring between eight and nine meters long. It’s probably the largest hadrosaur known from the Allen Formation. Like other hadrosaurs, it would’ve spent most of its time down on all fours as it grazed for food, but it retained the ability to rear up on its hind limbs to feed from tree branches or survey its surroundings. Most southern hadrosaurid dinosaurs are classified within a clade called the Austrokritosauria, which appears to have been the case with Kelumapusaura. The name of this group invokes that of North America’s Kritosaurus. The austrokritosaurs, like Kritosaurus, often had high-arched nasal bones, which may’ve supported some kind of display structure in life, or possibly air sacs. Austrokritosaurs may’ve been close relatives of Kritosaurus itself, which belonged to the hadrosaur tribe Kritosaurini.

Guanlong

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Tyrannosauroidea > Proceratosauridae

Overview: By the end of the Cretaceous, the tyrannosaurs had become the dominant land predators of the northern hemisphere, mainly in Asia and North America. They had also obtained considerable size, with Tyrannosaurus itself being among the largest known theropods. However, the tyrannosaur lineage had humble beginnings. They first appear in the fossil record during the Middle Jurassic. Guanlong itself lived towards the start of the Late Jurassic and, like most of its early tyrannosaur relatives, it wasn’t overly large. In terms of length, Guanlong was about three or four meters long at most and lightly built, quite unlike the tyrannosaur bruisers that would evolve some fifty to sixty million years later. Scientists usually classify Guanlong within a family known as the Proceratosauridae, which contained some of the most basal and earliest known members of the tyrannosauroid superfamily. Despite their name, they weren’t close relatives of Ceratosaurus.

Proceratosaurids shared a few features in common that set them apart from their later relatives, in addition to their (typically) smaller size. The earliest tyrannosaurs had rather long arms and retained three distinct, clawed digits on each hand. Fragile, bony head crests were also common to the proceratosaurids, growing along their snouts. Guanlong itself possessed such a crest, which took on a rounded form and was probably used for visual display. The creature’s name actually refers to this feature, being derived from the Mandarin Chinese words for “crown” and “dragon”. Guanlong fossils are known from China’s Shishugou Formation, located in what is now the western territory of Xinjiang. Like other Jurassic tyrannosaurs, Guanlong lived in the shadow of much larger predators. In its case, this was likely the allosauroid Sinraptor or the more basal Monolophosaurus. Guanlong probably occupied a niche pursuing smaller prey than either dinosaur.

Yinlong

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Marginocephalia > Ceratopsia > Chaoyangsauridae

Overview: Long before the enormous Triceratops roamed North America in the Late Cretaceous, its tiny ancestors did the same in Asia. Yinlong, while perhaps not a direct ancestor of Triceratops and its kin, is a prime example of an early ceratopsian dinosaur. Ceratopsians first appear in the fossil record only a few million years prior to Yinlong, near the start of the Late Jurassic. At that time, most were only around a meter long. They were bipeds, with quadrupedal locomotion only appearing among the ceratopsians in the Late Cretaceous. Later quadrupedal ceratopsians, such as Triceratops, often had gigantic heads sporting prominent bony frills, spikes and horns. Yinlong had the beginnings of a frill, taking the form of a ridge of bone along the back of its head, but had a tiny skull and entirely lacked horns. Both early and later ceratopsians possessed prominent beaks. Lacking any form of armor or actual horns, the only real defense this dinosaur possessed was said beak.

Phylogenetic studies usually classify Yinlong as a member of the family Chaoyangsauridae, named for the genus Chaoyangsaurus. Chaoyangsaurids are only known from China, with ceratopsians most likely having first evolved in central or eastern Asia. They would remain very successful in the region, though they would also migrate into North America and probably to Europe, though they don’t appear to have found much success in the southern hemisphere. Yinlong was probably a mostly or entirely herbivorous animal, though insects may’ve made up part of its diet. It retained teeth towards the front of its jaws, which the most derived ceratopsian dinosaurs lacked. Remains are known from the Shishugou Formation, in the Xinjiang region of western China. Described as a new genus in 2006, the generic name of Yinlong, meaning “hidden dragon” in Mandarin, refers to it having been discovered near to where the movie Crouching Tiger, Hidden Dragon was filmed.

Patagosaurus

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda > Gravisauria > Eusauropoda > Cetiosauridae

Overview: True sauropods evolved towards the end of the Late Triassic, but it was during the latter part of the Early Jurassic that they began to truly flourish and diversify. One major clade to evolve in this time was the Eusauropoda, from which most of the popularly known sauropods evolved from; dinosaurs like Diplodocus, Brachiosaurus and the titanosaurs. Patagosaurus was a basal or “primitive” member of the eusauropod lineage. Specifically, it’s usually classified within the family Cetiosauridae. Cetiosaurids were more basal than some other eusauropod families, such as the mamenchisaurids. As a member, Patagosaurus was of course a close relative of Cetiosaurus itself, though it’s debated as to which other basal eusauropods truly belonged to the family. Both had a lot in common, though Patagosaurus lived in Argentina, while Cetiosaurus roamed England.

Cetiosaurids were mostly medium-sized sauropods. Patagosaurus grew to be about fifteen meters long and a handful of tons in weight. While this is considerably smaller than some other sauropods that evolved later, it was still one of the largest land animals of its time. Patagosaurus probably fed upon medium to high-level vegetation, with conifers making up a fair bit of its diet. It itself may’ve been a food source for theropods like Asfaltovenator and Piatnitzkysaurus. All three genera are based on fossils unearthed from Argentina’s Cañadón Asfalto Formation. While an obscure genus to the public at large, Patagosaurus is decently well known as far as fossils go. Up to a dozen or more partial specimens have been found, representing different growth stages. Patagosaurus was described in 1979 by the renowned José F. Bonaparte, its name referring to the region of Patagonia.

Falcarius

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Maniraptora > Therizinosauria

Overview: Attributing one diet or lifestyle to broad groups of dinosaurs (or any animal) will lead to inaccurate assumptions. Theropods, which included dinosaurs like Megalosaurus, Allosaurus and Tyrannosaurus, were mostly carnivorous when we exclude modern birds. However, some non-avian theropods occupied quite different niches. Some were omnivores or even herbivorous, as we can see with Falcarius. Falcarius belonged to the clade Therizinosauria, which contained some of the most bizarre theropods known to science. They’re known for their unusual leg and hip bones, semi-upright posture, potbellied profiles, elongated necks and enormous arms. Their hands usually sported long claws. Some, such as Therizinosaurus itself, were quite enormous, but Falcarius was considerably smaller, as well as much more “primitive” within the Therizinosauria. Even among the more derived therizinosaurs, there was a lot of variation in body size.

Falcarius possessed a number of the traits for which the therizinosaurs are famed, though to a less exaggerated degree. Among the confirmed therizinosaurs, it’s usually classified as the most basal member of the clade, falling outside both the therizinosauroid superfamily (which included genera like Beipiaosaurus) and the family Therizinosauridae (to which Therizinosaurus belonged). Falcarius was more slenderly built and held its body more horizontally. The latter trait was the norm for most theropods, so it retained something its later relatives would evolve away from. Even as a basal therizinosaur, Falcarius had already evolved a herbivorous diet, though its ancestors would’ve been carnivores. Falcarius lived during the Early Cretaceous and is known from the lower levels of the Cedar Mountain Formation, in what is now Utah. While not a predator itself, other local theropods certainly were, most notably Utahraptor – the largest known dromaeosaurid dinosaur.

Gargoyleosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Thyreophora > Thyreophoroidea > Eurypoda > Ankylosauria > Euankylosauria > Nodosauridae (?)

Overview: With a modest understanding of dinosaurs, one might be tempted to see the stegosaurs (Stegosaurus, Kentrosaurus, etc.) as a strictly Jurassic group of herbivores and the Ankylosaurs (Ankylosaurus, Euoplocephalus, etc.) as a Cretaceous lineage. In reality, there is a fair bit of temporal overlap. The former would persist for a while into the Early Cretaceous, while the ankylosaurs actually first appear in the fossil record during the Middle to Late Jurassic. Taxa in both groups occasionally coexisted. Gargoyleosaurus is a good example of this. Fossils of the animal are known from the famous Morrison Formation of North America, which also contains the remains of the famous Stegosaurus. The two dinosaurs shared the same environment, likely having different ecological roles or niches. Gargoyleosaurus probably fed entirely on low-level plant life.

While the stegosaurs usually had upward-standing plates along their backs and spiked tails, the armor on ankylosaurs like Gargoyleosaurus was quite different. This dinosaur had rows of bony scutes running along its back, sides, tail and neck. It was well protected from all but the largest of theropods. Unfortunately for Gargoyleosaurus, it lived among a variety of large theropods, namely taxa like Ceratosaurus, Torvosaurus and Allosaurus. Gargoyleosaurus lacked a club on the end of its tail, which could imply it was a nodosaurid ankylosaur – ankylosaurids usually had such clubs. However, it could’ve been an early member of the Ankylosauridae, as we can assume the most basal of them didn’t have such a trait, which fossil evidence does support. Gargoyleosaurus was formally named as a genus in 1998, its generic name referring to the gargoyle like appearance of its holotype fossil.

Spectrovenator

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Ceratosauria > Neoceratosauria > Abelisauroidea > Abelisauridae

Overview: Described by paleontologists in 2020, the generic name of Spectrovenator is taken from Latin, intended to mean “ghost hunter”. Spectrovenator was a carnivore and presumed predator, so the name is fitting in that respect, but it was mainly chosen due to the circumstances in which its holotype specimen was found. It was buried beneath the remains of a much larger dinosaur, namely a titanosaurian sauropod, so it was “hidden” and could’ve been easily missed. The name can also be seen in a less literal way, referring to how Spectrovenator fills in a gap within the evolutionary history of its close relatives. Known fossils of the animal include its skull, neck, parts of the spine, pelvis, legs and a portion of the tail. Its upper torso and arms have yet to be found or described in detail. Spectrovenator was overall a remarkable and important find. All known fossils originate from Argentina’s Quiricó Formation.

Researchers classify Spectrovenator within the family Abelisauridae – a part of the larger clade Ceratosauria. Confirmed abelisaurid fossils first appear in the Early Cretaceous, Spectrovenator being an example of such an early and basal member. There are some fossils from the Jurassic that possibly belong to the Abelisauridae, specifically those of a genus called Eoabelisaurus, though it’s more often classified nowadays outside the family in the larger abelisauroid superfamily. Most of the better known abelisaurids lived in the Late Cretaceous, when they were notably successful in the southern hemisphere. They often had tiny arms, deep snouts and grew to be fairly large. In the case of Spectrovenator, however, it was notably small, at between two and three meters long. Its arms are unknown, but they may’ve been longer than those of its later relatives. The skull was longer and less deep than those of some more derived abelisaurids like Carnotaurus.

Kosmoceratops

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Marginocephalia > Ceratopsia > Neoceratopsia > Coronosauria > Ceratopsoidea > Ceratopsidae > Chasmosaurinae

Overview: The official description of Kosmoceratops as a new genus was published in 2010, based on a set of fossil remains unearthed in the state of Utah. Its generic name is derived from Greek, translated as “ornate horned face”. Ceratopsids like Kosmoceratops all possessed “ornate” faces, or more accurately, skulls overall. Said skull was enormous, bearing a long, beaked snout. The nasal horn was low, blade-like and oddly rectangular, as opposed to the longer and pointed horns usually seen on the snouts of its relatives. Kosmoceratops had much longer brow horns, which curved out towards the sides. The animal’s most distinctive feature was its frill, the top of which was curled forward and fringed by pointed hornlets. As with most ceratopsids, this frill was probably used for visual display purposes, but its horns may’ve played an additional defensive role. Ceratopsids differ little in their overall anatomy, but their cranial ornamentation helps to distinguish them.

Kosmoceratops belonged to the ceratopsid subfamily Chasmosaurinae, making it a somewhat close relative of Triceratops. It was more basal than Triceratops, however, being closer to genera like Pentaceratops and Spiclypeus. All of these animals were generally low-browsing herbivores, using their beaks to snap branches and snip stems, while tightly-packed shearing teeth lined the back of the jaws, well suited for processing even tougher vegetation. Some ceratopsids are known to have lived in groups, which was possibly the case with Kosmoceratops. Known fossils of this taxon include most of the skull, its ribcage, the spine, its pelvis and some leg bones. All were found within Utah’s Kaiparowits Formation, dated to the Late Cretaceous. It coexisted with other herbivores like Parasaurolophus and Nasutoceratops, as well as the carnivorous Teratophoneus (a tyrannosaurid). Deinosuchus, a large crocodilian or close kin to crocodilians, was also present.

Hadrosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Ornithopoda > Iguanodontia > Ankylopollexia > Styracosterna > Hadrosauriformes > Hadrosauroidea > Hadrosauridae

Overview: The vast majority of dinosaur fossils described from the United States come from the western part of the country. Exposed rock layers dated to the Mesozoic Era are more common there, unlike in the east, where erosion in some areas has left only pre-Mesozoic rocks exposed and sediment buildup lower down has covered them. Still, a number of dinosaurs have been found and described from the east, including some of the first ever to be named in North America. One of these dinosaurs was Hadrosaurus, described in 1858 by the renowned paleontologist Joseph M. Leidy. The first remains of the animal were found a couple decades earlier in New Jersey, quite on accident by a local man excavating for marlstone. More fossils would later be recovered at this same site, making Hadrosaurus one of the more completely known dinosaurs at the time.

Hadrosaurus would prove a significant discovery. Its limb bones showed it was capable of walking on its hind limbs, which had implications for earlier restorations of dinosaurs like Iguanodon (most famously depicted previously at Crystal Palace Park in London), which had been interpreted as entirely quadrupedal animals. The actual situation with Hadrosaurus was a bit more complex, as it belonged to a lineage that could actually walk in both fashions. Hadrosaurus would go on to become the namesake of both the family Hadrosauridae and the superfamily Hadrosauroidea – the clades containing the famous “duck-billed” dinosaurs. Among the hadrosaurids, Hadrosaurus itself is usually treated as a basal member. It was average-sized, at about eight meters long. Like all of its kin, it possessed both a beak and a large number of teeth lining the back of its jaws.

Khaan

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Maniraptora > Pennaraptora > Oviraptorosauria > Caenagnathoidea > Oviraptoridae > Heyuanniinae

Overview: Khaan takes its generic name from the Mongolian word for “lord” or “ruler”, the same title held by the infamous Genghis Khan. The name, coined in 2001, was chosen to honor Mongolia and its history. Khaan itself wasn’t an especially imposing or regal dinosaur by any means. It grew to be only one or two meters long and was, generally, lightly built. All known fossils of the animal come from the rock layers of Mongolia’s Djadochta Formation, so Khaan would’ve lived about seventy-five million years ago. Today, the Djadochta is exposed in the form of dramatic, reddish cliffs in the Gobi Desert. According to evidence, these rocks preserve what was once a somewhat similar environment, with Khaan and other Djadochta dinosaurs having inhabited a landscape of sand dunes with little water. Contemporaries included Velociraptor and Protoceratops.

The overall anatomy of Khaan reveals it to have been a member of the family Oviraptoridae, so it was a close relative of the famous Oviraptor, fossils of which are also known from the Djadochta. Oviraptorids belonged to a larger clade known as the Oviraptorosauria, which were classified somewhat close to the lineage that gave rise to birds. They were superficially very birdlike animals, evidence suggesting they possessed extensive feather coverage. Often, oviraptorid skeletons are found brooding over their own nests, their arms outstretched like the wings of birds to insulate and keep their eggs warm. Khaan and Oviraptor are distinguished by a few detailed traits. For the most part, Khaan retained more “primitive” traits than those of Oviraptor. Oviraptorids are usually seen as omnivores, having possessed short, toothless beaks that were well suited to such a diet.

Austroposeidon

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda > Gravisauria > Eusauropoda > Neosauropoda > Macronaria > Titanosauriformes > Somphospondyli > Titanosauria > Lithostrotia

Overview: Growing to be up to twenty-five meters long and weighing a few dozen tons, this genus is a contender for the largest known Brazilian dinosaur. Fossils of the animal originate from rocks belonging to the Presidente Prudente Formation, layers of which are today exposed in the south of Brazil. The first of these bones were found in the 1950’s, though Austroposeidon wouldn’t be fully described and properly named until 2016. Its generic name means “southern Poseidon” or “Poseidon of the south”, referring to it originating in the southern hemisphere, but also to the Greek sea deity Poseidon. Poseidon was thought to be the origin of earthquakes, so the name is often invoked for large sauropod genera. North America’s Sauroposeidon is one such example.

Austroposeidon was a member of the famous titanosaurian lineage of sauropods, making it related to dinosaurs like Paralititan and Argentinosaurus. It belonged to the clade Lithostrotia, so it was somewhat derived as far as titanosaurs go. Some believe it may’ve belonged to the Lognkosauria, which made Argentinosaurus a potential close relative, but this is debated. As large as this dinosaur was, some lognkosaurs, such as Argentinosaurus itself, grew to even greater proportions. Fully grown Austroposeidon were probably safe from most predators, though the young, weak and elderly would’ve been vulnerable to attacks from large theropods. Most of the traits that set this dinosaur apart from other titanosaurs could be seen on its vertebrae.

Kileskus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Tyrannosauroidea > Proceratosauridae

Overview: Kileskus is one of the oldest known tyrannosaurs from the fossil record, rivaled only by the genus Proceratosaurus, from what is now England. Kileskus itself lived in what is today central Russia, its fossils having been found in the region of Krasnoyarsk, in Siberia. Said fossils came from the Itat Formation, meaning Kileskus would’ve lived around one hundred and sixty-six million years ago. Both it and Proceratosaurus belonged to the family Proceratosauridae, which is generally seen as the most basal lineage within the tyrannosauroid superfamily. During the Middle Jurassic, tyrannosaurs were only minor predators, usually going after smaller prey. They themselves may’ve been preyed on by larger theropods, mainly megalosaurids or early allosaurs. Kileskus is currently the only dinosaur described from the Itat, though fossils of other dinosaurs have been found there. Described in 2010, its scientific full name, Kileskus aristotocus, translates as “lizard of noble origin”, partly derived from the local Khakas language.

Paleontologists estimate Kileskus to have been about four or so meters long, so it was a far cry from the behemoths that would later emerge from its superfamily. This was average for a member of the Proceratosauridae. Kileskus is primarily known from bits of its skull and some bones from the hands and feet. What we know about other proceratosaurids can help us to reconstruct its life appearance. These dinosaurs tended to have long arms, unlike the most derived tyrannosaurs, as well as three clawed digits. Kileskus likely had a bony crest over its snout, as we can see on both Proceratosaurus and Guanlong. We have direct fossil evidence that these early tyrannosaurs had simple proto-feathers over most of their bodies, but later tyrannosaurs may’ve lost them due to obtaining such huge sizes. The proceratosaurid Yutyrannus, from the Early Cretaceous of China, was decently large for its time and still had proto-feathers, however.

Oryctodromeus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Thescelosauridae > Orodrominae

Overview: Growing to be about two meters long, Oryctodromeus wasn’t a large dinosaur by any means, or exciting from a first glance. It was a bipedal herbivore that probably subsisted mainly on low-level shrubbery and ferns. What is unique about it was its potential lifestyle. The holotype of Oryctodromeus is one of the first known examples of evidence for burrowing behavior among the Dinosauria. Skeletal remains of the animal were actually found within their burrows, closely packed together and smothered with infilled sediments. The anatomy of the bones themselves also supports such a lifestyle. Its forelimbs and shoulders were modified to allow for prolonged digging, in a somewhat similar fashion to that of some modern burrowing animals.

Living in burrows would’ve granted a few key advantages to Oryctodromeus. It gave it a place to flee from larger predators and a place to more safely raise its young. As with other dinosaurs, it would’ve laid clutches of eggs, so multiple hatchlings probably lived together with their parents inside these burrows. We can assume, however, that some predators may’ve adapted in turn to this behavior, as there are some predators in our own time that specialize in pursuing burrowers. Described in 2007, the generic name of Oryctodromeus means “digging runner”, which refers to both its lifestyle and to its smaller, more agile form. The genus belonged to a family of mainly small-bodied ornithischians called the Thescelosauridae, Orodromeus being a close relative.

Unenlagia

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Maniraptora > Pennaraptora > Paraves > Dromaeosauridae (?) > Unenlagiinae

Overview: Unenlagia was long seen as a somewhat mysterious dinosaur. Fossils of the animal were first described in 1997, having been found in Argentina’s Portezuelo Formation, dated to the Late Cretaceous. Recovered bones included the pelvis, some leg bones, bits of vertebrae, ribs and part of the shoulder. All of these remains shared traits in common with birds, so Unenlagia was seen by its describers as one of the non-avian dinosaurs most closely related to proper birds. This idea is reflected by its generic name, meaning “half-bird” in the local Mapuche language of the region. In some later studies, Unenlagia was associated with the “raptor” family Dromaeosauridae, so for a time, many reconstructions depicted it as a creature similar in appearance to dinosaurs such as Deinonychus or Velociraptor, but this has since been proven inaccurate.

Currently, paleontologists classify Unenlagia within a subfamily called the Unenlagiinae. Other taxa probably belonging to the group includes Austroraptor and Buitreraptor. Most are known from the southern hemisphere, but a few potential northern members are also known. Unenlagiines are often classified as an early diverging subfamily within the aforementioned Dromaeosauridae, but in some studies, they are seen as relatives of dromaeosaurs, but not as true members of the family. If the latter is true, then they are a subfamily within the proposed family Unenlagiidae (which may also contain dinosaurs like Halszkaraptor, in their own subfamily). Either way, unenlagiines are set apart by a few unique traits. They tended to have long, slender jaws, lightly built bodies and shorter arms. Traditional dromaeosaur traits like enlarged toe claws were also present on these animals.

Tratayenia

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Tyrannosauroidea (?) > Megaraptora > Megaraptoridae

Overview: Tratayenia is known from partial skeletal remains, mainly comprised of vertebrae and some other bones, but enough features are readily observable to determine what kind of dinosaur it was in life. Tratayenia was probably a member of the family Megaraptoridae, which is itself a part of the larger clade Megaraptora. Megaraptorans are a mysterious group of theropods, with many studies over the years placing them in widely different lineages, largely due to them having a mix of derived and basal theropod traits. In years prior, megaraptorans were classified as allosaurs, more derived than Allosaurus itself and closely related to the carcharodontosaur Neovenator. The most recent studies, however, have instead classified megaraptorans within the Coelurosauria, so they were likely closer to birds and possibly related in some way to the tyrannosaurs.

Length estimates for Tratayenia put it at eight or so meters long, which made it one of the larger known megaraptorans and a decently large theropod overall. Notably, megaraptorans were often a lot more lightly built than other theropods of comparable lengths, which was likely the case with Tratayenia itself. This implies they were somewhat more agile. Megaraptorans tended to have long, narrow jaws, but quite enormous arms, often equipped with enlarged, curving claws. Unlike some other theropods, such as Tyrannosaurus, Tratayenia and its kin mainly relied upon their arms to capture their prey. Described in 2018, Tratayenia takes its name from the fossil site of Tratayén, a part of Argentina’s Bajo de la Carpa Formation, where it was originally found. It was probably one of the top predators of its local environment, possibly competing with some abelisaurids.

Huayangosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Thyreophora > Thyreophoroidea > Eurypoda > Stegosauria > Huayangosauridae

Overview: Back in the late 1970’s, in what is now the Chinese province of Sichuan, remains of a few armored dinosaurs were discovered. Chinese researchers would publish a description of the animal in 1982, naming it Huayangosaurus, deriving its name from the Mandarin term “Huayang”, which can historically be applied as a name for the region of Sichuan. It was found in some of the deeper layers of the Shaximiao Formation, dated to the Middle Jurassic. The discovery proved quite significant, as Huayangosaurus was at that time one of the oldest known members of the clade Stegosauria, to which the famous Stegosaurus belonged. In many respects, the two dinosaurs were very similar. Both possessed twin rows of enlarged osteoderms growing along their backs and long spines on the end of the tail. They walked on all fours and are assumed to have largely lived on low-level plant life. Huayangosaurus, however, was smaller than Stegosaurus.

Length estimates for Huayangosaurus put it at maybe four meters long at most, with it being much lighter than Stegosaurus as well. These weren’t the only differences. Huayangosaurus had far more robust forelimbs, comparatively, than its famous cousin, as well as a proportionately larger skull. Even more notable, Huayangosaurus retained teeth towards the front of its mouth, which among later stegosaurs, were absent (though they retained teeth farther back in the mouth). Most agree it was, fitting for its age, one of the most basal of the known stegosaurs, probably belonging to a family separate from that of Stegosaurus. Said family is referred to as the Huayangosauridae and may contain other notable Chinese stegosaurs like Chungkingosaurus. Huayangosaurus had rather narrow dorsal plates and possibly a set of long spikes on the shoulders. These spines were likely used to ward off predators, such as Gasosaurus or the larger Yangchuanosaurus.

Serikornis

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Maniraptora > Pennaraptora > Paraves > Anchiornithidae

Overview: The casual modern observer would probably mistake Serikornis for a bird at first sight. It certainly shared a lot in common with one, with direct fossil evidence revealing that its body was covered in various types of feathers. Key differences, however, included its long and bony tail, the teeth lining its jaws, lack of a beak and distinct clawed digits on each of its hands. Phylogenetically, Serikornis was closely related to modern birds, belonging to the clade Paraves, which also contains the dromaeosaurid “raptor” dinosaurs and the troodontid family. Serikornis, usually, is classified as a member of the family Anchiornithidae. Other notable anchiornithids includes Aurornis, Xiaotingia, Eosinopteryx and Anchiornis, the family’s namesake. This particular family saw its greatest success in the Late Jurassic, primarily in what is now the northeast of China.

Serikornis is based on some decently preserved remains. Its holotype specimen was unearthed from the rocks of the Tiaojishan Formation, in China’s Liaoning Province. This dates the creature to around one hundred and sixty million years ago, or towards the start of the Late Jurassic. Fossilized remains from the Tiaojishan are often very well preserved, with many small theropods being found with feather impressions around the body. This was the case with Serikornis, showing it had long wing feathers on the arms and simpler, filamentous feathers over most of its body. Fluffy feathers were particularly notable on the legs, covering much of their length. This invokes the image of modern Silkie chickens, which inspired the creature’s generic name, meaning “silk bird”. Serikornis likely fed upon insects and small reptiles. Some believe it is actually the same genus as Anchiornis.

Rhabdodon

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Ornithopoda > Iguanodontia > Rhabdodontomorpha > Rhabdodontoidea > Rhabdodontidae

Overview: Rhabdodon is the namesake of the family Rhabdodontidae, as well as to a larger clade known as the Rhabdodontomorpha. These dinosaurs were iguanodontian ornithopods, so they were relatives of the famous Iguanodon, though they were a lot more “primitive” in comparison, though most confirmed rhabdodontids lived towards the end of the Cretaceous. They likely split off from other iguanodonts much earlier. Most of the rhabdodontids were small to medium-sized ornithopods, probably subsisting on ground-level vegetation. They often had simple, yet sturdy teeth and deep jaws. Unlike more derived iguanodonts, Rhabdodon and its kin appear to have been strictly bipedal animals, their arms being quite short, though they could probably crouch to graze. Rhabdodontids were at their most successful in Europe during the Late Cretaceous.

The first known fossils of Rhabdodon were found in the 1840’s, when a team of workers in France were excavating what would later be a railroad tunnel. Scientists at first associated the find with the genus Iguanodon, but it would soon become clear the animals were fairly different. In 1869, a formal description of Rhabdodon as its own genus was published. The name of the creature means “fluted tooth”, referring to a prominent grove than ran along its teeth. Similar features can be seen on some of its rhabdodontid relatives. Potential kin included genera like Mochlodon and Zalmoxes, though the status of the latter has recently been called into question (one species of Zalmoxes has since been reclassified as a new ceratopsian genus, unrelated to Rhabdodon). Fossils attributed to Rhabdodon are mainly known from geological formations in France and Spain.

Acrocanthosaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Carnosauria > Allosauroidea > Carcharodontosauria > Carcharodontosauridae

Overview: In the 1940’s, two partial skeletons belonging to a large theropod were unearthed in the state of Oklahoma. Recovered fossils included limb bones, parts of the skull, bits of pelvis, ribs and a series of vertebrae. The latter bones stood out due to their rather tall and fairly wide neural spines, which would later inspire the creature’s generic name – Acrocanthosaurus, the “high-spined lizard”. The holotype material came from Oklahoma’s Antlers Formation, so the fossils have been dated to the Early Cretaceous. Other similarly-aged geological formations in the United States, such as the Twin Mountains Formation and Cloverly Formation, have also yielded the fossilized remains of Acrocanthosaurus. It apparently had a wide range over North America during its heyday. Isolated teeth from as far east as Maryland could possibly belong to Acrocanthosaurus, originating from the Arundel Formation, though we can’t be certain if this is the case. In some older studies, Acrocanthosaurus was associated with the megalosaurids or even the spinosaurids, but in most modern studies, it’s classified as a basal member of the Carcharodontosauridae, making it an allosauroid theropod close to Carcharodontosaurus and Giganotosaurus.

Acrocanthosaurus was a massive theropod, with some specimens measuring up to eleven or more meters long and weighing multiple tons. Indeed, it’s often cited as the largest theropod known from North America prior to the appearance of Tyrannosaurus. Acrocanthosaurus was likely the top predator in North America at that time, going after juvenile sauropods, armored ankylosaurs and iguanodonts. It also coexisted with many smaller theropods, notably the dromaeosaurid genus Deinonychus. Its neural spines probably supported a ridge of muscle or fat along its back, possibly to store energy or to make it appear even larger than it already was. Its jaws were lined with blade-like teeth, while its arms were quite powerful despite their shorter length. Three clawed digits were sported by each hand. Allosauroid mega-theropods like Acrocanthosaurus were the top predators in most parts of the world during the Early Cretaceous and into the first half of the Late Cretaceous. Eventually, as dinosaurs like Acrocanthosaurus died out, the tyrannosaurs would begin to grow larger and fill in these now vacant niches (at least in the northern hemisphere). So far, Acrocanthosaurus is the only confirmed carcharodontosaurid known from North America.

Ingentia

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda (?) > Lessemsauridae

Overview: Sauropodomorphs first appear in the fossil record in the Late Triassic, with most of the earliest members of the group being small, lightly built, bipedal and probably omnivorous (evolving from carnivorous dinosaur ancestors). However, prior to the end of the period, they would begin to diversify quite a bit. We can see this with the genus Ingentia. Unlike its earlier relatives, it was a quadrupedal, herbivorous and decently large animal. Its size didn’t quite reach that of some later sauropodomorphs from the Jurassic, but during its time, Ingentia and its closest relatives were some of the largest land animals the world had yet seen up until that point. Fully grown adults were likely safe from all but the largest of predators, which in that time, usually weren’t theropod dinosaurs. Theropods were still mostly smaller and lightly-built animals, while terrestrial crocodile relatives, namely the “rauisuchids”, dominated that top predator role in most regions. Ingentia itself would’ve subsisted on plants like conifers and cycads.

In order to support its great bulk, Ingentia had to have strong limbs, which fossil evidence suggests it certainly had. These limbs, however, weren’t the highly specialized, pillar-like limbs seen on some of its more derived relatives. For the most part, they retained some of their more “primitive” shape, but in a slightly altered form, still well suited for bearing its body weight. Evolving columnar legs would allow later sauropodomorphs to reach such stupendous sizes. Such dinosaurs belonged to the Sauropoda, which some studies suggest Ingentia also may’ve belonged to, representing one of the earliest examples of a true sauropod. It belonged to the family Lessemsauridae, alongside other dinosaurs like Lessemsaurus, Antetonitrus and Ledumahadi. Other studies, however, have placed the Lessemsauridae just outside the Sauropoda, as a sister clade – related, but not truly belonging to it. Described in 2018, the full scientific name of this dinosaur, Ingentia prima, means “first huge one”, referring to its size at a time when that was still rare for dinosaurs.

Sinoceratops

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Marginocephalia > Ceratopsia > Neoceratopsia > Coronosauria > Ceratopsidae > Centrosaurinae

Overview: Ceratopsian dinosaurs first evolved at some point in the Jurassic, starting out as mainly tiny, bipedal herbivores. Remains of such creatures are known from Asia, suggesting ceratopsians trace their roots to the continent. The largest and most derived of the ceratopsians, known from the Late Cretaceous, were the members of the family Ceratopsidae. Despite ceratopsians as a whole originating from Asia and having a lot of success there, Asian ceratopsids specifically were once almost entirely unheard of. Currently, only Sinoceratops has been described from Asia, its fossils having been described from the Shandong Province in eastern China. Paleontologists fully described and named the animal in 2010, giving it the name “Chinese horned face”, in honor of its country of origin. Its discovery proved that ceratopsids, while perhaps rarer there than in North America, were still present in Asia towards the end of the Cretaceous. Its fossils are known from the Hongtuya Formation, so it may’ve coexisted with Shantungosaurus or Zhuchengtyrannus.

Sinoceratops is mainly based on skull material. Pieces recovered include part of its face and around the eye, the base of a nasal horn and a fair bit of the frill that projected from the back of the head. It appears to have had a fairly deep snout in life and, while its full shape is unknown, probably a fairly large nasal horn. Sinoceratops lacked horns over the eyes, though it may’ve had low ridges on the same spot. The frill was relatively short and rounded in profile, bearing two large openings. In life, these openings were covered by tissue and skin (unlike in some media portrayals of the genus). All along the rim of the frill were prominent, forward-curving hornlets. These features were mainly for visual display, though the horn likely doubled as a defensive weapon. Longer nasal horns and deeper snouts are traits associated with the ceratopsid subfamily Centrosaurinae. Sinoceratops is usually considered to have been a basal member of this clade. In other words, Sinoceratops was a lot more closely related to dinosaurs like Styracosaurus than it was to the famous Triceratops (a chasmosaurine ceratopsid).

Zanabazar

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Maniraptora > Pennaraptora > Paraves > Troodontidae > Troodontinae

Overview: Zanabazar was officially described as a genus in 2009, based on fossil material found in the Nemegt Formation of Mongolia. Its generic name was chosen to honor the historical Zanabazar – a Mongolian Buddhist religious authority from the seventeenth century. Some other Mongolian dinosaurs are named after Buddhist figures or traditions, in recognition of its great influence over the country’s history (mainly transmitted from Tibet). Studies of the Nemegt Formation date it to around seventy million years ago, seeming to preserve a relatively humid region crossed by rivers and marshlands. Drier conditions were also present, however, with the formations directly beneath the Nemegt (like the Djadochta Formation) preserving desert-like environments. There is some debate over how distinct each of these formations are, with it being suggested that there was overlap with regards to which dinosaurs lived there at different times.

With a length of two or three meters, Zanabazar wasn’t a very large dinosaur, though it was in the normal size range for its family – the Troodontidae. Troodontids were a highly birdlike group that first appears in the fossil record back in the Late Jurassic, but they saw their greatest success in the Late Cretaceous, as we can see with Zanabazar itself. Scientists place the troodontids close to modern birds, being members of the clade Paraves, which also made them relatives of the famed Dromaeosauridae, or the “raptor” family. Like the “raptors”, Zanabazar and its kin had enlarged talons on the second toe of each foot, probably used to hook into and pin down small prey. The whole of the Paraves were extensively feathered, likely with advanced, veined feathers. Other notable traits seen on troodontids were their enormous eyes and proportionately large brains, so it’s possible they had both keen senses and relatively advanced social behaviors.

Leaellynasaura

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Ornithopoda > Iguanodontia > Elasmaria

Overview: One of the most fossil-riche sites in Australia is “Dinosaur Cove”, located near the coast of southern Victoria. Rocks from the site are dated to the Early Cretaceous and contain fossils of a myriad of different dinosaurs and other Mesozoic animals. Many of these fossils are those of small ornithopods, some of which are attributed to the genus Leaellynasaura. The creature was officially described and named back in 1989 by the paleontologists Thomas H. Rich and Patricia A. Vickers-Rich – a married couple best known for their work at Dinosaur Cove. Leaellynasaura was named after their own daughter, Leaellyn, with another more obscure dinosaur called Timimus being named after their son Timothy. The latter genus was also found at Dinosaur Cove and probably belongs to some kind of coelurosaur, though its exact status is debated. Leaellynasaura is known from more, but still fragmentary fossils. Both come from Australia’s Eumeralla Formation, so they had to contend with decently large megaraptoran or allosauroid predators.

The holotype specimen of Leaellynasaura consisted of portions of the skull, but some other skeletal remains found at Dinosaur Cove probably belong to it, though some studies have pointed out we can’t be certain if they’re from the same animal or not. Either way, it was clearly not a very large dinosaur. Length estimates put it at maybe two meters long and light in weight. The creature was a bipedal, low-browsing herbivore with a short, pointed snout. If the known skeletal material does belong to Leaellynasaura, then it had a remarkably long tail for an ornithopod of its size. The southern part of Australia was, in its time, much farther south. Leaellynasaura had to contend with prolonged periods of cold and darkness, even though the world overall was warmer than now. Its tail may’ve been an adaptation for these conditions, used to wrap around itself or its young. The holotype’s eyes were large, which implies decent night vision, though it may not have been fully grown. Leaellynasaura likely belonged to the clade Elasmaria, meaning it belonged to a lineage of mostly small to medium-sized iguanodonts from the southern hemisphere.

Alpkarakush

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Carnosauria > Allosauroidea > Metriacanthosauridae > Metriacanthosaurinae

Overview: Alpkarakush was probably the top predator of its own local region, some one hundred and sixty-five million years ago. Growing to be eight or so meters long and weighing up to a few tons, it was decently large and probably powerful enough to take on some larger herbivores and those with armor. Alpkarakush fossils are known from the Balabansai Formation, in what is now the nation of Kyrgyzstan in Central Asia. It’s one of the very few dinosaurs described from the Balabansai, though remains of sauropods and early stegosaurs show it lived among a variety of different taxa. Both types of animals were potential sources of prey. Described in 2024, Alpkarakush takes its generic name from that of a giant bird, most famously appearing in the Epic of Manas – an epic poem kept and shared among the Kyrgyz people for at least the past millennia. While a theropod, Alpkarakush itself wasn’t particularly closely related to actual birds.

Some of the fossils described so far from Alpkarakush include its pelvis, most of the legs, some finger bones, vertebrae, ribs and parts of the skull. Much of its torso and arms have yet to be described and the tail, so far, is entirely missing. Still, we have enough to get an idea as to how it would’ve appeared in life, as well as to how it should be classified. Alpkarakush was an allosauroid theropod belonging to the family Metriacanthosauridae. Metriacanthosaurids were a group of apex predators, mainly from Asia and Europe, which saw their peak of success during the Middle to Late Jurassic. They often had short, but very deep snouts, as we can see with Alpkarakush itself. Its arms were short, but probably fairly strong, bearing three digits on each hand. Metriacanthosaurids often had low crests of bone running along their snouts and over their eyes, which were particularly prominent on Alpkarakush, giving it a very distinct appearance.

Diabloceratops

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Marginocephalia > Ceratopsia > Neoceratopsia > Coronosauria > Ceratopsoidea > Ceratopsidae > Centrosaurinae

Overview: Diabloceratops was a medium-sized ceratopsid dinosaur from what is now Utah. The fossils of the animal date back to over eighty million years ago, which makes it one of the earliest confirmed members of the Ceratopsidae – the ceratopsian lineage leading to Triceratops and its ilk. As one would expect, Diabloceratops is also one of the most “primitive” of the ceratopsids, though it has enough distinct features to classify it. Scientists usually place it as one of the most basal known members of the subfamily Centrosaurinae. It was closer to dinosaurs like Styracosaurus or Centrosaurus than it was to the chasmosaurine ceratopsids like Triceratops. Most of the later and better known centrosaurines tended to have little or no brow horns, but long nasal horns or other such structures on the snout. Diabloceratops, like many early centrosaurines, lacked a nasal horn and had relatively long, curving brow horns over each eye.

While its horn arrangement was different than that of later centrosaurines, Diabloceratops did share the same deep, beaked snout. Some speculate that centrosaurines were less selective feeders than the narrow-snouted chasmosaurines. Centrosaurines also tended to have shorter frills, which we can see on Diabloceratops itself. Its frill was not only short, but also fairly narrow and bore large openings (covered with skin in life). The animal’s most famous feature was the pair of long, outwardly curved hornlets at the top of its frill. Described in 2010, those hornlets would inspire its generic name – “devil horned face”. These horns and hornlets probably doubled as display and defensive features. Diabloceratops was in need of defense, as it lived in the same time and region as the early tyrannosaurid Lythronax, both being known from the Wahweap Formation. The same formation also contains hadrosaur, ankylosaur and more ceratopsid fossils.

Olorotitan

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Ornithopoda > Iguanodontia > Ankylopollexia > Styracosterna > Hadrosauriformes > Hadrosauroidea > Hadrosauridae > Lambeosaurinae > Lambeosaurini

Overview: There are two major subfamilies in the family Hadrosauridae – the Saurolophinae and the Lambeosaurinae. Saurolophines, which included dinosaurs like Maiasaura and Edmontosaurus, usually had long and wide-billed snouts, while the snouts of lambeosaurines were shorter and more narrow. Bony head crests appeared among members of both subfamilies, but it those of the Lambeosaurinae were often far larger and more extravagant. We can see this with Olorotitan – the “giant swan”. Its crest, formed out of part of its nasal bones, took the form of a large, backward-pointing hatchet blade-like structure. The crest was hollow and linked to the respiratory system, so some believe Olorotitan, and other lambeosaurines with similar crests, could use their crests as resonating chambers to make loud calls. They also would’ve been used for courtship purposes.

Olorotitan was a decently large hadrosaurid, growing to be about eight or nine meters long, so in the same size range as related genera like Parasaurolophus or Lambeosaurus. These dinosaurs all evolved from bipedal ancestors. While they could walk up on their hind legs, most of their time would’ve been spent down on all fours, with low-level vegetation making up a fair bit of their diet. Olorotitan, like all hadrosaurids, had highly efficient jaws and teeth for dealing with even tougher plant material. Fossils attributed to Olorotitan are known from the Amur region of Russia, in the far eastern part of the country, along the Amur River, which forms a border with the northeast of China. Much of its skeleton has been recovered, with all bones being found within the rock layers of the Udurchukan Formation, meaning Olorotitan lived towards the end of the Cretaceous.

Scutellosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Thyreophora

Overview: Most of the armored dinosaurs best known to the general public were decently large animals. This included the plate-backed Stegosaurus and the tanky Ankylosaurus. Their relatives were similarly impressive or were at least moderately large by today’s standards, but they all shared a common ancestor that was much smaller. Scutellosaurus itself probably wasn’t the direct ancestor of either the stegosaurs or ankylosaurs, but it was closely related to such a creature and gives us a good idea as to what it would’ve looked like. It was a basal member of the Thyreophora, which was a clade containing stegosaurs, ankylosaurs and other armored dinosaurs. Remains that are attributed to Scutellosaurus come from the Kayenta Formation of Arizona, which formed back in the Early Jurassic. Scutellosaurus lived much earlier than its famous, more derived cousins. The creature’s longer legs suggest it even retained the bipedal stance of earlier ornithischian ancestors. Its fossils were first found in the 1970’s, leading to its 1981 description.

Armored dinosaurs were protected by bony lumps, or osteoderms, which grew imbedded within the skin, often forming rows along the back, sides, neck and tail. Osteoderms are far from unique to thyreophorans, having appeared among some sauropods for instance, but they also aren’t unique to dinosaurs as a whole. Crocodiles and alligators, for instance, also have osteoderms over much of their backs. Those on Scutellosaurus were small and rounded in some spots, but somewhat spinier in others, though fairly modest. Its generic name, meaning “small-shielded lizard”, refers to these osteoderms. Dermal armor was of great use to it, since it wasn’t able to ward off predators by sheer size alone. Scutellosaurus was only a little over a meter in length as an adult. In addition to its armor, evasion and camouflage may’ve played some added role in its defense. The same region was also home to the theropod Dilophosaurus, which likely preyed on this animal.

Piatnitzkysaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Megalosauroidea (?) > Piatnitzkysauridae

Overview: So far, there are two known fossil specimens of Piatnitzkysaurus, from both a subadult and probable adult, both represented by partial skeletons. These remains were found within Argentina’s Cañadón Asfalto Formation during the mid to late 1970’s. Described by the renowned Argentine paleontologist José F. Bonaparte, the generic name of Piatnitzkysaurus is meant to honor the geologist Alejandro M. Piatnitzky. Piatnitzkysaurus would go on to become the namesake of its own family, the Piatnitzkysauridae, which may contain other notable taxa like Marshosaurus and Condorraptor. Members are mainly known from either South or North America, though a genus from China may also belong to the family according to some studies. Piatnitzkysaurid fossils are mainly known from the Early to Late Jurassic.

According to its remains, Piatnitzkysaurus wasn’t a particularly large theropod. It measured four to five meters long, so it could be considered medium-sized. Asfaltovenator, another theropod from the Cañadón Asfalto, was a bit larger. Said genus may’ve been the region’s top predator, while Piatnitzkysaurus hunted smaller herbivores. None of the piatnitzkysaurids grew to be all that large, so this mid-tier role was their main strong suit. The same region was also home to sauropod dinosaurs like Patagosaurus. Adults of said genus were probably too large for this dinosaur to take down, at least when healthy. In many studies, the Piatnitzkysauridae is placed within the larger superfamily Megalosauroidea, making them related to Megalosaurus. Some recent studies suggest they were closer to Allosaurus, in the superfamily Allosauroidea.

Sphaerotholus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Neornithischia > Cerapoda > Marginocephalia > Pachycephalosauria > Pachycephalosauridae > Pachycephalosaurinae

Overview: As with many other pachycephalosaurs, most of the fossils we’ve collected so far from Sphaerotholus consist of skull material, namely pieces of its thickened dome. This bone was solid in comparison to other parts of the skull or skeleton, so it’s little wonder that these domes have been so commonly preserved intact. We can fill in the gaps of this creature’s appearance with the fossils of other related animals. Sphaerotholus, like most pachycephalosaurs, was both a biped and mainly or entirely herbivorous. The tips of the jaws would’ve sported a narrow beak, while small teeth were situated farther back in the mouth. Growing to be about two meters long, Sphaerotholus was an average-sized pachycephalosaur. It was much smaller than the famous Pachycephalosaurus, a close relative, which was probably the largest known pachycephalosaur. In some regions, depending on the time period, the two genera may’ve coexisted.

The generic name of Sphaerotholus, coined in its 2002 description, is Greek for “spherical dome”, referring to its skull morphology. Paleontologists debate the exact purpose of pachycephalosaur skull domes, though it’s often assumed that they engaged in direct head butting behavior, similar to bighorn sheep or other such modern animals. Some are skeptical, pointing out that the anatomy of pachycephalosaur necks were ill suited for dealing with the stresses this would cause. It’s possible that Sphaerotholus competed with other members of its species by delivering side-to-side strikes with the head. The type specimen of this genus was found in New Mexico’s Kirtland Formation, but fossils now known to have belonged to it were actually found as early as the 1940’s, in Alberta. Sphaerotholus seems to have had a wide range over North America and across a decent amount of time. Fossils are even known from the Hell Creek Formation, so some species of Sphaerotholus may’ve had to contend with the infamous Tyrannosaurus as a potential predator.

Kentrosaurus

Phylogeny: Dinosauria > Ornithischia > Genasauria > Thyreophora > Thyreophoroidea > Eurypoda > Stegosauria > Stegosauridae

Overview: Kentrosaurus, not to be confused with the ceratopsid Centrosaurus, was a stegosaurian dinosaur that roamed what is now Tanzania in the Late Jurassic. The first recorded specimens of the creature were discovered by German paleontologists a few years before World War I. At that time, Tanzania was within the territory of German East Africa, a part of Germany’s larger colonial empire. Kentrosaurus was officially described as a genus in 1915, taking part of its name from the Greek root word for “prickle” or “sharp point”. This word is also used as the basis for the name of the aforementioned Centrosaurus, though with different spellings, both names are valid. In the case of Kentrosaurus, its generic name refers to its striking dermal armor. As with many other stegosaurs, this armor was probably used both defensively and for visual display purposes.

Within the Stegosauria, Kentrosaurus is usually referred to the family Stegosauridae, though most classify it as more basal that the famed Stegosaurus. It was also smaller than Stegosaurus, growing to maybe four or five meters long. All stegosaurs possessed enlarged dorsal osteoderms, though in many cases, they took on widely different forms between species. Those on Kentrosaurus took on the form of narrow plates over its neck and over the shoulders, but gradually grew into elongated spines towards its hips and down the tail. Two additional spines were once placed on its hips, but most modern reconstructions put them over its shoulders. Kentrosaurus was a primarily low-browsing herbivore. Its fossils are known from the famous Tendaguru Formation, so it would’ve lived alongside dinosaurs like Giraffatitan, Dicraeosaurus, Tornieria and Veterupristisaurus.

Patagotitan

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda > Gravisauria > Eusauropoda > Neosauropoda > Macronaria > Titanosauriformes > Somphospondyli > Titanosauria > Lithostrotia > Colossosauria > Lognkosauria

Overview: In 2010, a farm laborer in Argentina’s Chubut Province came across the fossilized leg bone of a gigantic sauropod. After the find was reported, paleontologists descended on the site to search for more fossils, in the end securing a partial skeleton. Recovered bones included pieces of the pelvis, shoulder bones, some ribs and a few vertebrae from the neck, back and upper tail. This specimen would become the holotype for the genus Patagotitan, described in 2017. Patagotitan is named for the region of Patagonia, much of which stretches over southern Argentina. Its name is also a reference to the Titans of Greek myth – fitting considering its enormity. While size estimates have varied, Patagotitan was probably over thirty meters in length and weighed as much as sixty or so standard tons. This easily made it a contender for the largest land animal known to science. The animal’s sheer size alone would’ve warded off all but the largest of predators.

Two other specimens of Patagotitan have been described, mainly consisting of some limb bones or vertebrae. While known fossils are limited, Patagotitan is actually pretty well represented for a sauropod of its size. The related Argentinosaurus, which is one of its only rivals in terms of body size, is also known from fairly limited, but enormous fossils (the vertebrae and limb bones alone of these dinosaurs were as tall as or even taller than a human being). Both dinosaurs belong to a lineage called the Lognkosauria, which represents some of the most derived members of the Titanosauria. Futalognkosaurus, another gigantic sauropod, was also a member of this group (and its namesake). Patagotitan was undoubtedly a high-browser, feeding on tall conifers and other such trees. Being known from the Cerro Barcino Formation, it would’ve shared its environment with the enormous carcharodontosaurid Tyrannotitan, which probably preyed on its young or weaker individuals.

Dromaeosaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Maniraptora > Pennaraptora > Paraves > Dromaeosauridae > Eudromaeosauria > Dromaeosaurinae

Overview: Dromaeosaurus is the namesake of the Dromaeosauridae – the famous “raptor” family of dinosaurs. One would expect the namesake of such a family to be a well understood genus, but Dromaeosaurus is known from much fewer fossils than its more famous relatives like Deinonychus, Velociraptor or Utahraptor. The first of these limited fossils were described in 1922, having been unearthed by the American paleontologist Barnum Brown, in what is now Alberta, Canada. These fossils were found within the Dinosaur Park Formation, meaning that particular specimen would’ve lived about seventy-six million years ago. Some isolated fossils and teeth from other formations like the Horseshoe Canyon and Hell Creek have also been referred to Dromaeosaurus, extending its range towards the end of the Cretaceous, but there is some doubt over if they really belong to it. This animal lived in the shadow of larger predators like Daspletosaurus and Gorgosaurus.

Growing to be about two or so meters long, Dromaeosaurus would’ve been about the same length as Velociraptor, but it had a somewhat different appearance. Dromaeosaurus seems to have been somewhat bulkier and its skull was far more robust, with deeper jaws and larger teeth. While both of these dinosaurs possessed enlarged “killing claws” on the second toe of each foot, it appears that Dromaeosaurus relied more heavily on its jaws than the slender-snouted Velociraptor. Both of these dinosaurs are the namesakes of their respective dromaeosaurid subfamilies – Velociraptor for the Velociraptorinae and Dromaeosaurus for the Dromaeosaurinae. Dromaeosaurines tended to be more heavily built and had stronger jaws. The dromaeosaurids as a whole are known to have been feathered based on direct fossil evidence. While unable to fly, Dromaeosaurus could’ve used its plumage to help insulate clutches of eggs, for visual display or to give it a boost when running.

Amargasaurus

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda > Gravisauria > Eusauropoda > Neosauropoda > Diplodocoidea > Dicraeosauridae

Overview: In the mid-1980’s, a team of Argentine paleontologists came across the partial skeleton of a highly unusual sauropod. This same expedition would also yield the fossils of the well known abelisaurid Carnotaurus, though the two genera didn’t live together and were separated by many tens of millions of years (Carnotaurus lived towards the end of the Cretaceous). The sauropod came from rocks dated to the Early Cretaceous, or around one hundred and twenty-five million years ago, in Argentina’s La Amarga Formation. The formation would inspire the creature’s generic name, Amargasaurus, coined in its 1991 description. Fossils of other sauropods, a theropod, and a potential stegosaur are also known from the La Amarga. Amargasaurus likely filled the niche of a low to mid-level browsing herbivore, given its size and neck length. The original holotype specimen of Amargasaurus remains the only one found thus far, which included shoulder bones, pieces of the pelvis, limb bones, and numerous vertebrae from the tail, spine and neck. The latter set of vertebrae contained the animal’s most distinctive traits.

Similar to some of its relatives, the neural spines sticking out from the neck vertebrae were double-pronged, instead of being only one process like in most animals. However, on Amargasaurus, these prongs were particularly elongated, the longest being up to sixty centimeters. These ran all along its neck and on the upper back. Scientists at first proposed these spines supported sails of skin, the purpose of which was unclear. Later, some studies would note features that suggested this wasn’t the case, with the spines being more like horns, bearing sheathes of keratin. Recently, the original idea has come back into favor. Either way, why it had this feature is still being debated. Spines could be useful for defense and display, but twin sails could’ve had some thermo-regulating function. Its close relative, Bajadasaurus, also had similar neck spines, though its were raked forwards, while those of Amargasaurus were raked towards the back. Both dinosaurs belonged to a family called the Dicraeosauridae, itself a part of the larger sauropod superfamily Diplodocoidea. As with most dicraeosaurids, Amargasaurus wasn’t very large for a sauropod, being medium-sized at best.

Procompsognathus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Coelophysoidea > Coelophysidae

Overview: Described in 1913 by the renowned German paleontologist Eberhard Fraas, the generic name of Procompsognathus literally means “before Compsognathus”. The name, from a certain point of view, is quite accurate. Procompsognathus did indeed live before Compsognathus, being found within Late Triassic-aged rocks in Germany. Compsognathus, on the other hand, is known from Late Jurassic-aged formations, also in Germany. Fraas would chose this name as he thought it may’ve been an ancestral form of Compsognathus, or was at least related to it. Current studies of both animals find no support for this idea, however. Compsognathus was either a basal member of the Coelurosauria, making it a distant relative of birds, or is possibly based on the juveniles of some large theropod of megalosauroid or allosauroid origins. Procompsognathus was a much more basal theropod genus, not belonging to any of these lineages or groups. Indeed, it may’ve belonged to one of the oldest branching lineages of confirmed theropods.

Most recent studies classify Procompsognathus as a basal neotheropod, specifically within the superfamily Coelophysoidea. It may’ve belonged to the family Coelophysidae, making it a close relative of Coelophysis itself. Coelophysids tended to be slender-bodied theropods with narrow skulls perched on semi-elongated necks. Like its relatives, Procompsognathus probably went after smaller vertebrates or insects. Procompsognathus itself was fairly small, measuring only a meter or so in length, even as an adult (confirmed by studies of the type specimen). It may’ve been a source of food itself for larger theropods like Liliensternus. Both dinosaurs are known from the rocks of Germany’s Löwenstein Formation, alongside the basal sauropodomorph Plateosaurus. Most in the wider public may recognize Procompsognathus for being featured in the Jurassic Park franchise. In the original 1990 novel, the writer Michael Crichton took some liberties and portrayed it as both a pack hunter and venomous. No evidence for such behavior or adaptations exists, however.

Skorpiovenator

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Ceratosauria > Neoceratosauria > Abelisauroidea > Abelisauridae > Brachyrostra

Overview: Known primarily from the southern hemisphere, the Abelisauridae was one of the most successful theropod families of the Late Cretaceous. They often lived alongside other large-bodied theropods, some of which were quite a bit larger than the abelisaurids themselves. For this reason, they may’ve specialized at hunting smaller or medium-sized herbivores. Skorpiovenator itself lived in the same time and region as the large carcharodontosaurid Mapusaurus, which probably preyed on large sauropods like Argentinosaurus, or at least their young. Smaller sauropods or ornithopods may’ve been the main prey for Skorpiovenator, though it also probably scavenged the carcasses of the larger animals. All of these dinosaurs are known from the Huincul Formation in Argentina, with an estimated age of around ninety-five million years ago.

Described in 2009, Skorpiovenator was discovered on a farm in west-central Argentina. The name of the animal means “scorpion hunter”, referring not to its actual diet in life, but to the numerous scorpions that plagued the dig site. This dinosaur represents one of the more completely known genera in the Abelisauridae, with only some portions of its skeleton being absent on the holotype. It was an average-sized abelisaurid, being about six meters long. Like its close relatives, it sported a short, but deep set of jaws lined with relatively small teeth. The arms were extremely small, with no obvious use, making them potentially vestigial remnants (like the pelvic bones on some modern whales). Skorpiovenator belonged to the abelisaurid subgroup Brachyrostra, which makes it a close relative of the famous Carnotaurus, though it was a bit more “primitive” than said genus.

Brontosaurus

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda > Gravisauria > Eusauropoda > Neosauropoda > Diplodocoidea > Diplodocidae > Apatosaurinae

Overview: For much of the past century, if a scientifically-based list of dinosaurs were made, this incredibly famous genus probably wouldn’t have been included. It was first established as a genus in 1879 by the renowned American paleontologist Othniel C. Marsh. The creature’s name, meaning “thunder lizard”, is meant to invoke its impressive size. Indeed, it was one of the largest dinosaurs known at that time. Two years prior, Marsh had described a very similar dinosaur, which he named Apatosaurus. Upon studying Brontosaurus, he concluded the two were distinct taxa. In 1903, a far more detailed re-description of both dinosaurs was published by Elmer S. Riggs, in the same year he described the genus Brachiosaurus. Riggs found that Brontosaurus and Apatosaurus were far too similar to be distinct genera, though he recognized them as separate on the specific (species) level. He officially reassigned the species Brontosaurus excelsus and Brontosaurus parvus to the genus Apatosaurus. Apatosaurus, being described first, had priority if they were the same genus. Most paleontologists would agree with Riggs, so the name Brontosaurus fell out of official use.

It would be a different story in popular culture, however. Two years later, in 1905, a fully mounted skeleton would be unveiled in the American Museum of Natural History, labeled as Brontosaurus. It would cause the name to spread among the general public, eventually to a point that most people came to refer to all sauropods as “brontosaurs”. The skeleton was also mounted with an incorrect skull, that of a Camarasaurus, but contrary to popular myth, this had nothing to do with why the generic name was changed. Eventually, all mounted skeletons of all referred Apatosaurus species were labeled as such. Brontosaurus being a junior synonym of Apatosaurus would remain the popular consensus among researchers until the 2010’s. In 2015, a study was published that found the species originally referred to Brontosaurus, along with one since referred to Apatosaurus (now B. yahnahpin), had enough differences to warrant them being in a separate genus. Not all agreed with this conclusion, but it has since found quite a bit of support among researchers.

Even if Brontosaurus was distinct from Apatosaurus, the two animals were clearly closely related, bearing most superficial features in common. Both belonged to the sauropod family Diplodocidae, a part of the larger neosauropod superfamily Diplodocoidea. This made them related to genera like Barosaurus and Diplodocus. Brontosaurus and Apatosaurus, however, were a bit more basal, belonging to a subfamily called the Apatosaurinae. Apatosaurine diplodocids tended to be bulkier, with thicker necks than those of the diplodocine diplodocids. Between the two, Apatosaurus was the more heavily built genus, with Brontosaurus typically being a bit smaller. Still, it was one of the largest animals in its local environment. Brontosaurus and Apatosaurus are both known from the Late Jurassic-aged Morrison Formation in the western United States. Some Brontosaurus species are known from deeper layers of the Morrison than those of Apatosaurus. These animals lived in a semi-arid, seasonal environment, alongside many other sauropods, stegosaurs, ornithopods and a few large theropods like Allosaurus, Ceratosaurus and Torvosaurus.

Monolophosaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda (?) > Carnosauria (?)

Overview: Fossilized remains belonging to this medium-sized theropod are known from the region of Xinjiang, in western China. The holotype and so far only specimen of Monolophosaurus consists of a decent portion of the animal’s skull and body, excavated over the course of the 1980’s. Rock layers surrounding the bones date to the Middle Jurassic, geologists attributing them to the larger Shishugou Formation, preserving a semi-arid and seasonal environment. Monolophosaurus may’ve lived alongside notable dinosaurs like Limusaurus, Guanlong and Sinraptor, though these dinosaurs are often attributed to slightly younger sections of the Shishugou. With a length of about five to six meters, Monolophosaurus was probably going after smaller and medium-sized herbivores. Sources of food may’ve included smaller theropods, stegosaurs, early ceratopsians and young sauropods.

Monolophosaurus, the “single-crested lizard”, was named in reference to its most notable trait – a large, bony crest growing along the length of its snout. This crest was low, thin and bore a handful of openings, which in life may’ve been covered by keratin or other such tissues. Its exact purpose is unknown, but the keratin covering it may’ve been brightly colored or patterned, allowing it to signal its health and vitality to potential mates or ward off rivals. The aforementioned Guanlong, which was a fair bit smaller than Monolophosaurus, also had a similar crest. Some speculate that it was a juvenile of this genus, but the fossils referred to Guanlong were from a fully grown individual. On top of that, Guanlong is usually seen as an early tyrannosaur. Monolophosaurus has a somewhat more unstable phylogenetic status. It has been classified as a basal tetanuran, a relative of the allosauroids, related to the megalosauroids or even close to the spinosaurids.

Brachiosaurus

Phylogeny: Dinosauria > Saurischia > Sauropodomorpha > Plateosauria > Massopoda > Sauropodiformes > Sauropoda > Gravisauria > Eusauropoda > Neosauropoda > Macronaria > Titanosauriformes > Brachiosauridae

Overview: One of the most recognizable of the sauropods, Brachiosaurus was first described as a genus in 1903 by the American paleontologist Elmer S. Riggs. He based the description on a set of partial remains discovered a few years earlier in Colorado. Recovered fossils included limb bones, vertebrae, ribs, and part of the hips. Its generic name means “arm lizard”, referring to how its front limbs were longer than its hind limbs, which at that time was quite unusual for a sauropod. Also notable for that time was its size, with it being the largest sauropod described up until that point. It was poorly known from fossils, however. This would seemingly change in 1914, when a supposed new species of Brachiosaurus was named from Tanzania; Brachiosaurus brancai. It was based on a far more complete set of remains. The general image most people have of Brachiosaurus is heavily based on this African species. Its tall, arched nasal bone and short torso are commonly pictured elements of its physique. Older artistic depictions of Brachiosaurus draw heavily from its anatomy, which is unfortunate, as Brachiosaurus brancai is no longer classified under Brachiosaurus at all.

Differences between the type species, B. altithorax, and those of B. brancai were noted as early as the 1980’s. The latter was officially recognized as a separate genus, called Giraffatitan. Some of its notable differences were its far narrower chest, shorter torso, shorter tail, and overall lighter build than that of Brachiosaurus. Definitive skull material from Brachiosaurus also showed that its nasal arch, while somewhat tall, was not as dramatic as that of Giraffatitan. The two were still relatives, being classified in the family Brachiosauridae. They were also some of the largest animals in their respective environments and were alive at around the same time. Brachiosaurus is currently only known from the Morrison Formation in the United States. It coexisted with other sauropod genera like Camarasaurus and Diplodocus, probably feeding from much taller branches than either, being able to reach up to nine meters or more in the air. While no longer the largest known dinosaur, it was still large enough to ward off most predators as an adult, which may’ve included theropods like Allosaurus or Torvosaurus. Brachiosaurus fossils, unfortunately, remain quite rare even now. Some scattered remains continue to be found, with a few suggesting some rare individuals grew to be notably larger than others.