Late Cretaceous

Time-Span: 100 – 66 million years ago

Overview: Compared to some other denoted boundaries between Mesozoic epochs, that of the Early to Late Cretaceous transition is a bit more arbitrary. The boundary itself wasn’t punctuated by a major mass extinction, though one would occur a few million years later. This event, called the Cenomanian-Turonian extinction, occurred around 93 million years ago. Prior to then, many animal groups were a lot like those that came before, but animals afterward were often considerably different. The continents of the world were continuing to break apart and drift towards their modern positions. South America and Africa started out still connected, but they would finally split, connecting what is now the northern and southern Atlantic. Madagascar and India would also eventually split apart, though it wouldn’t be until after the Mesozoic Era that the latter would drive into the Asian mainland, creating the Himalayas. Sea levels, especially in the early half of the Late Cretaceous, were notably very high, covering much of Europe and forming the shallow Western Interior Seaway, running north to south in North America. Both of the poles were ice free, with the world experiencing nearly uniform warmer weather.

Long-necked sauropod dinosaurs were somewhat less common in certain parts of the world during the Late Cretaceous. For example, sauropods are rare or even nonexistent in North America’s fossil record for a long time in this epoch, though they would reappear in the form of titanosaurs like Alamosaurus, which may’ve migrated from South America across some land connection. In South America, they were far more abundant and highly successful. Some titanosaurs during the early Late Cretaceous, such as Argentinosaurus, grew to be enormous, but European titanosaurs were often quite small. This was due in part to them living on European island chains with limited resources. Some non-titanosaurian groups would survive into the early part of the Late Cretaceous, in the form of rebbachisaurid diplodocoids like Rebbachisaurus, but by the latter part of the epoch, only titanosaurs remained throughout the world. Whether or not sauropods were present in a given region often had major impacts on other dinosaur groups living there, particularly herbivores, some of which grew larger and took over those niches. In many regions, titanosaurian sauropods would thrive right up until the end of the Cretaceous Period.

Many of the theropod groups present at the start of the Late Cretaceous were like those seen in the earlier half the period. This included giant carcharodontosaurs like Giganotosaurus in Argentina or its North African relative Carcharodontosaurus, as well as Spinosaurus – the largest known member of the family Spinosauridae. After the Cenomanian-Turonian extinction, however, these two groups would disappear or entered a mark decline, opening up their niches. Tyrannosauroids, coelurosaurs closer to birds than said theropods, would come to take over those niches in the northern hemisphere. By 80 million years ago, some of them in the family Tyrannosauridae were truly enormous. This included taxa like Gorgosaurus, Tarbosaurus and of course Tyrannosaurus. Other more derived coelurosaurs like the “raptor” dromaeosaurids and troodontids continued to thrive. True birds were well established by this point, diversifying into many different niches. Only they would survive the mass extinction at the end of the Cretaceous. In the southern hemisphere, the more primitive abelisauroid ceratosaurs like Carnotaurus and Majungasaurus were often the dominant predators, alongside megaraptorans (possible tyrannosaurs)

By the end of the Early Cretaceous, the Stegosauria appears to have entirely vanished, while the other armored lineage, the Ankylosauria, continued right up until the end of the Late Cretaceous. Some, like Ankylosaurus itself, grew to be fairly large. Said genus belonged to the family Ankylosauridae, members of which often coexisted with the Nodosauridae – the other major ankylosaur family. Ornithopods were particularly successful, with the Late Cretaceous seeing the emergence of the first true hadrosaurids, or “duck-billed” dinosaurs. These were split into two major subfamilies – the crested lambeosaurines and the mainly non or smaller-crested saurolophines. Such dinosaurs were a major food source for the new tyrannosaur lineages in the northern hemisphere. Ceratopsian dinosaurs would reach their peak in the Late Cretaceous, with the appearance of the Ceratopsidae. This included enormous, four-legged forms like Styracosaurus and the famous Triceratops. Some lineages of smaller ceratopsians still existed. One other notable group was the Pachycephalosauria; dome-headed bipeds closely related to ceratopsians, which likely evolved earlier, but saw their greatest success during the Late Cretaceous.

Pterosaurs, the flying reptiles closely related to dinosaurs, continued to exist and flourish during the Late Cretaceous, though many smaller niches were being overtaken by birds. Some of the largest pterosaurs known to science existed in this period. Earlier on this included the famous crested Pteranodon, which made its home along the Western Interior Seaway, which would eventually narrow and vanish as the period went on. Pteranodon had a wingspan of over seven meters, but azhdarchid pterosaurs such as Quetzalcoatlus and Hatzegopteryx were even larger, exceeding ten or eleven meters. These latter forms are believed to have been able to fly, but were mainly “terrestrial stalkers”, hunting like giant “ground storks”. On some European islands, where dinosaurs were smaller, some of these pterosaurs reigned as apex predators. In the oceans, the Cenomanian-Turonian event saw the final demise of the long-lasting ichthyosaurs, but plesiosaurs would persist up until the end of the Cretaceous. They had competition, however, as this epoch saw the rise of the mosasaurs – large bodied marine relatives of modern monitor lizards and snakes. On land, crocodilians and mammals became more and more similar to those of today.

Famously, the end of the Late Cretaceous would see the final demise of the non-avian dinosaurs, as well as many other major large-bodied reptile lineages. Pterosaurs would die out completely, along with the plesiosaurs and mosasaurs in the oceans. All of this was caused by the Cretaceous-Paleogene extinction, which while not as devastating as the Paleogene-Triassic event, still saw the demise of almost all large land animals. Why exactly this took place is debated, though we know several events occurred prior to or around 66 million years ago. The retreat of the Western Interior Seaway would kill of a lot of animals that replied upon it, and pronounced volcanism in India may’ve contributed to harsh climatic changes. The most devastating event was the Chicxulub asteroid impact off the Yucatan Peninsula in Mexico, which was at least the coup de grace for non-avian dinosaurs, setting off global wildfires and sending debris into the atmosphere, blotting out most sunlight, possibly for years. Dying plant life would kill off most large herbivores, in turn killing off large theropods. Smaller forms like birds or mammals may’ve made it through by feeding on buried seeds, though they too lost a large number of species.

Notable Late Cretaceous Dinosaurs: Spinosaurus, Carcharodontosaurus, Aegyptosaurus, Giganotosaurus, Mapusaurus, Argentinosaurus, Australovenator, Nothronychus, Zuniceratops, Megaraptor, Lythronax, Maiasaura, Dromaeosaurus, Stenonychosaurus, Daspletosaurus, Gorgosaurus, Lambeosaurus, Parasaurolophus, Corythosaurus, Styracosaurus, Chasmosaurus, Centrosaurus, Euoplocephalus, Struthiomimus, Pentaceratops, Pinacosaurus, Velociraptor, Oviraptor, Protoceratops, Dreadnoughtus, Albertosaurus, Saurolophus, Edmontonia, Ornithomimus, Pachyrhinosaurus, Tarbosaurus, Deinocheirus, Therizinosaurus, Nemegtosaurus, Gallimimus, Mononykus, Nanuqsaurus, Alamosaurus, Majungasaurus, Masiakasaurus, Carnotaurus, Edmontosaurus, Thescelosaurus, Ankylosaurus, Pachycephalosaurus, Leptoceratops, Triceratops, Torosaurus, Anzu, Tyrannosaurus, Nanotyrannus, etc.

Notable Late Cretaceous Formations: Bahariya (Egypt), Douira (Morocco), Huincul (Argentina), Candeleros (Argentina), Winton (Australia), Moreno Hill (United States), Portezuelo (Argentina), Wahweap (United States), Two Medicine (United States), Oldman (Canada), Dinosaur Park (Canada), Kirtland (United States), Djadochta (Mongolia), Cerro Fortaleza (Argentina), Horseshoe Canyon (Canada), Nemegt (Mongolia), Prince Creek (United States), Ojo Alamo (United States), Maevarano (Madagascar), La Colonia (Argentina), Scollard (Canada), Lance (United States), Laramie (United States), Hell Creek (United States), etc.