Early Cretaceous

Time-Span: 143 – 100 million years ago

Overview: Unlike some previous transitions in the Mesozoic, the Late Jurassic and Early Cretaceous boundary wasn’t punctuated by any particular mass extinction, though there were some more drawn out changes in weather patterns, flora and fauna. Pangaea was long gone by this point, with Laurasia and Gondwana being fully separated. The average person could probably pick out the landmasses that would become the modern continents, though these were still in different positions and often closer together. Africa and South America, for example, remained directly connected during the early half and into the middle of the Cretaceous Period, while Europe to the north was mainly comprised of different island chains. Madagascar and India in the far south were still connected and separate from either Africa or Asia. Farther west, the Atlantic was still fairly narrow, but was fully established. Weather-wise, the world experienced less pronounced monsoon seasons and was dryer overall. Notably, flowering plants would become firmly established in this epoch, as well as many social, pollinating insects alongside them – bees being the most notable. Other insects included some wasps, ants, moths, and butterflies.

Changes in weather and vegetation would have some effects on herbivores. Many groups of sauropods that had been abundant during the Late Jurassic would see a marked decline. Diplodocids, for example, would persist for a few million years, but they would soon die out. Dicraeosaurids like the sail-necked (or spine-necked) Amargasaurus and rebbachisaurids like Nigersaurus were close relatives, sharing the same diplodocoid superfamily. These groups saw more success in the Early Cretaceous, but they too were becoming somewhat less common. So too were more basal macronarians, slowly replaced by more “advanced” taxa in the clade Titanosauriformes. This included giant sauropods like Sauroposeidon, as well as some of the first true titanosaurian sauropods, which varied greatly in size. Towards the end of the Early Cretaceous, some titanosaurs were becoming enormous. Some primitive sauropod lineages, such as the Turiasauria, would actually make it a ways into the Early Cretaceous, though this group is absent from the latter epoch of the Cretaceous. Some sauropods, including the aforementioned Nigersaurus, would become specialized for niches entirely new for sauropods.

Theropods saw some notable turnovers after the end of the Late Jurassic. Megalosaurid theropods like Torvosaurus had largely disappeared, but the long-snouted spinosaurids, which may’ve been relatives, would become quite abundant throughout the Early Cretaceous. These were largely fish-eating animals that lived along rivers, lagoons and coasts. Examples included England’s Baryonyx or the slightly larger Suchomimus from North Africa. Certain allosauroid groups like the allosaurids and metriacanthosaurids saw rapid declines and eventually died out, but their relatives, the carcharodontosaurs, would gradually become the dominant land predators in most regions. In the latter part of the epoch, some of these like Acrocanthosaurus and Tyrannotitan, had grown to be among the largest theropods of their time. Birds and their more primitive coelurosaurian kin continued to thrive. Some major groups like the “raptor” family Dromaeosauridae and the closely related Troodontidae were fully established by this time. In some regions, especially in Gondwana, the early branching ceratosaur-line of theropods was still going strong. Abelisaurid ceratosaurs, known for their tiny arms and deep snouts, were fairly numerous.

As for the ornithischian dinosaurs, they would undergo rapid and permanent changes in the Early Cretaceous. Stegosaurs, which had been abundant and highly successful in the Jurassic, would enter a rapid decline, eventually dying out. Some appear to have held on for slightly longer in Asia. The other major armored thyreophoran clade, the Ankylosauria, would go on to flourish and diversify, with the non-clubbed nodosaurids and club-tailed ankylosaurids being firmly established by the end of the Early Cretaceous. Ornithopods – the group that would later give rise to “duck-billed” dinosaurs – were already doing well by the end of the Jurassic, with the iguanodontian lineage seeing rapid diversification into the Cretaceous. This included Iguanodon itself as well as more distantly related forms like Ouranosaurus. It’s around this time that the hadrosauroids, close relatives of true “duck-bills”, would first evolve. Two other notable iguanodont groups that emerged in this time included the rhabdodontomorphs and the elasmarians, the latter being known mainly from the southern hemisphere. Early ceratopsians, relatives of the true horned dinosaurs, also flourished and diversified, though they remained small and bipedal.

Great changes occurred among non-dinosaurian reptile groups during the Early Cretaceous. The older long-tailed line of pterosaurs would vanish, being fully replaced by the short-tailed pterodactyloid line. Some of these retained teeth, but some advanced groups were toothless. This included the remarkable crest-headed tapejarid pterosaurs. Some pterosaurs were beginning to reach considerable wingspans, with Tropeognathus from South America possibly exceeding eight meters, though most were smaller. Pterosaurs didn’t lack for competition in this time, however, as birds would begin to compete with them in smaller-bodied niches. Marine reptiles saw considerable overturns, with certain plesiosaur lineages as well as most sea-going crocodylomorphs dying out during this period. Some plesiosaurs, such as those in the long-necked elasmosaurid line, would continue to see success. Ichthyosaurs, however, appear to have suffered a kind of bottleneck at this time, with only a few different forms making it through the Early Cretaceous. Among non-reptiles on land, mammals would see a significant occurrence – the split between marsupials and placental mammals. With some exceptions, most remained small, however.

Notable Early Cretaceous Dinosaurs: Utahraptor, Falcarius, Wuerhosaurus, Mierasaurus, Baryonyx, Iguanodon, Mantellisaurus, Hypsilophodon, Polacanthus, Neovenator, Eotyrannus, Ornithopsis, Amargasaurus, Psittacosaurus, Concavenator, Beipiaosaurus, Caudipteryx, Yutyrannus, Mei, Jinzhousaurus, Sinosauropteryx, Microraptor, Fukuiraptor, Moabosaurus, Acrocanthosaurus, Sauropelta, Deinonychus, Tenontosaurus, Sauroposeidon, Europelta, Suchomimus, Nigersaurus, Ouranosaurus, Leaellynasaura, Muttaburrasaurus, Kunbarrasaurus, Borealopelta, Tyrannotitan, Patagotitan, etc.

Notable Early Cretaceous Formations: Cedar Mountain (United States), Ejinhoro (China), Weald Clay (United Kingdom), Sainte-Barbe Clay (Belgium), Wessex (United Kingdom), La Amarga (Argentina), Oshih (Mongolia), La Huérguina (Spain), Yixian (China), Jiufotang (China), Kitadani (Japan), Cloverly (United States), Antlers (United States), Twin Mountains (United States), Escucha (Spain), Elrhaz (Niger), Eumeralla (Australia), Mackunda (Australia), Allaru (Australia), Clearwater (Canada), Cerro Barcino (Argentina), etc.