Early Jurassic

Time-Span: 201 – 174 million years ago

Overview: Current findings place the start of the Jurassic Period at around 201 million years ago, arriving with the Triassic-Jurassic extinction event. While not as devastating as the Permian-Triassic event, said mass extinction had severe consequences on the history of life to follow. Large-bodied pseudosuchians, the “proto-crocs” like Postosuchus, were very nearly wiped out. Only the crocodylomorph line, to which modern crocodilians belong, survived. Crocodylomorphs would go on to be highly successful in their own right, but most of the niches once occupied by the pseudosuchians were taken over by dinosaurs. It could be said that the Triassic-Jurassic extinction directly caused the rise of the dinosaurs, though it’s not entirely certain why they made it through so successfully. Some speculate their more dynamic body posture made them more agile, or it could have something to do with their metabolisms. Other major Triassic groups like the crocodile-like phytosaurs, most large amphibians, and the last remaining therapsids mammal relatives would also vanish from the fossil record. Plant groups remained mostly the same, though some of the seed ferns would die out, while the cycads went on to be abundant.

Some dinosaur lineages, including the first potential sauropods like Lessemsaurus, were already starting to grow large by the Late Triassic, but none were truly gigantic. Carnivorous theropods would start out the Early Jurassic being similar in form to Triassic taxa like Coelophysis – mainly small and slender-bodied. They would, however, gradually grow larger throughout the Early Jurassic. For instance, North America’s Dilophosaurus and Cryolophosaurus from Antarctica could reach lengths of up to six or even seven meters, though they were still usually fairly lightly built. Regardless, they were likely the top predators of their local regions. Both of them, along with a number of other theropods of the time, had striking head crests. Bulkier forms would evolve towards the end of the Early Jurassic among certain lineages. Theropods in general diversified greatly in the Early Jurassic, with some newly emerging groups including the ceratosaurs and the more “advanced” tetanurans (the lineage that later gave rise to taxa like Allosaurus, Tyrannosaurus and modern birds). Some of the bulkier theropods that would evolve later in the Early Jurassic included Asfaltovenator and Piatnitzkysaurus, both from Argentina.

Theropods weren’t the only dinosaur group to undergo diversification in the Early Jurassic. Two-legged, more primitive sauropodomorphs like Lufengosaurus and Yunnanosaurus would survive a ways into the epoch, but they would eventually die out, giving way entirely to the larger, quadrupedal true sauropods. Some early confirmed sauropods from this time included India’s Barapasaurus and Vulcanodon from Zimbabwe. These earlier sauropods often lacked the specialized column-like limbs seen on their later kin. Barapasaurus possibly belonged to the clade Eusauropoda, which would later give rise to most of the major sauropod lineages. Some early eusauropod groups included the cetiosaurids like Patagosaurus; a herbivore that was likely prey for Asfaltovenator. The earliest sauropodomorphs from the Late Triassic may’ve been omnivores or even carnivores, but by the Early Jurassic, all sauropodomorphs were primarily or entirely herbivorous. They would evolve ever longer necks, proportionately smaller heads and even larger body sizes. Larger bodies meant they would require more and more energy, but such bulk also granted moderate protection against theropods, which were also obtaining greater sizes.

Ornithischian dinosaurs, which would go on to include “duck-billed”, horned and armored varieties, are believed to have originated in the Triassic. Silesaurids, usually classified as close relatives to dinosaurs, are thought by some to actually be examples of Triassic ornithischians, but this is debated. The earliest confirmed ornithischians come from the Early Jurassic. This included South African taxa like Eocursor or the tusked Heterodontosaurus. They were generally small, bipedal animals, some of which may’ve had bristle-like integument over their bodies, and were largely herbivorous. By the end of the Early Jurassic, they had diversified into two major lineages – the Neornithischia and the Thyreophora. The latter was the group to which the armored dinosaurs belonged. Early armored dinosaurs included North America’s Scutellosaurus, a small bipedal creature, as well as Scelidosaurus from England, which grew to be a fair bit larger, but was still very primitive. By the following epoch, the first true stegosaurs and ankylosaurs would emerge. Scutellosaurus, Scelidosaurus and their later kin were protected by osteoderms – lumps of bone that grew imbedded in the skin, forming their dermal armor and often other features.

The flying pterosaurs are often incorrectly referred to as dinosaurs, though the two groups were closely related. They started out quite small in the Late Triassic, which would largely remain true in the Early Jurassic, though they would also diversify to a great extent. Some notable pterosaurs form this epoch included Dimorphodon, Campylognathoides, and Dorygnathus. They would expand their range during this time, moving into new habitats. Marine reptiles continued to thrive in the sea, though some groups like the nothosaurs died out. Plesiosaurs, which were close relatives, would begin to become more common in the Early Jurassic. Ichthyosaurs continued to be successful, exemplified by large predators like Temnodontosaurus and the smaller Ichthyosaurus itself. Some crocodylomorphs would also take to the sea in this time, in the form of the thalattosuchians. Pangaea would continue to break apart during the Early Jurassic, though the northern and southern continents had yet to separate into the two distinct landmasses of Laurasia (north) and Gondwana (south). When this did occur, it would further influence the evolution of dinosaurs and other animals going into the following Middle Jurassic epoch.

Notable Early Jurassic Dinosaurs: Heterodontosaurus, Massospondylus, Megapnosaurus, Eocursor, Lesothosaurus, Sinosaurus, Lufengosaurus, Scutellosaurus, Scelidosaurus, Anchisaurus, Mussaurus, Jingshanosaurus, Yunnanosaurus, Barapasaurus, Dilophosaurus, Cryolophosaurus, Patagosaurus, Piatnitzkysaurus, Asfaltovenator, etc.

Notable Early Jurassic Formations: Upper Elliot (South Africa), Lufeng (China), Charmouth Mudstone (United Kingdom), Portland (United States), Laguna Colorada (Argentina), Kota (India), Kayenta (United States), Hanson (Antarctica), Cañadón Asfalto (Argentina), etc.