Middle Jurassic

Time-Span: 174 – 161 million years ago

Overview: No especially notable mass extinction occurred at the boundary between the Early and Middle Jurassic, though some die offs did occur prior to the transition. These occurred mainly in the oceans, likely influenced by increased volcanism as the continents continued to break apart and with following changes in weather patterns. Plants would of course see notable changes due to these conditions, largely adapting to deal with less available moisture. New varieties of ferns and taller conifers appear to have been far hardier than those that came before. Trees grew taller, which of course would push the long-necked sauropod dinosaurs to grow longer necks and even larger body sizes. All of the more primitive, bipedal non-sauropod sauropodomorphs – the “prosauropods” – had died out by this point. Most if not all sauropods of the Middle Jurassic were members of the Eusauropoda, with the cetiosaurids continuing to thrive throughout the epoch in the form of Cetiosaurus or perhaps others. Far larger were the mamenchisaurids of Asia, famed for their extraordinary necks – proportionately long even for sauropods. The more advanced lineage Neosauropoda would also appear in this time.

Many of the more primitive, slender-bodied theropods like Dilophosaurus would not survive into the Middle Jurassic, though theropods as a whole saw an amazing degree of diversification in this period. Ceratosaurs continued to thrive, retaining a number of primitive traits, but new groups including the birdlike coelurosaurs and bulkier megalosauroids also evolved. Megalosaurid theropods, including Megalosaurus itself, were among the most successful theropods of this epoch, especially in Europe. More derived allosauroid theropods, however, would also emerge – the latter group eventually going on to outcompete the former. Some allosauroids grew to be surprisingly massive for the time. This included China’s Yangchuanosaurus, which could reach ten to eleven meters long. Coelurosaurs rapidly split into different groups, with most being smaller-sized predators. Some of them retained more stereotypically theropod-like body plans, including the earliest tyrannosauroids like Guanlong, but some remarkably birdlike coelurosaurs also emerged, many belonging to the group Paraves. This was especially true towards the end of the Middle Jurassic. Coelurosaurs seem to have been ancestrally feathered.

Diversification and rapid change wasn’t only occurring in the saurischian lineages, but also among the ornithischian dinosaurs. Some of the ornithischians would retain their bipedal posture and smaller bodies, but some would become far more specialized. The armored thyreophorans in particular were becoming strikingly weird by comparison. Earlier forms were also bipedal with simple armor, but they trended towards quadrupedal posture during the Middle Jurassic, meaning they could grow to be much larger. Both the Stegosauria and Ankylosauria appear in the fossil record at this time. Some potential early members of both groups evolved in North Africa, though many Middle Jurassic stegosaurs lived in China – the primitive genus Huayangosaurus is an example. Stegosaurs tended to have striking dorsal plates and spiked tails, but ankylosaurs were more heavily armored. Another major ornithischian lineage that appeared was the Cerapoda, from which the ornithopods and the horned ceratopsians would later evolve. Ornithischians in general were set apart from other dinosaurs by their beaks and a specialized bone called the predentary, located at the very tip of their lower jaws, anchoring the lower beak.

Non-dinosaurian contemporaries would also evolve along with climatic and geographical changes. In the seas, new groups of plesiosaurs emerged. Some of these were long-necked, others short-necked with enormous skulls (pliosaurs). Among the ichthyosaurs, the more specialized fish-like forms continued to thrive. As for the sea-going crocodylomorphs, a major group known as the Metriorhynchidae would also evolve; being more specialized for swimming than other thalattosuchian groups. Pterosaurs would remain the dominant flying vertebrates, though they largely remained small, long-tailed and toothed. Some potential transitional pterosaurs, however, may be known from the Middle Jurassic specifically, revealing they too were undergoing notable changes. One change on a global scale was the final demise of Pangaea as a single landmass, creating a continuous sea between the northern landmass of Laurasia (North America, Europe, Asia) and Gondwana in the south (South America, Africa, Australia, Antarctica, etc.). This would send the diversification of different dinosaur groups into overdrive. Overall, the Middle Jurassic is fairly mysterious, as relatively few geological formations are documented from the epoch.

Notable Middle Jurassic Dinosaurs: Kulindadromeus, Cetiosaurus, Megalosaurus, Proceratosaurus, Wiehenvenator, Atlasaurus, Spicomellus, Spinophorosaurus, Jobaria, Afrovenator, Xinjiangtitan, Alpkarakush, Agilisaurus, Omeisaurus, Huayangosaurus, Yangchuanosaurus, Shunosaurus, Monolophosaurus, Eustreptospondylus, etc.

Notable Middle Jurassic Formations: Ukureyskaya (Russia), Rutland (United Kingdom), Taynton Limestone (United Kingdom), Ornatenton (Germany), Guettioua (Morocco), El Mers III (Morocco), Irhazer Shale (Niger), Tiourarén (Niger), Qiketai (China), Balabansai (Kyrgyzstan), Lower Shaximiao (China), Shishugou (China), Oxford Clay (United Kingdom), etc.