The Jurassic Period

Article by: J. D. Dixon
Edited by: Harry T. Jones

The Jurassic Period started 201.3 ± 0.2 million years ago (Ma) and ended ~ 145.0 Ma. The period is named after the Jura Mountains, which are made up of limestone deposited during this time interval. The Jurassic is the second period of the Mesozoic Era, and throughout it, the continental break-up of Pangaea continued, extending the Tethys Ocean and creating the Proto-Atlantic. Until the Mid-Jurassic, global temperatures were warm, and so carbon dioxide levels are thought to have been relatively high (∼2225 parts per million by volume at the start of the period). During the later period, temperatures cooled and seasonality emerged.

The plants of the Triassic began to expand into new species in the Jurassic, and fossils from Northern England have revealed the true range of Jurassic vegetation. Plant genera belonging to conifers, cycads, ferns, and ginkgoes governed the floral biosphere. Assortments of arthropods, such as dragonflies and horseshoe crabs, were present. Many extant orders of insects, like beetles, diversified, and it’s even possible that insects aided in the pollination of some plants, much as they do today.

Due to the dissolution of Pangaea, continental shelves were generated globally, making way for all new forms of marine life. Numerous invertebrate groups, such as bivalves, echinoderms, gastropods, and scleractinian corals, diversified greatly after the Triassic-Jurassic extinction. Crustaceans became common, and many kinds of ammonites and belemnites populated seas all over the world. There were even twenty-four Jurassic ammonite turnovers (origination and extinction events), which exhibited a 75% correlation to sea-level fluctuations.

The Hexanchiformes (six-gilled) sharks had appeared by the Early Jurassic, while some sharks, such as Hybodus, were very successful throughout the period. One of the largest fish ever, the filter-feeding Leedsicthys problematicus, has been recovered from numerous European sediments of Jurassic age. Nevertheless, of the Jurassic aquatic animals, the most notable are the marine reptiles. There was a vast array, including gigantic pliosaurs (such as Liopleurodon); dolphin-like ichthyosaurs; crocodilians; and a plethora of turtles, spanning international oceans. Many of these groups co-existed for over 50 million years as a result of dietary niche partitioning (meaning they didn’t compete for food), and large-prey specialists may have diversified at times with elevated sea-levels. Across the Pliensbachian–Toarcian (Early Jurassic), a multi-phase extinction event caused the decline of many of these marine communities, including the extinction of 84% of bivalve species.

At this time, moderately sized pterosaurs, such as the long-tailed Rhamphorhynchus and short-tailed Pterodactylus, dominated global skies. Pterosaurs weren’t the only vertebrates to take to the air, as evidence from Germany shows feathered dinosaurs such as Archaeopteryx also emerged.

The most famous of the Jurassic vertebrates, the dinosaurs, diversified massively. Theropods differentiated into a variety of ferocious forms, from the formidable British Megalosaurus bucklandii to the basal tyrannosauroid Guanlong. Perhaps the most prominent predator of the Late Jurassic is the 12 metre long killing machine Allosaurus. Also by the Late Jurassic, ornithischian dinosaurs had developed from small bipeds such as Lesothosaurus and Scutellosaurus in the Early Jurassic to much larger and more impressive types like Stegosaurus and Camptosaurus . Small sauropodomorphs such as Anchisaurus soon gave way to massive sauropods like Vulcanodon and the widely-known genus Diplodocus.

Other terrestrial vertebrates included amphibians such as the frog Prosalirus and the burrowing Eocaecilia. Most importantly for us, the Jurassic saw the first diversification of early mammal groups, including the eutherians (placental mammals). These mammals were mostly small (no larger than a human hand), and probably similar in appearance and behaviour to modern rats. Examples of these mammals include Sinoconodon and Asfaltomylos.

At the end of the Jurassic there was another minor extinction, occurring during the Tithonian Stage. This event led to a possible 5-6% fall in marine families, but its effects were mainly seen in Europe. There is no evidence of a dinosaur extinction event during this stage as seen from the Morrison Formation, North America. Some have even hypothesised this event as representative of a fossil preservation bias in Upper Jurassic sediments as opposed to a true extinction signal, but only future excavation will reveal if this is the case.

Image References
[1] The Promised Land. Two Scutellosaurus reach the promised land in Arizona (196 million years ago). Artwork by John Conway.
[2] A mother Plesiosaurus dolichodeirus gives birth while her mate circles defensively. Artwork by Jack Wood.
[3] Megalosaurus bucklandii devouring a carcass. Artwork by Mark Witton.

Information References and Further Sources
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[2] Benson, R. B. J. (2010). ‘A description of Megalosaurus bucklandii (Dinosauria: Theropoda) from the Bathonian of the UK and the relationships of Middle Jurassic theropods’, Zoological Journal of the Linnean Society, 158 (4), pp. 882-935. Accessed 25^th October 2020. Click Here.
[3] Benson, R. B. J., Barrett, P. M., Powell, H. P., and Norman, D. B. (2008). ‘The Taxonomic Status of Megalosaurus bucklandii (Dinosauria, Theropoda) from the Middle Jurassic of Oxfordshire, UK’, Palaeontology, 51 (2), pp. 419-424. Accessed 25^th October 2020. Click Here.
[4] Breeden III, B. T., and Rowe, T. B. (2020). ‘New specimens of Scutellosaurus lawleri Colbert, 1981, from the Lower Jurassic Kayenta Formation in Arizona elucidate the early evolution of thyreophoran dinosaurs’, Journal of Vertebrate Paleontology. Accessed 25^th October 2020. Click Here.
[5] Chart drafted by K. M. Cohen, D. A. T. Harper, P. L. Gibbard, and J.-X. Fan (c) International Commission on Stratigraphy, March 2020. To cite: Cohen, K. M., Finney, S. C., Gibbard, P. L. & Fan, J.-X. (2013; updated). The ICS International Chronostratigraphic Chart. Episodes 36: 199-204. Accessed 25^th October 2020. Click Here.
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