Sauropterygia: Plesiosauria

Taxa on This Page

1. Plesiosauria X

Plesiosauria: Introduction and History

Macroplata, in the figure at right, was a fairly typical Early Jurassic plesiosaur. The Plesiosauria the name means "near lizards,") were an important order of Mesozoic marine reptiles, members of the superorder Sauropterygia. The Plesiosauria include both short and long-necked forms. The largest short-necked forms reached enormous sizes (lengths of over 10 meters, weight of 20 tonnes or more), while some of the later long-necked forms, although only marginally longer and not as heavily built,  developed the greatest number of neck vertebrae of any animal. Plesiosaurs evolved from animals related to pachypleurosaurs and nothosaurs during the Middle Triassic. They remained somewhat rare until the end of that period, when they underwent an amazing evolutionary radiation. These reptiles then dominated the seas throughout the Jurassic and Cretaceous periods, continuing to flourish right up until the end of the Mesozoic Era.

Plesiosaurs were among the earliest large prehistoric creatures to be described. Victorian British accounts are full of references to "antediluvian monsters".   Most plesiosaur material was found and described in the Nineteenth Century, much of it located and prepared by an early paleontologist named Mary Anning (1799-1847). A life-long resident of Lyme Regis, England, Anning made a remarkable series of important discoveries in the Early Jurassic seaside cliffs and limestone quarries within walking distance of her home. Collectors quickly depleted these coastal exposures. Sadly, by about 1910 most limestone quarries had become mechanized, this severely limiting the collection of fossil material before it was destroyed.   After this date very little new material was found, and paleontologists in this region were limited to re-classifying, redescribing, and reviewing old material.

Meanwhile, important discoveries were being made in  North America, where sediments from the Late Cretaceous inland sea contain the remains of many large marine reptiles, including both long and short-necked plesiosaurs. This historical accident has led to the belief that plesiosaurs evolved and flourished in Europe during the Jurassic, and became rare there in the Cretaceous, spreading at that time to North America, and finally attaining world-wide distribution at the end of the period.

This opinion could not be more false. As with all large, ocean-going animals, there is no doubt that plesiosaurs had world-wide distribution virtually from the very start, even as early as the Triassic. Certainly Early Jurassic plesiosaurs are known from China, South America, and Australia as well as Europe. As with so much of paleobiology, it is misleading to make sweeping assumptions on the basis of patchy and incomplete geological preservation. MAK

Descriptions

Plesiosauria: plesiosaurs > nothosaurs.

Notes: There is still a school of thought that plesiosaurs laid eggs on land.

Image: (right) Plesiosaurus in the British Museum, modified from the former Donald Nute's Dinosaur Gallery site (former page), by permission.

References: Carroll & Gaskill 1985) [CG85]; Holmes et al. (2008) [HCW08]. ATW061217, rev'd ATW080330.

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