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Unit 220: Lepidosauromorpha |
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The Vertebrates |
400: Plesiosauria |
Abbreviated CladogramEUREPTILIA |--ARCHOSAUROMORPHA | Lepidosauromorpha |--Sauropterygia | |--Pachypleurosauridae | `--+--Nothosauridae | `--+--Pistosaurus | `--Plesiosauria | |--Corosaurus | `--+--Pliosauroidea | `--Plesiosauroidea | |--Elasmosauridae | `--Cryptocleidoidea `--Lepidosauriformes |--RHYNCHOCEPHALIA `--SQUAMATA |
Contents220.000: Overview |
Plesiosauria:
Introduction and HistoryMacroplata, 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
Plesiosauria:
plesiosaurs > nothosaurs.
Range: Middle Triassic to Late Cretaceous
Phylogeny: Eusauropterygia::: Pistosaurus + *: Pliosauroidea + Plesiosauroidea.
Characters: Up to 13m; nostrils high, just preorbital; $ nasals
absent; large eyes located on sides
of head; palate less specialized than nothosaurs; palate consists
mainly of vomers, large palatines & pterygoids; ectopterygoid extends to
cheek & secures palate [CG85]; interpterygoid
vacuities retained, sharp, jagged teeth set in sockets at the edge
of the jaw; heavy, rigid trunks; $ presence of nutritive foramina
in vertebral centra, on underside of cervical and caudal, on sides of dorsal
centra, and on base of neural canal (either tubes joining neural canal to
underside of spinal column, or openings into a space filled with some specialized
tissue within the body of the centrum); single-headed ribs; $ relatively short tail;
$ gastralia present and well-developed (ballast?); $ both girdles elaborated ventrally, with
massive ventral plates; space between girdles filled with thick gastralia,
giving them almost continuous ventral bone surface;
anterior and posterior 
limbs similar, $ hyperphalangy; $ illium
does not contact pubis; presumed rowing or "flying" locomotion; propulsive stroke,
but no dorsoventral control?
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.
Links: Plesiosauria Translation and Pronunciation Guide Introduction; Plesiosaur Skeleton; Marine Reptiles (NOT Dinosaurs); Plesiosaurs- Enchanted Learning Software; plesiosaurs; The UnMuseum - Sea Reptiles; List of fossils; Marine Reptiles II: Plesiosaurs - Suite101.com; PLESIOSAURIA; Untitled Document; plesiosauria (excellent, but rather incomplete site); Plesiosauria after O'Keefe, 2001 (Mikko's Phylogeny); The Plesiosaur Site (Best on the Web -- Richard has finally fixed up this site so that it really works, and the result is a very complete database); Nathis Fauna Reptilelen Mariene Reptielen; The CDM's Earth Sciences Resource Site- Elasmosauridae; Fauna (detailed image of vertebrae); The Journal of Vertebrate Paleontology (abstract).
References: Carroll & Gaskill (1985) [CG85]. ATW061217.
Corosaurus: C. alcovensis Case 1936a.
Range: Early Triassic to Late Triassic of North America
Phylogeny: Eusauropterygia : (Simosaurus + (Nothosauridae + (Pistosaurus + Plesiosauria))) + *.
Links: The Journal of Vertebrate Paleontology; The Plesiosaur Site - Species.
ATW020627.
PistosaurusThe Pistosaurs were morphologically intermediate between nothosaurs and the Plesiosauria proper. See image at right by J. Aragón from PLESIOSAURIOS. Pistosaurus is presently known only from the Middle Triassic (Ladinian epoch). Until recently, Pistosaurus was the only known genus in its family, to which we may now add the American pistosaur, Augustasaurus. Pistosaurus had a nothosaur-like body with a plesiosaur-like head. In Linnean systematics, it is variously classified under the Nothosauridae and the Plesiosauria, depending on what characteristics one uses to define membership of each group. Even if it is a nothosaur it is acknowledged to be closely related to the ancestors of plesiosaurs.
These may have been ocean going creatures. They are known from both sides of Pangea (Europe/Tethyan province) and North America) so they seem to have had a wide distribution. Nobody has as yet done an analysis on the way these animals moved so we do not know whether it swam like a crocodile (typical nothosaurid) or paddled like a turtle or sea lion (plesiosaurian). In view of the nothosaurid body it is likely that with Pistosaurus the characteristic plesiosaur locomotor pattern had not yet evolved. MAK991210
Pistosaurus:
P. grandaevus von Meyer 1855, P. longaevus von Meyer 1839
Range: Middle Triassic of Europe & North America, Ladinian, Upper Muschelkalk
Phylogeny: Eusauropterygia::: Plesiosauria + *.
Characters: ~3m; vomeronasal organ present; very long neck; body form streamlined; more rigid spine as in plesiosaurs; gastralia present; limb girdles not strongly bound together as in Plesiosauria; limbs formed as paddle-shaped flippers.
Note: Pistosaurus is now the flagship genus of the Pistosauridae, the other member of the family being Augustasaurus hagodorni Sander, Rieppel & Bucher 1997. See The Journal of Vertebrate Paleontology.
Links: Nothosaurus- Enchanted Learning Software; A Manual of the Anatomy of Vertebrated Animals (1881); Dinosaurios: Pistosaurus; PLESIOSAURIOS (Spanish J. Aragón); The Plesiosaur Site - Species; The Journal of Vertebrate Paleontology; pistosaurus. ATW020630.
checked ATW050109
