Palaeos		Sauropterygia
Vertebrates		Plesiosauria

Sauropterygia: Plesiosauria

DIAPSIDA
|--ARCHOSAUROMORPHA
|
LEPIDOSAUROMORPHA
|--SAUROPTERYGIA
|  |--Placodontia
|  |--Pachypleurosauridae
|  `--+--Nothosauridae
|     `--+--Corosaurus 
|        `--+--Pistosaurus  
|           `--Plesiosauria
|              |--Pliosauroidea
|              `--Plesiosauroidea
|                 |--Elasmosauridae
|                 `--Cryptocleidoidea
`--Lepidosauriformes
   |--SPHENODONTIA
   `--SQUAMATA                

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.

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. 

Range: Middle Triassic to Late Cretaceous

Phylogeny: depending on which cladistic node one wants to identify as Plesiosauria there is a choice of Eusauropterygia :Nothosauridae + * : Corosaurus + (Pistosaurus + (Pliosauroidea + Plesiosauroidea))  [HCW08] - for the stem-base definition (Nothosaurus <n; Plesiosaurus)
or Eusauropterygia : Nothosauridae + (Corosaurus + (Pistosaurus + * : Pliosauroidea + Plesiosauroidea for the node-base definition (common ancestor of Pliosaurus brachydeirus and Plesiosaurus dolichodeirus (or substitute preferred crown group taxa, this is just for the sake of example) and all their descendents). Both definitions are equally valid. Of course, this is nothing next to the problem of defining tetrapoda! MAK111103

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]; Holmes et al. (2008) [HCW08]. ATW061217, rev'd ATW080330.