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plesiosaurs
plesiosaurs Plesiosaurs are one group of tetrapods that returned from the land to a wholly aquatic existence. In the process of doing this the reptilian features that had made them efficient land animals had to be modified in order for them to become efficient aquatic animals. Plesiosaurs first appear in the Triassic, where they apparently developed from a group known as the nothosaurs, which show specializations for propulsion in water, particularly reduction in the limbs and development of the tail vertebrae to support a laterally flattened muscular tail. They probably moved as modern crocodiles do, folding the limbs against the body and progressing by lateral undulations of the tail and body.
Plesiosaurs became numerous and worldwide in their distribution during the Jurassic and Cretaceous periods as they further developed the basic nothosaurian pattern. They were large animals, ranging from 3 to 15 m in length with short stocky bodies, large flippers, and short tails. Although many of them had long necks and small heads, one group, the pliosaurs, had short necks and large heads. Both groups had long jaws with rows of narrow conical teeth which were adaptations for a piscivorous (fish-eating) way of life. In both forms, the abdominal region was strengthened by the development of additional ventral ribs termed gastralia. The pectoral and pelvic girdles were also greatly expanded ventrally, although the dorsal parts of the girdles were reduced. This adaptation provided large areas for the attachment of the powerful muscles that pulled the flippers backwards and forwards. Analysis of the range of motion of plesiosaur flippers shows that these animals swam in a way that is comparable to that of modern sea lions. These animals create lift by an initial downward and then backward stroke, and can also create lift during the forward feathered part of the stroke. Although sea lions use their front flippers only in propulsion, plesiosaurs had pectoral and pelvic flippers that were almost equal in size and probably used them out of phase to provide continuous motion. It seems probable that the long-necked plesiosaurs hunted by swimming among shoals of fish and using their long necks to dart from side to side and strike at their prey. The pliosaurs, which were faster swimmers, might have pursued and caught larger prey. One trend seen in the plesiosaurs during the Cretaceous was increase in size. The pliosaur Kronosaurus from the Cretaceous of Australia had a skull about 3.5 m long. The plesiosaur Elasmosaurus from the Cretaceous of North America had a neck that was twice as long as the body and contained 60 vertebrae. Although the plesiosaurs were a vigorous and widespread group, they became extinct at the end of the Cretaceous together with the dinosaurs and many other groups. However, despite the fact that these animals have no fossil record after the Cretaceous their modern presence has frequently been put forward to explain sightings of the ‘Loch Ness Monster’ and similar phenomena. David K. Elliott |
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Cite this article
PAUL HANCOCK and BRIAN J. SKINNER. "plesiosaurs." The Oxford Companion to the Earth. 2000. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. PAUL HANCOCK and BRIAN J. SKINNER. "plesiosaurs." The Oxford Companion to the Earth. 2000. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O112-plesiosaurs.html PAUL HANCOCK and BRIAN J. SKINNER. "plesiosaurs." The Oxford Companion to the Earth. 2000. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O112-plesiosaurs.html |
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Plesiosauroidea
Plesiosauroidea (plesiosaurs) Suborder of aquatic reptiles which enter the fossil record in the late Triassic, and which are common in many Jurassic and Cretaceous sediments. In appearance they were likened by Buckland to ‘a snake strung through the body of a turtle’, and some grew up to 15 m in length. There were also short-necked types, as well as the swannecked. The former are defined as the superfamily Plesiosauroidea (e.g. Plesiosaurus and Muraenosaurus), the latter as the superfamily Pliosauroidea (e.g. Pliosaurus and Trinacromerum), the two superfamilies making up the suborder.
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Cite this article
AILSA ALLABY and MICHAEL ALLABY. "Plesiosauroidea." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. AILSA ALLABY and MICHAEL ALLABY. "Plesiosauroidea." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O13-Plesiosauroidea.html AILSA ALLABY and MICHAEL ALLABY. "Plesiosauroidea." A Dictionary of Earth Sciences. 1999. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-Plesiosauroidea.html |
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plesiosaurus
plesiosaurus one of a genus of extinct marine reptiles. XIX. f. Gr. plēsíos near + saúros lizard.
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Cite this article
T. F. HOAD. "plesiosaurus." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. T. F. HOAD. "plesiosaurus." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O27-plesiosaurus.html T. F. HOAD. "plesiosaurus." The Concise Oxford Dictionary of English Etymology. 1996. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O27-plesiosaurus.html |
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Plesiosauroidea
Plesiosauroidea See PLESIOSAURIA.
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Cite this article
MICHAEL ALLABY. "Plesiosauroidea." A Dictionary of Zoology. 1999. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. MICHAEL ALLABY. "Plesiosauroidea." A Dictionary of Zoology. 1999. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O8-Plesiosauroidea.html MICHAEL ALLABY. "Plesiosauroidea." A Dictionary of Zoology. 1999. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O8-Plesiosauroidea.html |
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plesiosaurs
plesiosaurs See PLESIOSAUROIDEA.
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Cite this article
AILSA ALLABY and MICHAEL ALLABY. "plesiosaurs." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. AILSA ALLABY and MICHAEL ALLABY. "plesiosaurs." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O13-plesiosaurs.html AILSA ALLABY and MICHAEL ALLABY. "plesiosaurs." A Dictionary of Earth Sciences. 1999. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-plesiosaurs.html |
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plesiosaurs
plesiosaurs See PLESIOSAURIA.
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Cite this article
MICHAEL ALLABY. "plesiosaurs." A Dictionary of Zoology. 1999. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>. MICHAEL ALLABY. "plesiosaurs." A Dictionary of Zoology. 1999. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O8-plesiosaurs.html MICHAEL ALLABY. "plesiosaurs." A Dictionary of Zoology. 1999. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O8-plesiosaurs.html |
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