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ICHTHYOSAURUS

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Originally appearing in Volume V14, Page 271 of the 1911 Encyclopedia Britannica.
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ICHTHYOSAURUS , a See also:

fish or See also:porpoise-shaped marine reptile which characterized the Mesozoic See also:period and became See also:extinct immediately after the deposition of the See also:Chalk. It was named Ichthyosaurus (Gr. fish-See also:lizard) by C. See also:Konig in 1818 in allusion to its outward See also:form, and is best known by nearly See also:complete skeletons from the See also:Lias of See also:England and See also:Germany. The large See also:head is produced into a slender, pointed snout; and the jaws are provided with a See also:row of conical See also:teeth nearly See also:uniform in See also:size and deeply implanted in a continuous groove. The See also:eye is enormous, and is surrounded by a See also:ring of overlapping " sclerotic From See also:British Museum See also:Guide to Fossil See also:Reptiles and Fishes, by permission of the Trustees. See also:Skeleton of Ichthyosaurus communis, with outline of See also:body and fins, from the See also:Lower Lias of Lyme Regis, See also:Dorset; See also:original nearly four metres in length. plates," which would serve to protect the eye-See also:ball during diving. The vertebrae are very numerous, See also:short and deeply biconcave,. imparting See also:great flexibility to the backbone as in fishes. The See also:neck is so short and thick that it is practically absent. There are always two pairs of See also:paddle-like limbs, the hinder pair never disappearing as in porpoises and other See also:Cetacea, though often much reduced in size. A few specimens from the Upper Lias of See also:Wurttemberg (in the museums of See also:Stuttgart, See also:Tubingen, See also:Budapest and See also:Chicago) exhibit remains of the skin, which is quite smooth and forms two triangular median fins, one in the See also:middle of the back, the other at the end of the tail. The dorsal fin consists merely of skin without any See also:internal skeleton, while the tail-fin is See also:expanded in a See also:vertical See also:plane and has the lower See also:lobe stiffened by the tapering end of the backbone, which is sharply See also:bent downwards.

Immature individuals are sometimes observable within the full-grown skeletons, suggesting that this reptile was viviparous. The largest known See also:

species of Ichthyosaurus is I. trigonodon from the Upper Lias of Banz, See also:Bavaria, with the head measuring about two metres in length and probably representing an See also:animal not less than ten metres in See also:total length. I. platyodon, from the See also:English Lower Lias, seems to have been almost equally large. I. intermedius and I. communis, which are the commonest species in the English Lower Lias, rarely exceed a length of three or four metres. The species in rocks later than the Lias are known for the most See also:part only by fragments, but the remains of Lower Cretaceous See also:age are noteworthy for their very wide See also:geographical See also:distribution, having been found in See also:Europe, the See also:East Indies. See also:Australia, New See also:Zealand and See also:South See also:America. Allied Ichthyosaurians named Ophthalmosaurus and Baptanodon, from the Upper See also:Jurassic of England and See also:North America, are nearly or quite toothless and have very flexible broad paddles. The earliest known Ichthyosaurians (Mixosaurus), which occur in the Trias, are of diminutive size, with paddles which suggest that these marine reptiles were originally descended from See also:land or See also:marsh animals (see REPTILES).

End of Article: ICHTHYOSAURUS

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