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A Reinterpretation of the Upper Triassic Ichthyosaur Shonisaurus

Chris McGowan and Ryosuke Motani
Journal of Vertebrate Paleontology
Vol. 19, No. 1 (Mar. 15, 1999), pp. 42-49
Stable URL: http://www.jstor.org/stable/4523968
Page Count: 8
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A Reinterpretation of the Upper Triassic Ichthyosaur Shonisaurus
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Abstract

Shonisaurus, from the Upper Triassic (Carnian) of Nevada, is represented by some 40 individuals. However, many comprise only a few elements, and not one of the skeletons is complete. Furthermore, preservation is often poor, so information is limited. Reaching lengths of up to 15 m, possibly slightly more, Shonisaurus was about the size of a Gray Whale. The only other ichthyosaur of comparable size was a little known and unnamed ichthyosaur from the English Lower Jurassic, which may have reached 15 m (McGowan, 1996). Shonisaurus was long-bodied, with about 60 presacral vertebrae. Some of the centra, at about the level of the 97th vertebra, are wedge-shaped, indicating a tailbend. However, their cambers are small, and their combined angle of deflection is only about 5°, which hardly constitutes a tailbend. The paired fins were long and slender, with no more than three major digits in each, possibly only two. Contrary to previous descriptions, the emarginations on the free margins of many of the fin elements were probably preaxial rather than postaxial, as in all other ichthyosaurs. The coracoid does not have a hooked process distally, and is therefore much like that of Shastasaurus. The teeth are set in individual sockets, as previously described. Three species have been described, but S. popularis is here considered the only valid species.

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