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The forefin of Chensaurus chaoxianensis (Ichthyosauria) shows delayed mesopodial ossification

Published online by Cambridge University Press:  20 May 2016

Ryosuke Motani  and
Hailu You
Ryosuke Motani
Affiliation:
Department of Zoology, University of Toronto, 25 Harbord Street, Toronto, Ontario, M5S 3G5 Canada
Hailu You
Affiliation:
Department of Geology, University of Pennsylvania, Hayden Building, Philadelphia 19104
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Abstract

The hitherto poorly known forefin of Chensaurus chaoxianensis (Ichthyosauria) from the Lower Triassic (Spathian) is redescribed on the basis of the holotype and two new specimens. The humerus resembles that of Utatsusaurus hataii but is distinctive in having an emarginated anterior margin. The anteroproximal prominence of the radius is well developed, unlike that of other ichthyosaurs. All three specimens have five metacarpals and many phalanges, but only three carpals, which are identified as the ulnare, intermedium, and fourth distal carpal. These specimens show that delayed mesopodial ossification occurred in ichthyosaurs, at least in an early evolutionary stage. Because delayed mesopodial ossification is common among diapsids and is unknown in Jurassic ichthyosaurs, it was lost during the evolution of the Ichthyosauria. The osteogenic developmental axis appears to have continued into the fourth digit, as in other amniotes. The ossification pattern provides conclusive evidence to support the suggestion that the basal element of the fifth digit in Early Triassic ichthyosaurs is a metacarpal, rather than a carpal.

Type
Research Article
Information
Journal of Paleontology , Volume 72 , Issue 1 , January 1998 , pp. 133 - 136
Copyright
Copyright © The Paleontological Society 

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