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Science 24 September 1993:
Vol. 261. no. 5129, pp. 1716 - 1720
DOI: 10.1126/science.261.5129.1716
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Articles

The Origin of the Turtle Body Plan: Bridging a Famous Morphological Gap

Michael S. Y. Lee 1

1 University Museum of Zoology, Downing Street, Cambridge CB2 3EJ, United Kingdom

A restudy of pareiasaurs reveals that these primitive reptiles are the nearest relatives of turtles. The two groups share numerous derived characters, such as a reduced presacral count, an acromion process, and a trochanter major, which are absent in other basal amniotes. Many traits long thought specific to chelonians also occur in pareiasaurs and must have evolved before the distinctive turtle shell appeared. Evidence uniting captorhinid or procolophonoids with turtles is shown to be weak. The phylogeny proposed here also suggests that certain features of the earliest turtle (Proganochelys) that have been interpreted as specializations, such as the large supratemporal and robust metacarpals, are primitive for turtles. In pareiasaurs, the osteoderms represent the precursors of the chelonian shell and the morphology of the anterior region is consistent with the idea that the shoulder girdle in turtles has migrated posteriorly into the rib cage.

Submitted on March 19, 1993
Accepted on June 2, 1993


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