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Phalangeal fusion in schizotheriine Chalicotheres (Mammalia, Perissodactyla)

Published online by Cambridge University Press:  14 July 2015

Margery C. Coombs  and
Bruce M. Rothschild
Margery C. Coombs
Affiliation:
Department of Biology and Graduate Program in Organismic and Evolutionary Biology, University of Massachusetts, Amherst 01003,
Bruce M. Rothschild
Affiliation:
Northeast Ohio Universities College of Medicine, 5500 Market Street, Youngstown 44512, , and Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, Pennsylvania 15213
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Abstract

In four genera of schizotheriine Chalicotheriidae—Moropus, Tylocephalonyx, Phyllotillon, and Ancylotherium–proximal and middle phalanges of digit II of the manus fuse to form a duplex bone. Fusion of proximal and middle phalanges of digit II of the pes also occurs in many species of these genera. Radiographic and sagittal-sectional study of the fused duplex phalanges of Moropus elatus and Tylocephalonyx skinneri shows that the joint between the two bones formed fully prior to fusion but had little movement. Fusion proceeded by external enthesial interactions at the insertion of the ligament capsule and by disruption of cortical bone with trabecular bridging along the internal part of the original articular surface. Cortical disruption is more rapid on the middle than on the proximal phalangeal component of the duplex. In M. elatus, the degrees of external and internal fusion roughly correspond, and a ventral surficial groove sometimes traces the former facet, whereas in T. skinneri external fusion is much more developed than internal fusion. Phalangeal fusion seems to have strengthened the digit against strong forces applied along the medial (radial or tibial) side of the foot.

Type
Research Article
Information
Journal of Paleontology , Volume 73 , Issue 4 , July 1999 , pp. 682 - 690
Copyright
Copyright © The Paleontological Society 

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