Morphology and variation within the bony labyrinth of zhelestids (Mammalia, Eutheria) and other therian mammals

ABSTRACT

The morphology of the labyrinth of zhelestids, a group of eutherian mammals from the Late Cretaceous, agrees with that of other extinct eutherians, including Kulbeckia kulbecke, Ukhaatherium gobiensis, and Zalambdalestes lechei. Features of the labyrinth of zhelestids include a cochlea with one and a half turns and a secondary common crus, which are plesiomorphic for eutherians. Although isolated petrosals likely represent multiple species because several species of zhelestids are recognized based on dental characters, the degree of variation within the entire zhelestid petrosal sample is comparable to that observed within single extant species of therian mammals. Planarities of individual semicircular canals were the most variable measurements. Coiling of the cochlea and arc radii of the semicircular canals do not vary significantly in the specimens examined. The morphology of the bony labyrinth does not suggest any phylogenetic affinities of zhelestids, but the anatomy described will be useful for future studies of the evolution and physiology of the ear of mammals.

ACKNOWLEDGMENTS

The research reported here was conducted to fulfill partial requirements of the Ph.D. dissertation of E.G.E., for whom T.R. served as supervisor. Both authors thank UTCT for scanning of specimens. The UTCT laboratory is an NSF-supported shared multiuser facility (NSF EAR-0345710). Further financial support was provided by the Jackson School of Geosciences at The University of Texas at Austin in Austin, Texas. The petrosals of zhelestids were provided by A. Averianov and J. D. Archibald. The CT data for the two Zalambdalestes and Ukhaatherium specimens were provided by M. Novacek at the American Museum of Natural History in New York. Helpful comments and discussions throughout the completion of the manuscript were provided by H. Ahrens, C. Bell, M. Colbert, Z. –X. Luo, J. Olori, J. Sprinkle, T. Martin (editor), and two anonymous reviewers.

Handling editor: Thomas Martin.