Comparative morphology of the ilium of anurans and urodeles (Lissamphibia) and a re-assessment of the anuran affinities of Nezpercius dodsoni Blob et al., 2001

ABSTRACT

Ilia of anurans (frogs) and urodeles (salamanders) are commonly recovered from microvertebrate fossil localities. Ilia in these clades are distinctive when complete and articulated with the rest of the pelvic girdle, but when preserved as isolated and broken fossils they may appear superficially similar. Reliable identification of urodele ilia is further hampered by limited information about the basic structure and contacts of the bone and its variation within the clade. Here we demonstrate that the ilium is more variable among urodeles than previously realized and provide the first detailed inventory of features that are potentially useful for differentiating ilia of anurans and urodeles. Many of these features relate to differences between the two clades in the orientation of the ilium and its contacts with other bones and with soft tissues. Based on the results of our survey, we re-interpret the holotype and two referred ilia of Nezpercius dodsoni (Late Cretaceous; Montana, U.S.A.) as being from an indeterminate urodele, not an anuran as originally described. Additional examples of Late Cretaceous urodele ilia are documented to highlight some of the variation seen in fossil urodele ilia and to aid in the proper identification of such specimens.

ACKNOWLEDGMENTS

We thank the collections personnel and curators at the institutional collections listed in the introductory section for access to specimens in their care. We are especially grateful to J. Bolt, O. Rieppel, and W. Simpson (FMNH) for loans to J.D.G. of the Nezpercius type series, to F. Jenkins, Jr. (MCZ) and J. Whitmore Gillette (MNA) for allowing Z.R. to examine Prosalirus material, and to K. Leiper (TMP), J. Person (NDGS), and J. Whitmore Gillette for promptly issuing catalog numbers for many of the figured specimens. M. Švátora (Charles University) allowed us to dissect the thigh musculature of an Andrias davidianus specimen, J. McCabe (TMP) provided lab space to dissect and macerate other urodele specimens, G. Braybrook (University of Alberta) took the SEMs, and C. Scott and L. Davis (TMP) assisted with digital photographs and interlibrary loans, respectively. M. Venczel (Ţării Crişurilor Museum), J.-C. Rage (Muséum national d'Histoire naturelle), and D. Brinkman (TMP) encouraged us to explore the urodele affinities of Nezpercius and provided useful literature, specimens, and comments. The Royal Tyrrell Museum Cooperating Society provided financial support for Z.R.'s visit to Drumheller in 2006, at which time this project was initiated. J.T.S.'s work in North Dakota benefited from the advice and assistance of K. Johnson (Denver Museum of Nature and Science), D. Pearson (Pioneer Trails Regional Museum), and students from the South Dakota School of Mines and Technology and was supported by funding from the Earthwatch Institute (No. 4-70250), National Geographic Society (No. 7099-01), a Haslem post-doctoral fellowship at the South Dakota School of Mines and Technology (1999–2002), and NSF Epscor. NDGS 853 was collected in 2001 from property administered by the Bureau of Land Management under a permit issued to J. Hoganson (NDGS). J.D.G. thanks N. Marklund for her support and tolerance, especially while this project was in its final stages. We are grateful to D. DeMar and J.-C. Rage for their constructive reviews.