Abstract
The mammalian fossil record is largely composed of isolated teeth and tooth-bearing elements. In vertebrate microfossil assemblages with closely related, co-occurring species of mammals, it can be difficult to identify isolated teeth to species level because morphological differences among species may be slight and based on a single tooth position. Here we investigate the utility of the allegedly diagnostic lower fourth premolar (p4) for species-level identification in the genus Mesodma (Multituberculata, Neoplagiaulacidae). We conducted linear and geometric morphometrics on 86 p4s representing four Cretaceous-Paleogene (K-Pg) species of Mesodma that are common in deposits of the western interior of North America. Although Mesodma has been extensively discussed in the literature, these four species overlap considerably in p4 size and shape, making species-level identifications challenging. Using linear measurements, landmarks, and semilandmarks, we quantified p4 size and shape to understand morphological variation across the genus and uncover practical sources of morphological differentiation among the species represented here. Our results indicate (1) size is more important than shape for identifying p4s of Mesodma species; p4 shape varies across the genus, but cannot be used alone to identify isolated p4s to species; (2) M. garfieldensis and M. thompsoni cannot be distinguished from each other using p4 size or shape; we therefore subsume M. garfieldensis within M. thompsoni; and (3) M. formosa increased in size across the K-Pg boundary. In light of these results, we recommend that taxonomic diagnoses relying on isolated teeth incorporate quantitative analyses of morphology whenever possible to increase the accuracy of species-level identifications and paleofaunal studies that employ them.
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Acknowledgments
Funding for this research was provided by the University of Washington and the Hell Creek Project III. We would like to thank all previous Hell Creek field crews for their hard work and help with acquisition of specimens for this study. Permits to collect vertebrate fossils on state and federal land were provided by the U.S. Bureau of Land Management, Montana Department of Natural Resources, and Charles M. Russell Wildlife Preserve. We thank the University of California Museum of Paleontology (UCMP), especially Patricia Holroyd and Mark Goodwin, for access to specimens, and the UCMP Welles Fund, which enabled SMS to visit UCMP collections for this research. Special thanks to Anne Weil for digital images of UCMP 116621. We are also grateful to W.A. Clemens, members of the Wilson Lab (Alexandria Brannick, Jonathan Calede, Meng Chen, Lauren DeBey, and David DeMar, Jr.,), Ian Browne, and Yue Zhang for helpful comments and discussions concerning this research and manuscript.
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Smith, S.M., Wilson, G.P. Species Discrimination of Co-Occurring Small Fossil Mammals: A Case Study of the Cretaceous-Paleogene Multituberculate Genus Mesodma . J Mammal Evol 24, 147–157 (2017). https://doi.org/10.1007/s10914-016-9332-2
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DOI: https://doi.org/10.1007/s10914-016-9332-2