EVOLUTION OF CAMELID PODIALS AND CONSEQUENCES FOR ESTIMATION OF MIOCENE MAMMAL DIVERSITY IN THE GREAT BASIN

Prior work on camelid astragali and metapodials suggests that the bones of the manus and pes in camels are taxonomically diagnostic, at least to the generic and possibly even to the specific level. It is, as yet, unclear how the different bones of the hands and feet may vary in taxonomic utility, and whether other elements may prove more or less taxonomically useful. This study presents a morphometric analysis of other camelid podial elements in an effort to determine their taxonomic utility. Because of their high cortex-to-volume ratio and their location in low-nutrition portions of the anatomy, podial elements are preferentially preserved in the highly-scavenged, fluvial deposits of the Miocene of the Great Basin. At many sites, several families of artiodactyls are only known from podial and metapodial elements. With a better knowledge of the shapes and sizes of these elements from sites where the animals are known from skulls as well as podials, the results of this study should improve estimations of taxonomic richness in Great Basin fossil mammal faunas. Additionally, understanding the patterns of mosaic evolution of hand and foot bones is informative for understanding the locomotor evolution of camelids.

Astragali and metapodials have shown great promise for taxonomic identification because of their structural constraints. Both are relatively large podial elements, however, and tend to have a higher incidence of breakage than some of the smaller elements. These smaller elements contain less structural information but provide a larger average sample size. In combination, all of the elements can provide greater confidence in the taxonomic composition of a fauna.

There is some suggestion that lateral elements are more variable, and there is a weak correlation between number of articular surfaces and taxonomic utility. As expected, there is a weak inverse correlation between element size and relative abundance. There is no clear distinction in discriminant ability between carpal and tarsal elements or between left and right sides. As with astragali, the major genus-level taxonomic discriminant is overall size, with some evidence that shape differences can distinguish species within generic size-bands. Careful examination of these differences is beginning to elucidate the nuances of camel locomotor evolution.