DIRECTIONAL SELECTION, MACROEVOLUTIONARY RATCHET, AND THE DIVERSIFICATION AND EXTINCTION OF NORTH AMERICAN CREODONTA (MAMMALIA)

Creodonts were relatively abundant and diverse mammalian carnivores of the emerging eutherian fauna of the Paleogene. They exhibited a range postcranial morphologies leading to the hypothesis that they filled numerous ecological roles, from small-bodied predators capable of climbing to large, high-speed pursuit predators. In addition to hypotheses regarding their ecology, researchers have observed that changes in their postcranial skeleton and their eventual extinction may have been correlated with environmental and climatic shifts of the Paleogene of North America. Here, I examine patterns of disparity and test for directional selection in order to assess biotic and abiotic factors influencing the diversification and extinction of Creodonta.

Disparity and trait data were derived from linear morphometric data collected from the postcranial skeleton of both creodont families (Oxyaenidae and Hyaenodontidae), with Principal Coordinates Analysis used to visualize locomotor morphospace. Using a time-calibrated composite phylogeny of Creodonta and the principal coordinates scores, I tested two alternative models of trait evolution, directional selection and Brownian motion. Evolutionary modeling analyses supported a Brownian motion model of trait evolution over directional selection. Subsequent examination of morphospace occupation through time reveals the loss of early, generalized taxa with the latest-surviving taxa adapted for specialized locomotor behaviors. This pattern may be attributable to macroevolutionary ratchet and mirrors that of the dentition, in which there was a loss of generalized, centrally-located taxa within the morphospace. These patterns, along with the lack of support for directional selection, indicate that the decline in creodont diversity and disparity may have resulted from competition with another successful clade of eutherian carnivores, the carnivoramorphans.