EXAMINING THE BIOCHRONOLOGIC OCCURRENCES AND MORPHOLOGICAL RELATIONSHIPS OF LEPTAUCHENIINE OREODONTS: IMPLICATIONS OF THE EMIGRATION AND TAXONOMIC STATUS OF THE GENUS SESPIA

Oreodonts are notorious for exhibiting a convoluted history of taxonomic turbulences. The subfamily leptaucheniinae is one of the currently better resolved oreodont subgroups. However, no studies have elucidated interrelationships within the subfamily. This study focuses on the morphological relationships of Leptauchenia and Sespia along with the frequently inferred oreonetine ancestor Limnenetes, while noting stratigraphic and biogeographic occurrences. Some of the most conspicuous characters demonstrating the distinctions of Leptauchenia and Sespia include relative rostral length, jugal depth, presence or absence of molar styles, and a trend from slightly inflated to extremely inflated auditory bullae. There is an observable biochronologic cline of slightly inflated auditory bullae remaining separated from the paroccipital processes, extremely inflated bullae conjoining the paroccipital processes, and further inflation of the bullae while the paroccipital processes is deformed to fit the contour of the posterior margin of the bullae. These features are respectively seen in Limnenetes, Leptauchenia, and the biogeographically removed Sespia californica.

This morphologic trend is broken with the species of Sespia from the high plains, which retain reduced auditory bullae, well separated from the paroccipital processes, as also observed in Limnenetes. The corresponding morphologies in congruence with the fossil record could potentially mean a divergence of Leptauchenia and Sespia, as early as the latest Eocene, with S. nitida and S. ultima retaining some synapomorphies with Limnenetes. These morphologic trends between High Plains Sespia and S. californica seem to be counterintuitive to previous interpretations of direct relationship. A Whitneyan specimen of Sespia, strongly resembling a larger morphotype of S. californica, suggests the possibility that S. californica derived directly from Leptauchenia approximately near the later Whitneyan, slightly before the first appearance of S. californica in the early Arikareean of California. The examination of these characters and when they emerge suggest that a polyphyletic relationship within the genus Sespia may be a more parsimonious interpretation.