MORPHOMETRICS AND SYSTEMATICS OF HYOPSODUS (MAMMALIA, CONDYLARTHRA) FROM THE WASATCHIAN AND BRIDGERIAN EOCENE OF THE GREEN RIVER BASIN, WYOMING

Fossil teeth of Hyopsodus Leidy (1870) from the Green River Basin in southwestern Wyoming are described and analyzed using traditional morphometrics. Numerous (n=746) specimens of the posterior dentition are measured with attention to linear dimensions and cusp morphology and plotted as scatterplots of variation in morphospace. These data are aggregated by biostratigraphic zone spanning 6.5 m.y. from the latest Wasatchian (Lostcabinian Zone, Wa-7) through the late Bridgerian (Twinbuttean Zone, Br-3) of the Eocene. This study fills a stratigraphic gap in the history of the genus from an interval between the well-known older records from the Bighorn and Wind River basins and the records of Hyopsodus from the later Eocene. The results are generally consistent with those of previously published work regarding the tempo and mode of evolution within this common Eocene taxon.

In Wa-7, the first appearance of the small-sized H. minusculus is reported alongside H. miticulus, both of which continue into the Gardnerbuttean Zone, Br-1a. Evidence from the premolar series is suggestive of a continuation of the still larger H. walcottianus into Br-1a. Additionally, a last occurrence of H. miticulus is noted at the end of Br-1a, with H. minusculus continuing as the sole representative of the genus in the Early Blacksforkian Zone (Br-1b). The Late Blacksforkian Zone (Br-2) sediments preserve H. minusculus, the first appearance of H. paulus (the type species of Hyopsodus), and a possible precocious occurrence of the large-sized H. marshi (previously known from Br-3). Sample sizes from Br-3 are small, but the available evidence suggests that two smaller species of Hyopsodus accompany H. marshi, perhaps continuations of H. minusculus and H. paulus.

The presence of only one Hyopsodus species of small body size in Br-1b and the appearance of two additional species and an overall increase in body size among all lineages in Br-2 and Br-3 may be a reflection of climatic deterioration in the later Bridgerian.Geographic variation in Hyopsodus dentitions is also evident in this study. In the Br-2, body size is the smallest in specimens from localities near the basin-margin and increases toward the basin-center. In Br-1a, accessory cusps on the lower molars are variably present only in part of the basin-center of the Green River Basin.