Stratigraphy, paleocology, and habitat change in the middle Eocene of North America

This study was undertaken to estimate what the local habitats were like during the middle Eocene of North America (the Bridgerian and Uintan North American Land Mammal Ages [NALMAs]) and to evaluate if a significant habitat change occurred that could have influenced primate community decline during this interval. Habitat types were estimated for five fossil assemblages from the Bridger Formation (Green River Basin, Wyoming) and for three assemblages from the Uinta Formation (Uinta Basin, Utah). The eight fossil assemblages were generated from stratigraphically controlled fossil collections from both formations. These fossil assemblages were compared with 20 mammal communities from four major extant habitat types in the Neotropics. The analysis presented here showed that the four Neotropical habitat types were significantly different and statistically appropriate for comparison with fossil samples.; Recent evidence from new fossil collections in the Uinta Formation controlled by the newly described stratigraphic section suggest that the biostratigraphic transition between early and late Uintan faunas occurs high in the section than previously reported. The traditional marker bed (Amynodon sandstone) is not an appropriate lithostratigraphic boundary for the Uinta B-Uinta C transition and both the lithologic and faunal transitions are higher in the section.; Using ecological diversity analysis, the eight fossil assemblages were classified with the 20 Neotropical modern faunas using a discriminant function analysis. This comprehensive analysis demonstrated that assemblages from the Bridger Formation were primarily classified with woodland or forest communities. One Bridger Formation assemblage was classified with an open habitat community, but this anomalous. The earliest Uinta Formation assemblage was classified with a forest community and both of the later assemblages were classified with open country.; My results show that a distinct habitat change occurred from the Bridgerian to the Uinta, not at the transition between these NALMAs, but significantly later. In the Uinta Formation stratigraphic section, the earliest occurrence of this change was detected at the 98-meter level and the latest occurrence of this change was at the 134-meter level. This change to a more open habitat would have negatively affected primate communities, and is indeed correlated with a reduction in primate diversity and abundance.