The Eocene vertebrate assemblage from South Pass along the northeastern edge of the Green River Basin represents a basin margin fauna typified by high taxonomic diversity, unique species, great morphological disparity, and the co-occurrence of ancestor-descendant couplets. It has been hypothesized that marginal areas, because of their geographic isolation, topographic complexity, and greater habitat diversity, were centers of morphological innovation and speciation. Such hypotheses, however, lack a clear causal mechanism that would produce biotic perturbations under which relatively rapid evolution could occur.

Stratigraphic analyses of the South Pass section may offer a proximate causal explanation for the unusual vertebrate assemblage. Green River Formation (GRF) lake sediments interfinger with Wasatch (WF) and Bridger formation fluvial sediments in the South Pass section. Two major early Eocene lake transgressions occurred in the study interval. The first, represented by the Wilkins Peak/Tipton Shale members (GRF), occurs near the base of the section. The second, represented by the Laney Shale Member (GRF) occurs in the upper third of the study interval. These two major transgressive events separate the major faunal transitions: Wasatchian (Wa7) to basal Bridgerian (Br0/Br1a) to traditional early Bridgerian (Br1b). In addition, a minor transgressive event within the predominantly alluvial Cathedral Bluffs Tongue (WF), may separate the Br0-Br1a transition.

We propose that during the late Wasatchian and early Bridgerian, transgressive phases of Paleolake Gosiute would have inundated lowlands, forcing the inhabitants into basin margin areas along the periphery of their habitat ranges. Interactions between invading basin center taxa and resident upland forms could have resulted in increased competition for limited resources, producing selective pressures on both sets of vertebrate populations. In addition, chances that evolutionary innovations would be incorporated into the genetic structure of such small, isolated populations would be greater due to genetic drift.

Additional analyses of vertebrate diversification in relation to sea level change further suggest that “transgressive forcing” may be an important evolutionary mechanism at a variety of temporal and geographic scales.