Fossil mammals of the Twelvemile Bonanza Local Fauna, late Tiffanian of southwestern Wyoming

Abstract

Twelvemile Bonanza is a recently discovered Paleocene fossil mammal-bearing locality in southwestern Wyoming. The environment it preserves was most likely a relatively poorly drained, well-forested floodplain. The Twelvemile Bonanza Local Fauna is the only fauna that can be unambiguously assigned to the Ti5 interval outside of the Bighorn Basin; it therefore greatly adds to our knowledge of the mammals alive in North America during this time by providing additional geographic coverage of this poorly documented interval. The age of the fauna also puts the site in a prime position to test the effects of changing temperatures on the ranges, body sizes, and paleoecology of late Tiffanian mammals in Wyoming. The Twelvemile Bonanza local fauna is largely similar to the contemporary Princeton Quarry fauna in the Bighorn Basin. Where species differ, they differ most often among the smaller-bodied taxa of both sites, consistent with the modern observation that smaller species tend to have smaller ranges, suggesting they tend to become reproductively isolated more often when they do disperse. Taxa such as Aletodon conardae and Microsyopidae appear to have tracked warmer temperatures as their ranges expanded and contracted across Wyoming over the course of the late Paleocene. Furthermore, when comparing Twelvemile Bonanza to other sites in southwestern Wyoming immediately preceding it (Ti4) and following it (Cf1/Cf2) in time, two trends are noticeable: (1) carnivorous species decrease in body mass (a trend consistent with Bergman’s Rule), and in diversity, as temperatures rise; (2) the body mass distributions of the fauna increasingly tend to cluster around a few peaks as temperatures rise, rather than being more evenly spread out. One explanation for the latter trend may be that the fauna become more specialized as temperatures increase, allowing species of similar body size to coexist in the same ecosystem.