Cenozoic ichthyofaunas of the North American Western Interior, and palaeoclimatic and palaeoenvironmental reconstructions

The Cenozoic fluvial fish faunas of the North American Western Interior are poorly known in comparison to better preserved lake ichthyofaunas, such as that of the Green River Formation. However, fluvial fishes are typically represented by disarticulated material, which is more commonly preserved than articulated specimens, and floodplain riverine environments correspond to areas of maximal taxonomic diversity in modern freshwater environments. Therefore, disarticulated fluvial material should preserve a greater number of taxa than articulated lacustrine material, allowing a more complete understanding of the formation of the North American modern fish fauna since the Late Cretaceous. The freshwater fishes of four North American Western Interior assemblages are here described. These assemblages were recovered from the southern Saskatchewan mid-Miocene Wood Mountain and Eo–Oligocene Cypress Hills formations, as well as the middle and early Eocene Bridger and Wasatch formations of Wyoming. The diversity of these assemblages is documented and forms the basis of palaeoclimatic and palaeoenvironmental reconstructions, founded on the specific tolerance ranges of the fish taxa present. Both Canadian assemblages are highly diverse, typical of lowland, well-oxygenated and varied floodplain environments, and indicate warm-temperate to subtropical climates at time of deposition. The Wyoming assemblages, although also indicative of warm environments, are relatively less diverse. However, these indicate that the early Paleogene ichthyofauna of North America was similar to that of the Late Cretaceous. Based on the comparison of the assemblages described in this thesis with one another and with other described assemblages, the evolution of the North American freshwater fish fauna through the Cenozoic is reconstructed, from the Mesozoic to the present. The turnovers that led to the formation of the modern fauna appear to have been relatively uninfluenced by the K-Pg transition, but to have occurred in two phases instead, one in the mid-Paleogene and the other in the late Neogene. These coincide with the increase of seasonality in North America between the middle and late Eocene, and the gradual cooling of climates leading to the Plio-Pleistocene glaciations, respectively.

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