The rise of modern mammalian faunas: tempo and mode of faunal turnover in western Montana during the Oligocene
Calede, Jonathan Jean-Michel
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The ongoing biodiversity crisis affects almost one in four mammal species. The main threat affecting them is the loss of their habitat with changes in their environment. Paleontological data allow an exploration of the patterns and processes that regulate such faunal turnovers linked to environmental change. I use the fossil record of the Arikareean (ca. 30 to 25.5 Ma), which records a major environmental change concurrent with the rise of many mammalian taxa, to shed light on the link between environmental change, ecological filtering by habitat, and taxonomic turnover over evolutionary timescales. I focus my work on the Cabbage Patch beds of western Montana, an area geographically intermediate between the well-studied John Day Formation of Oregon and Arikaree Group of the Great Plains. My detailed study of the preservation and depositional environments of the mammalian fossils from the Cabbage Patch beds demonstrates that many of the well sampled vertebrate microfossil assemblages from the beds can be considered isotaphonomic for the purpose of comparing biodiversity through time. Geometric morphometric analyses of the upper third molars and lower fourth premolars of entoptychine gophers, a very abundant group of mammals through the beds, demonstrate that even worn and isolated cheek-teeth can be used to identify these rodents to the genus, or even the species-level. These results, along with a detailed analysis of the taxonomic affinities of almost 1,000 specimens of fossil mammals through the beds, allow the assembly of the dataset necessary for an analysis of mammalian biodiversity in this area of the Rocky Mountains during the Arikareean. The study of the taxonomy and diversity of the mammalian fauna through the beds suggest that the timing of the faunal turnover of the Arikareean in the northern Rocky Mountains was more similar to the timing in Oregon than in the Great Plains. This pattern supports an eastward spread of the faunal transition of the Arikareean and a time transgressive mammalian turnover across the western United States that is associated to a stronger immigration from Eurasia than previously suggested. My analysis of the changes in dietary and locomotory ecologies through time at the scale of communities in the Cabbage Patch beds and a comparative dataset from Nebraska show evidence for a transition from assemblages dominated by mammals with affinities for forested and mesic environments to assemblages dominated by mammals with affinities for open environments ca. 27-26 Ma . Differences in ecologies across taxa suggest that these changes in the ecological composition of Arikareean mammalian assemblages were linked to taxonomic turnover from immigration and in situ evolution. These results suggest that the link between environmental change, ecological filtering, and taxonomic turnover acting over evolutionary timescales led to the rise of modern mammalian faunas.
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