Conceptualizing vertebrate faunal dynamics : new perspectives from the Triassic and Eocene of Western North America




Stocker, Michelle Renae

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Conceptualizations of actual biological patterns as preserved in the fossil record must accommodate the results of biotic and abiotic drivers of faunal dynamics. However, those conceptualizations also may reflect cognitive biases resulting from foundational philosophical stances. Whether fossils are conceptualized as the remains of biological entities or as geological objects will affect both taxonomic identifications and secondary inferences derived from those identifications. In addition, operational research bias centered on relativistic views of ‘importance’ of particular components (i.e., taxonomic or skeletal region) of the assemblage results in preferential documentation of some taxa and marginalization of others. I explored the consequences of those specific cognitive and operational biases through examination of Triassic and Eocene faunal assemblages in western North America. For the Triassic I focused on taxonomic and systematic treatments of Paleorhinus, a group of phytosaurs important for the establishment of biochronologic correlations. Specimen-level reexamination of Paleorhinus supported a restricted usage of Paleorhinus as a clade, dissolved a biochronologic connection between terrestrial and marine deposits, and indicated a prior compression of the early part of the Late Triassic as a result of previous conceptualizations of species. I reexamined the Otis Chalk tetrapod assemblage in light of new specimens and modern phylogenetic frameworks. My examination supported a restricted usage of the Otischalkian for biochronologic correlation of the Late Triassic, and emphasized the importance of apomorphic character-based specimen examinations in conjunction with detailed lithostratigraphy prior to the development of biochronologic schema. For the Eocene I focused on undocumented terrestrial reptiles from the late Uintan fauna of West Texas. Specifically I discovered new taxa and new geographic occurrences of amphisbaenians and caimanine crocodylians. The amphisbaenians represent the southernmost record of the clade in the North American Paleogene, and, when combined with other amphisbaenian records, document that the clade responded to late Paleogene climatic changes in ways different from the inferred mammalian response. The new taxon of caimanine crocodylian represents a new geographic and temporal record of that clade. That new record indicates that the biogeographic range of extant caimans represents a climate-driven restriction from a formerly more expansive range, and suggests that the previous geographic and temporal gap in paleodistribution data is related to sampling biases and is not a solely a biological phenomenon. These data indicate that reliable characterization of vertebrate faunal dynamics requires open acknowledgment and appropriate documentation of cognitive and operational biases that affect interpretations of paleontological data.



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