Browsing by Author "Campbell, James Alexander"

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    Osteology and Histology of Plesiosaurs (Reptilia: Sauropterygia) from Non-marine (Fluvial to Estuarine) and Marine Depositional Environments
    (2019-12-19) Campbell, James Alexander; Anderson, Jason S.; Theodor, Jessica M.; Henderson, Donald M.; Caldwell, Michael W.; O'Keefe, Frank Robin; Dutchak, Alex R.
    Plesiosaurs are a highly diverse group of secondarily-aquatic reptiles that lived from Late Triassic to Late Cretaceous time. Plesiosaurs are known primarily from marine deposits, but also occur sparingly in non-marine units. Here, I describe the osteology and histology of plesiosaurs from both non-marine (Dinosaur Park Formation, DPF) and marine (Bearpaw and Pierre Shale Formations; BF, PSF), Upper Cretaceous-aged deposits of western Canada. These sediments were deposited in the Western Interior Basin, which was inundated by a large marine corridor known as the Western Interior Seaway. Non-marine (fluvial to estuarine) sediments of the DPF have yielded a stratigraphically extensive collection of elasmosaurid plesiosaur specimens. These specimens are notable as they belong to individuals that are small-bodied relative to some elasmosaurid specimens from marine units. In this study, I test whether the small-bodiedness of DPF specimens is due to the presence of immature individuals or a small-bodied taxon. Analyses of select DPF elasmosaurid specimens indicate the presence of a small-bodied taxon. Furthermore, a well-preserved partial skeleton from this unit is recognized as the holotype of a new genus and species. The holotype and referred specimens span both estuarine and fluvial sediments and may indicate an example of niche-partitioning within the predominantly marine Plesiosauria. The small body size is also consistent with those of plesiosaurs from other non-marine units. I also describe select elasmosaurid and polycotylid specimens from the BF and PSF, respectively. One elasmosaurid belongs to a large-bodied taxon which may be closely-related to, or possibly conspecific with, the DPF taxon. Another elasmosaurid belongs to a small-bodied, likely new, taxon. The two polycotylids were originally considered to be a juvenile and adult of Dolichorhynchops bonneri, based on their similar morphologies and large discrepancy in size. However, both specimens are osteologically mature, possibly reflecting interspecific differences; alternatively, these body size differences may be due to sexual dimorphism. Finally, this dissertation constitutes one of the first histological studies on plesiosaurs from non-marine units and Canada. This study also contributes to our limited understanding of plesiosaur ontogeny.