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A re-assessment of the genus Mosasaurus (Squamata: Mosasauridae) Open Access


Other title
Type of item
Degree grantor
University of Alberta
Author or creator
Street, Hallie P.
Supervisor and department
Caldwell, Michael (Biological Sciences)
Examining committee member and department
Murray, Alison (Biological Sciences)
Holmes, Robert (Biological Sciences)
Proctor, Heather (Biological Sciences)
Bell, Christopher (Geological Sciences)
Department of Biological Sciences
Systematics and Evolution
Date accepted
Graduation date
Doctor of Philosophy
Degree level
Mosasauridae is a lineage of extinct marine squamates that inhabited the world’s oceans during the Late Cretaceous (100-66 Ma). The name Mosasaurus was given to the first described specimen, which was a fossil discovered in Maastricht, the Netherlands, during the 1770s. Naturalists of the time did not provide a precise diagnosis when describing or naming the specimen, and so the concept of the taxon remained poorly delineated. As has been the case for many of the early generic names erected for fossil reptiles (e.g. Iguanodon, Ichthyosaurus, Plesiosaurus), this lack of a precise diagnosis allowed Mosasaurus to become a wastebasket taxon, with dozens of species assigned to it over the past two centuries. Some paleontologists made efforts to more precisely diagnose Mosasaurus over the past fifty years, but the specimens these diagnoses were based on further confused what was being defined. The first goal of this study is to emend the diagnoses of Mosasaurus and its type species M. hoffmannii Mantell 1829 in order to develop a clear-cut characterization of these taxa. Provided with a precise diagnosis of the genus, the skeletal morphology of 12 potentially valid species is compared to that of M. hoffmannii in order to determine their agreement with the generic paradigm. The findings of these comparisons informed the choice of taxa included in the phylogenetic analyses. To date, no phylogenetic analysis of Mosasauridae has included more than three or four species of Mosasaurus, and this study rectifies that. The phylogenetic analysis, which was based on a taxon list including only those taxa determined to be both valid and distinct by the morphological comparison, found Mosasaurus to be composed of only four species and part of a discrete clade of closely related genera that differ from the rest of the Mosasaurinae. The morphological comparisons and hypothesis of relationships produced by the phylogenetic analyses are combined to develop the ultimate goal of this study, which is a complete systematic revision of Mosasaurus. The species that disagree with the generic paradigm are reassigned to other genera or serve as the basis for new genera within the clade Mosasaurini. After nearly two centuries, M. hoffmannii is diagnosed based on the morphology exhibited by the holotype specimen, and the diversity of taxa related to Mosasaurus is more clearly understood.
This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for the purpose of private, scholarly or scientific research. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
Citation for previous publication
Street, H.P., Caldwell, M.W. 2014. Reassessment of Turonian mosasaur material from the “Middle Chalk” (England, U.K.), and the status of Mosasaurus gracilis Owen, 1849. Journal of Vertebrate Paleontology. 34, 1072–1079. doi:10.1080/02724634.2014.846263

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