Date of Award


Degree Name

Biological Sciences


College of Science

Type of Degree


Document Type


First Advisor

F. Robin O’Keefe

Second Advisor

Suzanne Strait

Third Advisor

Andrew Grass


The polycotylids were a clade of plesiosaurs that proliferated during the Cretaceous period. Despite recent research efforts, evolutionary relationships among polycotylid species remain unresolved. In this study, a phylogenetic analysis incorporating a large taxon sampling from the Polycotylidae was used to parse out the evolutionary relationships among the taxa. The main focus was to assign two polycotylid specimens from the Wallace Ranch to a formal species. A study on the axial osteology of the juvenile Wallace Ranch polycotylid was also performed, as the development of the axial column of a polycotylid plesiosaur has been poorly understood and may have phylogenetic implications. This study revealed that the two Wallace Ranch specimens form a well-supported clade, and feature a mosaic of character states found in currently described species, and may represent a new species. The monophyly of the genus Dolichorhynchops, was not supported in the current analysis. D. bonneri, D. tropicensis, and the Wallace Ranch specimens are more closely related to Trinacromerum. Therefore a formal redescription of the two genera Dolichorhynchops and Trinacromerum is required. The study also revealed that the three skeletons attributed to the species Polycotylus latipinnis do represent members of the same species. However, the study did not support the close relationship between D. bonneri and P. latipinnis, and that the similarity seen between the two species is attributed to a convergence in large body size. The cranial osteology of the juvenile Wallace Ranch polycotylid provides a rare insight into the development and fusion pattern of a polycotylid skull, and reveals some possible ontogenetic characters, which should be excluded from future phylogenetic analyses. After analyzing the axial osteology of the juvenile Wallace Ranch specimen, the individual was most likely a neonate at the time of death. The understanding of the morphological changes during ontogeny provides better insight into intraspecific variation in a polycotylid species. The current study revealed, Polycotylidae feature a basal clade, a polyphyletic genus Dolichorhynchops, with an expanded Trinacromerum, and supports the previous assignment of three specimens to the species P. latipinnis.


Plesiosaurus -- Research.

Included in

Paleontology Commons