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

Paul Constantino


Understanding ontogenetic variation in plesiosaurs from the family Polycotylidae can provide insight into the evolutionary history of this group of extinct short-necked marine reptiles from the Cretaceous period. In this study, I analyzed a juvenile polycotylid plesiosaur from the University of Nebraska State Museum (UNSM 55810). UNSM 55810 came from the Pierre Shale (99-65 Ma) of Nebraska and possesses skeletal elements that include a partial skull, nearly complete pectoral and pelvic girdles, and an assortment of paddle elements. Based on several cranial characteristics, this specimen is probably referable to the genus Dolichorhynchops. Previous qualitative studies have included UNSM 55810 but a formal description has not yet been done. To understand the ontogenetic variations among polycotylids, I collected metric measurements of UNSM 55810 and compared them among several related plesiosaurs. The greatest amount of change occurred in the girdle elements, particularly along the anterior and posterior edges. In contrast, the clavicles exhibit relative conservatism throughout ontogeny. This may be due to their dermal origin versus the endochondral origin of the other girdle elements. The juvenile girdle elements lack ossification along their margins, which suggests that cartilage may have been present in order to provide support at the time of birth. Changes in girdle morphology during polycotylid development resemble the evolutionary girdle morphology that occurred in earlier sauropterygians,suggesting that polycotylid ontogeny recapitulates sauropterygian phylogeny. Understanding the morphological changes during polycotylid ontogeny will enhance information for improved identification of subadult specimens. In addition, it provides insight to the evolutionary and developmental history of plesiosaur ontogeny and evolution.