Department
Geosciences
Degree Name
Master of Science (MS)
Abstract
Tylosaurus nepaeolicus is a basal, medium-sized tylosaurine mosasaur that lived during the late Coniacian and early Santonian in the Western Interior Seaway. Despite being a well-studied species, there is debate over whether Tylosaurus kansasensis represents the juvenile form of T. nepaeolicus, or if the two species are distinct. Osteohistology can determine organism age at time of death and can be used to help resolve this debate and provide a better understanding of the growth of T. nepaeolicus. Thin-sections were taken from a humerus, rib, and vertebra of a single individual (FHSM VP-2209). All thin-sections show primarily parallel-fibered cortical bone and trabecular networks of varying extent with secondarily remodeled lamellar bone. Vascular canals are primarily longitudinal in the rib with longitudinal and radial orientations in the humerus and vertebra. Canals are open to the periosteal surface in all bones examined indicating the individual was actively growing at the time of death and had not yet reached somatic growth. Based on lines of arrested growth, the mosasaur was at least 12 years old at the time of death. Zygapophyseal growth rings support this age estimate, with 11 rings observed on the surface of the zygapophysis.
FHSM VP-2209 is interpreted as a sub-adult based on the extensive, but incomplete secondary remodeling, the lack of an external fundamental system, and the presence of vascular canals open to the periosteal surface. FHSM VP-2209 has previously been estimated to be 7.1 meters long and represents one of the largest T. nepaeolicus known, implying that the species may have grown larger than previously thought. If T. kansasensis is a junior synonym of T. nepaeolicus and represents the juvenile ontogenetic stage of the species, then known individuals identified as T. nepaeolicus should be skeletally mature. Otherwise, anatomical differences would represent phylogenetic, rather than ontogenetic, characters. Because this specimen has been consistently identified as T. nepaeolicus and was still growing at the time of death, it is not likely that T. kansasensis also represents a juvenile form of T. nepaeolicus. As a result, this histological survey of a large but sub-adult T. nepaeolicus specimen supports the validity of T. kansasensis as a distinct species.
Keywords
histology, marine reptiles, ontogeny, Niobrara, Cretaceous
Advisor
Dr. Laura Wilson
Date of Award
Spring 2025
Document Type
Thesis
DOI
http://doi.org/10.58809/VQRV7159
Recommended Citation
Cope, Carson, "A Histological Survey of a Large, Sub-Adult Tylosaurus nepaeolicus with Implications on the Validity of Tylosaurus kansasensis" (2025). Master's Theses. 3269.
DOI: 10.58809/VQRV7159
Available at:
https://scholars.fhsu.edu/theses/3269
Rights
© The Author
