RIB HISTOLOGY OF EDMONTOSAURUS AND TRICERATOPS: AN ANALYSIS ON SUITABILITY FOR DETERMINING ONTOGENY

Ornithoscelida, including genera like Triceratops and Edmontosaurus, is a well-studied group in the dinosauria clade. Lack of well-preserved long bone fossils and research have, however, rendered histological data lacking for multiple species. This data comes in the form of ontogenetic analysis and is vital in understanding skeletal maturity and growth. Long bones are preferential for histology due to their circumferential shape but are less numerous as skeletal elements, and so are less common than ribs. To address the lack of ontogenetic information within this group, my senior thesis research focuses on performing histological analysis on Edmontosaurus and Triceratops postcranial fossil material to test if rib histology is suitable for determining ontogeny in these groups. Ontogeny can be assessed by analyzing microosteological characteristics of the rib thin sections, such as osteocytes, osteons, and bone tissue. A key feature I will be looking to identify are lines of arrested growth (LAGs) because of their ability to record annual bone growth. Thin sections of the medial shaft of an Edmontosaurus rib were prepared at the petrographic facility at University of Iowa and ground approximately 70 μm thick. Microscope analysis of slides revealed two possible LAGs. Parallel, continuous lines are present before they are overlain by secondary remodeling that obscures most of the sections. Open channels in the cancellous bone are assumed to be a preemptive stage before transitioning into primary osteons. This, as well as high osteocyte density, is indicative of immature bone. While not conclusively identified as LAGs, these lines are promising growth markers and act as an interesting piece of data since growth record was assumed to be better preserved in the medial portion of the rib. More data are needed in addition to the growth markers found in Edmontosaurus on if LAGs are identifiable in rib material. With Edmontosaurus and Triceratops both being quadrupedal herbivores inhabiting the North American region during the late cretaceous, these similar lifestyles allow us to compare results in the future. While this individual test was not able to determine age range from lack of LAGs, there is promise that rib histology is a suitable method for establishing ontogenetic age in ornithoscelidan fossils.