Applications in Paleohistology: Osteohistology of Protostega gigas

Abstract

Protostega gigas was a large sea turtle that lived in the Late Cretaceous Western Interior Seaway of North America. Protostega fossils are found in the Smoky Hill Member of the Niobrara Formation in Kansas. Despite their abundance, little is known about how these turtles grew and aged. The purpose of this study is to expand the knowledge of sea turtles osteohistology and determine the ontogenetic stage of the specimen at the time of death. Osteohistology is the study of bone microstructure, and is used to study bone growth rates. Microstructure and growth rates directly relate to phylogeny (evolutionary relationships), ontogeny (growth stage), biomechanics (how a bone is used), and environment. The femur of the Protostega gigas specimen analyzed is spongiose, with large networks of vascular canals and little or no open medullary cavity. Vascular canal orientation is dominated by longitudinal and circumferential canals organized in concentric layers. Vascular canals increase in circumferential organization towards the outer cortex. Overall, the Protostega bone analyzed in this study has a similar microstructure to that that described in the leatherback sea turtle. Because there are vascular canals open to the periosteal surface of the bone, with no indications of reduced vascularity and cessation in bone growth, this specimen is interpreted to have been a juvenile at the time of death. ; Histologic results contradict previous research interpreting this specimen as an adult.