THORACIC RIB HISTOLOGY AND THE IMPLICATIONS FOR DOLICHORHYNCHOPS OSBORNI (SAUROPTERYGIA; PLESIOSAURIA) ONTOGENY

Dolichorhynchops osborni is a polycotylid plesiosaur from the Late Cretaceous Western Interior Seaway. FHSM VP-404 and MCZ 1064 are two nearly complete specimens of D. osborni from the Smoky Hill Chalk of the Niobrara Formation of Logan Co., KS. In previous studies the larger FHSM VP-404 was interpreted as a sub-adult or adult, while the smaller MCZ 1064 has been referred to as a juvenile of D. osborni. The purpose of this study is to test if differences in ontogeny can be assessed in two different sized individuals of D. osborni using the microstructure of thoracic ribs. Additional objectives are to test if the microstructure of the ribs changes based on which rib is used or where the rib is sectioned. An anterior, medial, and posterior rib from FHSM VP-404 were histologically sectioned and one posterior rib from MCZ 1064 was sectioned. All ribs were sectioned on the proximal shaft and the medial rib of FHSM VP-404 was sectioned an additional three times: at the head, the diaphysis, and the distal shaft.

In all sections, the medullary cavity is small with trabeculae composed of secondary lamellar bone. The cortex is compact and most primary bone has been extensively remodeled by secondary osteons. A modified Kerley method was used to quantify the difference in secondary osteon density between the two specimens and results shows that FHSM VP-404 has a significantly higher secondary osteon density. Previous studies show a correlation between increased secondary osteon density and age in other animals. This suggests that FHSM VP-404 is an older individual than MCZ 1064. Additionally, the distal most section of the medial rib from FHSM VP-404 retains the most primary bone of any section and preserves a 90 mm thick layer of avascular lamellar bone at the periosteal surface. This feature is interpreted as an external fundamental system (EFS), which signals skeletal maturity and is the first occurrence seen in the rib of a plesiosaur. The presence of an EFS in FHSM VP-404 and a significantly higher density of secondary osteons confirms that it is a skeletally mature example of D. osborni and that MCZ 1064 was not as old as FHSM VP-404.