Paleohistology of a new non-aristonectine elasmosaurid (Sauropterygia, Plesiosauria) from the Calafate formation (Upper Cretaceous), Santa Cruz province, Argentina

GARCIA MARSA, J.A.; D´ANGELO, J.; CERRONI, M.A.; NOVAS, F.E.

Congreso; Jornadas Argentinas de Paleontología de Vertebrados XXXII; 2018

The present contribution provides an attempt to describe the microstructure and paleohistology of a large elasmosauridspecimen (MPM-PV 10004) from the Maastrichtian beds of Santa Cruz province. The thin sections examined correspondto an indeterminate diapophysis, a rib, two gastralia (lateral and medial), femur and a phalanx. The primary bone is mainlycomposed of a fibro-lamellar tissue. The vascularization is high, with longitudinal and reticular canals configurated as primaryand secondary osteons. These features in the examined material are indicative of a rapid growth and a high metabolism.There are no sign of growth zones or annuli/LAGS in the cortical region. It is remarkably the presence of numerous Sharpey?sfibers in the rib section and the medial gastralia; as well as the presence of an External Fundamental System (EFS) in thosethin sections corresponding to both gastralia; which would indicate that the animal reached the somatic maturity. Interestingly,the gastralia displays osteoporotic bone ?sandwiched? between two layers of thick pachyostotic cortical tissue,especially in the medial gastralia. It is thought that pachyostosis, particularly of the ribs and other thoracic elements actsas hydrostatic ballast which increases stability. Therefore, the MPM-PV 10004 specimen is interpreted as an animal oflow swimming speeds and lack of agility restricted to living in shallow water environments, in where an increased stabilitywas advantageous.