Microanatomy and Osteohistology of a Juvenile Elasmosaurid Plesiosaur from the Upper Maastrichtian of Marambio (=Seymour) Island, Antartica

Abstract

We describe here the microanatomy and histology of several bones from a juvenile elasmosaurid from upper Maastrichtian beds of Marambio (=Seymour) Island, Antarctica. Transversal thin sections were obtained from the diaphyseal region of a propodial bone and from a scapula fragment. Microanatomical analyses were conducted using Bone Profiler© software to measure bone compactness in two propodials, a vertebra and scapula and pubis fragments. Our results show the following: (1) at the microanatomical level, we found bone matrix filling almost completely the medullar region of the analyzed bones with compactness index values between 0.858 - 0.993 (values from 0 to 1) suggesting a dense osteosclerotic-like skeleton. (2) At the histological level, we didn´t found any sign of growth zones or annuli/LAGS in the cortex region. Instead of that, we found a thin cortex composed by vascularized primary woven-fibered bone tissue with anastomosed radial vascular canals and a medullar region composed by few bony trabeculae filled with calcified cartilage with some resorption cavities and Howship´s lacunae (=evidence of chondroclastic activity). These osteo-histological features confirm the presence of osteosclerosis (probably having a "manatee-like" mode of life of a slow swimmer) and firmly suggesting that this specimen was a very juvenile at the moment of death.