Fossil Record 22(2): 91-110, doi: 10.5194/fr-22-91-2019
Microanatomy and growth of the mesosaurs Stereosternum tumidum and Brazilosaurus sanpauloensis (Reptilia, Parareptilia)
expand article infoNicole Klein, Antoine Verrière§, Heitor Sartorelli, Tanja Wintrich, Jörg Fröbisch|
‡ Abteilung Paläontologie, Institut für Geowissenschaften, Universität Bonn, Nussallee 8, 53115 Bonn, Germany§ Museum für Naturkunde – Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, 10115 Berlin, Germany| Evolutionary Studies Institute & School of Geosciences, University of the Witwatersrand, Private Bag 3, Johannesburg 2050, South Africa
Open Access

Histology and microanatomy of vertebrae, ribs, haemalarch, and humeri and femora of 10 individuals of Stereosternum and two dorsal ribs of1 individual of Brazilosaurus were studied. All individuals had achieved a body lengthof 50 cm (equal to 65 % of the maximum known body length) or larger. All sampledbones are highly osteosclerotic due to the reduction of medullary cavitiesand the filling of medullary regions by endosteal bone. Calcified cartilageoccurs – if at all – only locally in small clusters in the medullaryregions of midshaft and in higher amounts only in non-midshaft sections oflong bones and towards the medio-distal rib shaft, respectively. The primarybone tissue consists of highly organized parallel-fibred tissue and/orlamellar tissue, which is in most samples relatively lightly vascularized oreven avascular. If present, vascular canals are mainly longitudinallyoriented; some show a radial orientation. Simple vascular canals as well asprimary osteons occur. Some of the latter are secondarily altered, i.e. widened. Remodelling of the periosteal cortex is only documented by fewscattered erosion cavities and secondary osteons. The tissue is regularlystratified by lines of arrested growth (LAGs), which usually appear as double or multiple rest lines,indicating strong dependence on exogenous and endogenous factors. Because ofthe inhibition of periosteal remodelling the growth record is complete andno inner cycles are lost. Individuals of Stereosternum show a poor correlation of bodysize and number of growth marks, which might be the result of developmentalplasticity. Brazilosaurus shows a highly organized, avascular lamellar tissue and a highnumber of regularly deposited rest lines throughout the cortex of the ribs.The medullary region in the ribs of Brazilosaurus is distinctly larger when compared to ribsof Stereosternum. However, strong osteosclerosis is obvious in both taxa, pointing to ahigh degree of aquatic adaption. Ribs of Stereosternum, Brazilosaurus, and Mesosaurus are clearly distinguishablefrom each other by the distribution of the periosteal and endostealterritory. Furthermore, Brazilosaurus differs in its growth pattern (i.e. spacing of restlines) when compared to Stereosternum and Mesosaurus.