Microestructura ósea de Priosphenodon avelasi (Rhynchocephalia: Sphenodontia): implicancias paleobiológicas

Abstract

The rhynchocephalians are one of the groups of vertebrates with fossil representatives which shows a vast taxonomic diversity and worldwide distribution for the Mesozoic era. They have been the subject of studies on their phylogeny and morphology, but those focused on their paleobiology are scarce. In this final degree project, the bone microstructure was studied for eleven elements of the appendicular skeleton belonging to a specimen of Priosphenodon avelasi (MPCA-Pv 308), a Cenomanian-Turonian rhynchocephalian from Río Negro (Argentina) that represents a case of gigantism among Rhynchocephalia. Several aspects of its paleobiology were inquired, such as the sexual and somatic maturation, its relative growth rate, and the minimum age that the individual reached before dying. For this, thin transverse sections were made at the diaphysis level and were analyzed under a binocular optical and petrographic polarized light microscope, comparing the results with studies on other fossil forms, such as Gephyrosaurus and Palaeopleurosaurus, as well as its current relative, Sphenodon. The analyzed samples showed a predominance of pseudolamellar bone in the cortices, a conservative trait among the Rhynchocephalia and an indicator of a relatively low growth rate. However, this rate is not constant, alternating between slow and fast periods, as evidenced by the presence of strongly spaced growth marks which could be linked to the gigantism exhibited in Priosphenodon avelasi. In this respect, P. avelasi resembles more fossil forms than Sphenodon. The elements with a smaller medullary cavity and a higher compaction index were the humerus and the radius, which preserved a higher number of growth marks, evidencing a minimum age of ten years for the specimen MPCA-Pv 308 and proving that they are good bone elements for skeletochronological studies. In the same sense, it was possible to establish that the specimen MPCA-Pv 308 corresponds to a subadult individual, which died after reaching its sexual maturity, but before the attainment of somatic maturity.