Unexpected skeletal histology of an ichthyosaur from the Middle Jurassic of Patagonia: implications for evolution of bone microstructure among secondary aquatic tetrapods

M. TALEVI Y M. S. FERNANDEZ

NATURWISSENSCHAFTEN

SPRINGER

Año: 2012 vol. 99 p. 3 - 6

0028-1042

During the Mesozoic, one of the most significant evolutionary processes was the secondary adaptation of tetrapods to life in water. Several non-related lineages invaded from terrestrial realms the oceans of the entire world. Among these lineages, ichthyosaurs were particularly successful. Advance parvipelvian ichthyosaurs were the first tetrapods to evolve a fish-shaped body profile. The deep skeletal modifications of their bodies, as well as their biology, depict advance ichthyosaurs as the paradigm of secondary adaptation of reptiles to marine life. Functional inferences point to them as off-shore cruising forms, similar to living tuna, and some of them were capable of deep diving. Bone histology of some genera such as Temnodontosaurus, Stenopterygius, Ichthyosaurus and Caypullisaurus, characterized by overall cancellous bone, is consistent with the idea of fish-shaped ichthyosaurs as fast and far cruisers. Here we provide histological examination of the ribs of the Middle Jurassic parvipelvian Mollesaurus. Contrasting with the bone histology of other parvipelvian, Mollesaurus ribs are characterized by a compact and thick cortex. Our data indicate that the rib cage was heavy and suggest that not all advanced ichthyosaurs were fast cruisers. The compact and dense ribs in these parvipelvian, shows that advance ichthyosaurs were ecologically more diverse than previously thought, and that the lightening of the skeleton reversed, as also occurred in the evolution of cetacean, at least once along the evolutionary history of ichthyosaurs.