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Mollesaurus, ichthyosaur with heavy rib cage

From: Ben Creisler

A new advance online paper in Naturwissenschaften:

Marianella Talevi and Marta S. Fernández (2012)
Unexpected skeletal histology of an ichthyosaur from the Middle
Jurassic of Patagonia: implications for evolution of bone
microstructure among secondary aquatic tetrapods.
Naturwissenschaften (advance online publication)
DOI: 10.1007/s00114-012-0886-4

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 the terrestrial realms and
from 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 a 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 a 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 show 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.