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Houssaye, A. & Bardet, N. 2011: Rib and vertebral micro-anatomical characteristics of hydropelvic mosasauroids. Lethaia, Vol. 45, pp. 200–209.

Mosasauroids are squamates secondarily adapted to an aquatic life that dominated the sea during the Late Cretaceous. Mosasauroids display distinct types of morphologies illustrating steps in the adaptation of this lineage to increasing obligatory habits. Hydropelvic mosasauroids (sensu Caldwell & Palci) were the most highly adapted to an open-sea environment. Contrary to plesiopelvic forms, they are considered to have relied on a hydrodynamic, rather than hydrostatic buoyancy and body trim control strategies. This led previous authors to consider that these taxa would favour bone lightening (osteoporotic-like condition) rather than bone mass increase. Although an osteoporotic-like state was indeed described in Clidastes and Tylosaurus, bone mass increase was reported in Platecarpus. As a matter of fact, the new analysis of vertebral thin sections of various taxa combined with the reanalysis of the rib sections available in Sheldon’s PhD thesis in a micro-anatomical perspective reveals the absence of both bone mass increase and bone lightening in these organisms. These taxa in fact display a vertebral micro-anatomy much peculiar within squamates. It characteristically corresponds to a true network of thin trabeculae whose tightness varies between taxa, probably as a result of both species and individual size differences, particularly the latter. In addition, analysis of the pattern of vascularization as observed in hydropelvic mosasauroids, which is unique amongst squamates, suggests that large size in hydropelvic mosasauroids would mainly rely on protracted rather than faster growth rates. □Histology, hydropelvic mosasauroids, micro-anatomy, ribs, vascularization, vertebrae.