What vertebral microanatomy reveals about the ecology of juvenile mosasaurs (Reptilia, Squamata)

ABSTRACT

In mosasauroids, one of the major groups of Mesozoic marine reptiles, various ecological grades have been identified. However, no data are available concerning the ecology of the poorly known juveniles. Osseous microanatomical organization appears to rely mainly on the biomechanical constraints undergone by organisms. As such, it is considered as a valuable paleoecological marker. The vertebral microanatomy of several juvenile specimens of hydropelvic mosasauroids (whose adult forms are active pelagic swimmers) was analyzed. This study provides data about their growth mode and speed but, above all, reveals that juvenile hydropelvic mosasauroids display microanatomical features comparable to those of adults. This signifies that hydropelvic mosasauroids were already highly efficient swimmers at a very young age, contrary to plesiopelvic forms (i.e., those with a terrestrial-like pelvis and limbs), that were relying on hydrostatic (and not hydrodynamic) regulation of buoyancy and body trim. This study tends to support recent views challenging the old hypothesis of sheltered nurseries and shows that, in mosasauroids, the functional requirements for buoyancy and body trim control are not correlated with individual size.

ACKNOWLEDGMENTS

A.H. is particularly grateful to J. Ebersole (MacWane Science Center, Birmingham, Alabama, U.S.A.) for all his very precious help in organizing her visit to the collections of Alabama. A.H. also warmly thanks The Alabama Paleontological Society for their financial support of this visit and P. and M. Atkinson, S. Ebersole, D. C. Kopaska-Merkel, and J. Lamb for their help, hospitality, and kindness. We are also thankful to the European Synchrotron Radiation Facility (Grenoble, France) for providing beamtime and support, and to M. Geze and J. Jovet for providing and accommodating the use of Avizo software at CEMIM (Muséum National d’Histoire Naturelle, Paris, France). We also particularly thank N. Bardet (MNHN), J. Lindgren (Lund University, Lund, Sweden), and M. P. Sander (Bonn Universität, Bonn, Germany) for useful comments that improved the manuscript, and J. Müller and P. Barrett for editorial work.

Handling editor: Johannes Müller