Journal of Vertebrate Paleontology 32(5):1042–1048, September 2012 © 2012 by the Society of Vertebrate Paleontology ARTICLE WHAT VERTEBRAL MICROANATOMY REVEALS ABOUT THE ECOLOGY OF JUVENILE MOSASAURS (REPTILIA, SQUAMATA) ALEXANDRA HOUSSAYE *,1 and PAUL TAFFOREAU 2 1 UMR 7207 du CNRS, D ´ epartement Histoire de la Terre, Mus ´ eum National d’Histoire Naturelle, 57 rue Cuvier CP 38, 75005 Paris, France, houssaye@mnhn.fr; 2 European Synchrotron Radiation Facility, BP220, 6 rue Jules Horowitz, 38043 Grenoble Cedex, France, paul.tafforeau@esrf.fr 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 ple- siopelvic 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. INTRODUCTION Mosasauroids are a group of medium-sized (less than 2 m) to giant (up to 15 m) marine squamates that ranged in time from the Cenomanian to the end of the Maastrichtian (i.e., from 99 to 65 Ma). Including the paraphyletic ex-“Aigialosauridae” and the polyphyletic ex-“Mosasauridae” (see Bell and Polcyn, 2005), this group displays distinct morphologies illustrating various ecological grades from shallow-marine dwelling, relatively poor swimmers towards more open-marine living, active swimmers. Whereas plesiopedal and plesiopelvic mosasauroids (sensu Bell and Polcyn, 2005, and Caldwell and Palci, 2007; cf. Fig. 1A) dis- play limb and pelvic anatomy similar to that of terrestrial monitor lizards, hydropedal and hydropelvic forms (cf. Fig. 1D) are char- acterized by paddle-like limbs and a pelvis modified so that there is no sacrum (Bell and Polcyn, 2005; Caldwell and Palci, 2007). Juvenile mosasauroid remains are considered rather scarce in the fossil record, which led earlier workers to suggest that mosasauroids laid their eggs in shallow, freshwater environments protected from predators (Williston, 1914; Russell, 1967). How- ever, numerous specimens, consisting mainly of isolated verte- brae but also, to a lesser extent, of teeth, skull, and appendicular elements, have been discovered in the Campanian Mooreville Chalk Formation of Alabama (Bell and Sheldon, 1986; Sheldon, 1987; A.H., pers. observ.). This formation corresponds to de- posits laid down in shallow-marine environments in the Western Interior Seaway of North America (Kiernan, 2002). Moreover, immature specimens (represented by both skull and postcranial elements) have also been discovered in the Smoky Hill Chalk Member of the Niobrara Formation (late Coniacian–earliest Campanian) in western Kansas (Sheldon, 1990; Everhart, 2002, 2007), which is considered to have been deposited hundreds * Corresponding author. of kilometers from the nearest shore. The discoveries of these juvenile specimens in these marine sediments (sometimes far from shore) and their occurrence in the same beds as adults challenge previous hypotheses stating that mosasauroids gave birth in sheltered nurseries (Williston, 1904; Russell, 1967; Lindgren and Siverson, 2004), and is in accordance with recent studies that suggest that birth might have occurred in adult life environments (Everhart, 2002, 2007). It is also in accordance with discoveries suggesting that mosasauroids were livebearers (Bell, 1996; Caldwell and Lee, 2001; Bell and Sheldon, 2004). Although adult and juvenile mosasauroids might have lived in similar environmental conditions, it is possible that their ecol- ogy might have been distinct. One way to assess the ecology of these individuals is to examine bone microanatomy. Analysis of vertebral microanatomy has revealed two main microanatom- ical trends within mosasauroids. Pachyosteosclerosis (increase in bone volume and compactness; cf. Houssaye, 2009) is ob- served in plesiopelvic forms with relatively poor swimming abil- ities that rely on hydrostatic regulation of buoyancy and body trim (Houssaye, 2008; A.H., pers. observ.; cf. Fig. 1B–C). Con- versely, no change in compactness is observed in the more efficiently swimming hydropelvic forms (cf. Houssaye, 2008; Houssaye and Bardet, 2012), whose vertebrae are characterized by the development of a true trabecular organization adapted to confer lightness and resistance (Houssaye, 2008; Fig. 1E–F). The shift in microanatomical organization in this lineage accompa- nies a transition from a shallow- to a more open-marine life habit (Robaszynski and Christensen, 1989; Bardet et al., 2003; Caldwell and Palci, 2007). Following from this statement, it appears that if a shift in ecology occurs during ontogeny in hydropelvic forms, it should be recorded in bone microanatomy. In plesiosaurs, which are large (from 3 to 15 m long) marine reptiles that lived from the end of the Triassic to the end of the Cretaceous, Wiffen et al. (1995) noted a change in long bone and vertebral microanatom- ical features during ontogeny, from pachyosteosclerosis in 1042