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.