The form and function of the sacrum are of great relevance to understand the evolution of locomotion in tetrapods because it
is a key piece of the vertebrate skeleton. The sacrum connects the caudal and presacral regions of the vertebral column and
the hindlimbs through the pelvis. Here, we investigate sacrum shape evolution in pinnipeds (Carnivora: Pinnipedia) in relation
to terrestrial mammalian carnivorans (fissipeds), and we include crown and stem taxa to quantify the morphological changes
they experience in relation to the aquatic environment they inhabit. We use 3D geometric morphometric methods to explore
the morphological variability and disparity of the sacrum in a set of terrestrial and aquatic carnivoran species. Our results
show that the morphology of the sacrum of each pinniped family is remarkably different and that these differences may be
related to the aquatic mode of locomotion (pectoral or pelvic oscillation), the use of hindlimbs to support body weight on
land (otariids in contrast with phocids), and the presence or absence of a functional tail. In addition, disparity-through-time
analyses indicate that the sacrum of pinnipeds is less constrained than that of fissipeds, which suggests a gravitational origin
of such constraints in fissipeds. In conclusion, our results give further support to the important role played by this skeletal
structure in the locomotory adaptations of mammals.
