Investigating the land-to-sea transition in carnivorans from the evolution of sacrum morphology in pinnipeds

Abstract

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.

The influence of the land-to-sea macroevolutionary transition on vertebral column disparification in Pinnipedia

“…Therefore, we hypothesize that A. kelloggi employed a mixed type of locomotion, or that the thrust would not be produced entirely from the lumbar region. Instead, only the first lumbar vertebra was likely involved in exerting propulsive forces and a recent study revealed that the sacrum of Allodesmus was more like that of phocids than to that of otariids [ 16 ].…”

“…We also hypothesize that the axial evolution in phocids is subject to greater constraints compared to otariids, because the phocid vertebral column exhibit a rigid anterior region and a highly mobile posterior region, whereas otariids have evolved to maintain maximum flexibility in the entire vertebral column [72]. All vertebrae of phocids, particularly the thoracics and lumbars, are more disparate than the ones of otariids, which probably relates to the role of the vertebral column in swimming, and probably increases morphological variability among different regions depending upon its role in locomotion, such as providing rigidity (the case of posterior thoracics) or mobility (the case of the lumbars).…”

“…Our data are based on osteological collections of pinnipeds housed in museum collections. The specimen numbers and museum collections are detailed in electronic supplementary material, table S1 [ 73 ].…”

“…[4,8,15]). Therefore, the marine environment releases (in part) the axial skeleton from its role in body support under gravity [4,[16][17][18][19], and it allows oscillatory or undulatory movements [20]. On the other hand, this new physical environment imposed selective regimes leading to a major reorganization of their body plans, including the acquisition of streamlined, torpedo-shaped bodies, the transformation of limbs into flippers, and sometimes the appearance of caudal flukes [21][22][23].…”

Evolutionary ecomorphology for the twenty-first century: examples from mammalian carnivores

“…Tail length itself is associated with substrate, with arboreal mammals tending to maintain longer tails and tail loss generally occurring in terrestrial species [ 124 ]. Recently, the shape of the sacrum has been quantified using GMM in relation to the swimming strategies in extant pinnipeds [ 125 ]. The pinniped sacrum is characterized by a set of traits indicative of the low stresses it withstands because of the reduced time pinnipeds spent on land compared to terrestrial carnivorans.…”