Abstract
Terrestrial vertebrates from at least 30 distinct lineages in both extinct and extant clades have returned to aquatic environments. With these transitions came numerous morphological adaptations to accommodate life in water. Relatively little attention has been paid to the cervical region when tracking this transition. In fully aquatic cetaceans, the cervical vertebrae are compressed, largely because a loss of neck mobility reduces drag. We ask whether this pattern of cervical evolution is present in the more recently evolved semiaquatic pinnipeds. Here, we compare neck morphology and function in three families of pinnipeds, the Otariidae, Phocidae, and Odobenidae as well as between pinnipeds and their terrestrial arctoid relatives (ursids and mustelids). Using cranial CT scans, we quantified the occipital surface area for neck muscle attachment as well as vertebral size and shape using linear measurements. Results show that pinnipeds have a relatively larger occipital surface area than ursids and terrestrial mustelids, suggesting that marine carnivorans have enlarged their neck muscles to assist with head stabilization during swimming. Within pinnipeds, we found quantitative differences in cervical morphology between otariids and phocids that coincide with their locomotor style. Phocids are hindlimb-dominated swimmers that propel themselves with pelvic oscillations. Their necks are relatively stiff and their cervical vertebrae are compressed anteroposteriorly with reduced muscular attachment areas. By contrast, otariids are forelimb-dominated swimmers that locomote in water and on land using their pectoral limbs, often recruiting their neck to initiate turns underwater as well as assisting in "walking" on land. Consequently, otariids have stronger, more flexible necks than phocids, which is reflected in more elongate cervical vertebral centra with larger muscle attachments. The walrus (Odobenidae) has a cervical vertebrae morphology intermediate to that of phocids and otariids, consistent with a phocid swimming mode combined with a more muscular neck that likely functions in intraspecific conflict and haul-out behavior.