UCLA

Terrestrial vertebrates from at least 30 distinct lineages in both extinct and extant clades, including archosaurs, lepidosaurs, and mammals, have returned to aquatic environments. With these transitions came numerous morphological and physiological adaptations to accommodate life in water. The axial and appendicular skeleton are of particular interest in this transition due to their role in locomotion. Although several studies have focused on the limbs and thoracolumbar spine, less attention has been paid to the cervical region. In fully aquatic cetaceans, the cervical vertebrae are compressed or fused, largely because a loss of neck mobility reduces drag. We ask if this pattern of cervical evolution is present in pinnipeds that have more recently invaded a marine habitat but retain some terrestrial habits. Here, we quantitatively compare neck morphology and function in two groups of pinnipeds with different degrees of aquatic adaptation, the Otariidae and Phocidae, as well as between pinnipeds and their terrestrial arctoid relatives (ursids and mustelids). Using cranial CT scans of museum specimens, we quantified the occipital surface area for neck muscle attachment and also took linear measurements of the cervical vertebrae to capture vertebral size and shape. Results show that the pinnipeds have a relatively larger occipital surface area than ursids and terrestrial mustelids. This suggests that marine carnivorans have enlarged their neck muscles to assist with stabilizing the head during swimming. Within pinnipeds, there are functional differences in cervical morphology between otariids and phocids that coincide with their degree of aquatic adaptation. Otariids are more specialized for terrestrial locomotion than phocids and have relatively longer cervical vertebrae centra that allow for greater neck flexibility. By contrast, phocids are more specialized for aquatic locomotion and consequently have shorter cervical vertebrae and less flexible necks. Findings also suggest there is a direct relationship between neck flexibility and the habitat complexity of where these animals forage. The quantitative measures used in our analysis are applicable to fossil vertebrate taxa, such as Enaliarctos or Allodesmus, and enable the tracking of progressive adaptations to life in water during the transition from land to sea.