Morphofunctional analysis of pterygoid sinuses, cervical vertebrae and forelimb bones of the fossil beaked whales Messapicetus gregarius

Ziphiidae are a successful family of medium to large-sized toothed whales. Their modern representatives regularly perform deep dives beyond the photic zone to forage mainly on cephalopods. Conversely, at least part of the long-snouted stem ziphiids are interpreted as epipelagic predators, and hypothesis based on indirect evidence and the association of numerous epipelagic fish with a fossil specimen of Messapicetus gregarius from the Late Miocene of Peru. However, some aspects of this hypothesis remain unclear, due to the lack of clear morphological proxies to recognize regular deep divers. In the present work, we investigated the variation in the morphology and size of the hamular fossa of the pterygoid sinus (HF), cervical vertebrae, and forelimb bones of extant odontocetes as potential proxies to assess deep diving abilities in the fossil species M. gregarius. We reconstructed the musculature of the neck and forelimb of M. gregarius and compared it with other odontocetes. We also quantified variation in the proportions of the forelimb and the HF using 16 linear measurements. A Q-mode for phylogenetic regression was used to evaluate the degree of association between deep diving abilities in extant odontocetes and the linear measurements, while accounting for the phylogeny. Our reconstruction of the neck musculature of M. gregarius suggests that this species possessed a neck more flexible than most extant ziphiids, due to the lower degree of fusion of the cervical vertebrae and the large insertions for the M. longus colli and intertransversari ventrales cervicis. While neck rigidity might be related to deep diving, differences in the degree of neck flexibility among extant ziphiids indicates a more complex functional interpretation. Although we did not find a relationship between forelimb morphology and deep diving abilities in extant odontocetes, the proportions of the forelimb of M. gregarius are closer to modern ziphiids than to other odontocetes. Proportions of the HF as quantified in our analysis also revealed unsuccessful to evaluate deep diving abilities in odontocetes, but we suspect linear measurements to be insufficient to precisely capture interspecific variation in the volume of the hamular fossa. The lack of clear morphological proxies for deep diving abilities indicates that morphology alone should be used with caution to interpret diving abilities in extinct odontocetes.