The range of atlanto‐occipital joint motion in cetaceans reflects their feeding behavior

Abstract

The cetaceans display a wide variety of lifestyles, especially with regard to their feeding behavior. However, the evolutionary process of the feeding behavior in cetaceans is still poorly understood, in part because reconstructing the feeding behavior of extinct taxa remains difficult. In cetaceans, cranium mobility relative to the trunk largely depends on the range of motion permitted by the atlanto‐occipital joint, given the lack of flexibility of the cervical series. In this study, we examined 56 extant cetacean skeletal specimens from 30 species in 25 genera and nine families in order to investigate the relationships between anatomical traits and feeding behavior. Our results suggest that the range of dorso‐ventral motion allowed by the atlanto‐occipital joint (ROM) depends on prey habitat and the feeding technique of cetaceans. Cetaceans feeding on benthic/demersal prey had a relatively large ROM compared with those feeding on pelagic prey. In addition, ROM was largest in raptorial feeders, intermediate in suction feeders, and smallest in ram‐filter feeders. Among raptorial feeders, ROM tended to be larger in taxa that facultatively tear off the prey's flesh compared with taxa that swallow their prey whole. Therefore, we conclude that ROM is a powerful tool to reliably reconstruct the feeding behavior of extinct cetacean taxa.

Convergent Evolution of Swimming Adaptations in Modern Whales Revealed by a Large Macrophagous Dolphin from the Oligocene of South Carolina

“…Fossils of Ankylorhiza have not yet revealed evidence of sexual dimorphism, and perhaps procumbent incisor tusks were used during prey ''ramming'' (e.g., [29]) rather than combat as in ziphiids. Neck mobility measurements of Ankylorhiza correspond directly to the morphospace occupied by modern killer whales and false killer whales (Supplemental Information; [16]). Thus, multiple lines of evidence indicate that Ankylorhiza was the first large macrophagous odontocete, reoccupying a niche vacated by basilosaurids 4-5 My earlier at the close of the Eocene.…”

“…The temporal fossae are the largest among Odontoceti (greatest length = 24% of condylobasal length; greatest width = 36% of bizygomatic width). Vertical neck movement ranges from +4.4 to À21.2 [16]. At 4.8 m, Ankylorhiza tiedemani is the largest of a diverse assemblage of Oligocene odontocetes from the Charleston Embayment.…”

“…Feeding ecology was investigated by examining dental and rostral morphology [15,24,28], evaluating patterns of tooth wear and breakage [26], and measuring vertical range of motion at the craniovertebral junction [16]. Body length was estimated by combining skull length, vertebral length, estimated measurements of missing vertebra, and reconstructing the length of intervertebral disks (Methods S1: Feeding Morphology and Ecology).…”

A new dolphin with tusk-like teeth from the late Oligocene of New Zealand indicates evolution of novel feeding strategies

“…The unfused cervical vertebrae observed in N. matakoi and other basal odontocetes would have allowed a broader range of neck movements, including lateral, dorsoventral [63] and rotational movements [64–66]. Unfused cervical vertebrae and increased range in neck motion is also observed in extant odontocetes such as Delphinapterus leucas (belugas), Monodon monoceros (narwhals) and in river dolphins [63,64,66]. Additionally, the two main muscles used for lateral head movements are the m. sternocephalicus and m. brachiocephalicus [19].…”