Osteological correlates of evolutionary transitions in cetacean feeding and related oropharyngeal functions

Abstract

Teeth are often the first structures that anatomists and paleontologists examine to understand the ecology and morphology of feeding, both because teeth are highly specialized structures that provide precise information, and because they are among the best and most commonly preserved fossils. Unfortunately, many fragmentary fossil and recent specimens lack teeth, and some come from edentulous individuals and taxa, as in mysticete (baleen) whales. In our broad comparative review, we survey non-dental osteological features that, due to size, shape, arrangement, and surface features reflecting muscle attachments, provide useful clues to general or specific aspects of prey capture, intraoral transport, processing, or swallowing. We focus on hyoid, palatal, and pterygoid bones, mandibular symphyses and processes such as the coronoid, and the temporal fossa and zygomatic arch, as well as adjacent cranial bones relating to oral and pharyngeal anatomy. These bones relate to muscles of five general locations especially indicative of feeding: mandibular, hyoid, tongue, pharyngeal, and facial regions. Together these bones and muscles affect feeding and related activities including suckling and breathing. We discuss osteological correlates that provide special relevance to key transitions in cetacean evolutionary history, such as the shift from predominantly terrestrial to aquatic feeding, the shift from typical mammalian mastication to swallowing prey entirely or nearly whole, and the separation of respiratory and digestive passages. We also point to examples of modern specialists in these anatomical optima for different modes of prey capture, intraoral transport, processing, and swallowing. Although we focus on cetaceans, our approach is broadly relevant to all vertebrates, notably other marine tetrapods.

New Skeletons of the Ancient Dolphin Xenorophus sloanii and Xenorophus simplicidens sp. nov. (Mammalia, Cetacea) from the Oligocene of South Carolina and the Ontogeny, Functional Anatomy, Asymmetry, Pathology, and Evolution of the Earliest Odontoceti

“…Consistent with previous studies (e.g., [3,4]), there is good support for the unnamed taxon ChM PV4746 being the most basal xenorophid, with the clade excluding it supported by a bootstrap value of 92% and six unambiguous synapomorphies, including lacrimal expanded dorsally over the supraorbital process of the frontal (76:2), bilateral antorbital basins present (92:1), posterior end of maxilla aligned with gap between frontal and zygomatic process (111:3), supraorbital process roofs over temporal fossa (173:0), ventral window in supraorbital process that exposes overlying maxilla and premaxilla (174: 1), and posterior end of premaxilla abruptly widens (390:3). Sanders and Geisler [5] listed three of these synapomorphies (i.e., 76,92,174) in their diagnosis for Xenorophidae, but it is important to realize that some of the features do not occur in ChM PV4746, and are actually diagnostic of this more exclusive clade. As in previous studies, Xenorophidae (sensu [5]) is well supported (Figure 71), with a bootstrap of 92% and nine supporting synapomorphies, including ventral exposure of lacrimal + jugal is intermediate in size (80:1), posterior border of supraorbital process extends posteromedially from lateral end (85:0), side of postorbital process faces laterally Boessenecker et al [2] placed the then newly described Inermorostrum xenops as the sister-group to Cotylocara + Echovenator, but here we find it to be the second most basal xenorophid (Figure 71).…”

“…In Crown Mysticeti, the temporal fossa becomes transversely wide and anteroposteriorly foreshortened as well. Reduction in size of the temporal fossa, and hence volume and length of the temporalis, is correlated with decreasing bite force and mastication in Cetacea [174].…”

“…Size does not explain temporal fossa length within Crown Mysticeti, as there seems to be a ceiling (400 mm long temporal fossa) and floor (150 mm long temporal fossa) regardless of body size, with the envelope of these taxa occupying a horizontal zone on the plot. bite force and mastication in Cetacea [174].…”

New heterodont odontocetes from the Oligocene Pysht Formation in Washington State, U.S.A., and a reevaluation of Simocetidae (Cetacea, Odontoceti)

“…However, O. thalassodon , has closely spaced, larger teeth, as well as a relatively gracile, unfused hyoid apparatus ( Figs. 11 – 13A – 13C ; Johnston & Berta, 2011 ; Viglino et al, 2021 ; Werth & Beatty, 2023 ), which suggest that this taxon was instead a raptorial or combined feeder ( Fig. 22 ).…”