Skeletal Transformations and the Origin of Baleen Whales (Mammalia, Cetacea, Mysticeti): A Study on Evolutionary Patterns

Abstract

A review of the morphological patterns exhibited by all the main radiations of mysticete (baleen whale) cetaceans provided a broad assessment of the fundamental morphological transformations that occurred in the transition to the Mysticeti clade. Skull and postcranial characters were illustrated, described and compared, and their distribution was mapped on a combined phylogeny in the search for morphological support for the principal mysticete clades (i.e., Mysticeti, Chaeomysticeti and Balaenomorpha). In particular, characters of the skull (rostrum, vertex, temporal fossa, tympanic bulla and dentary) and the postcranial appendicular skeleton (scapula, humerus, radius and ulna) were all involved at different degrees in the process of morphological transformations leading to the modern-day mysticetes. Apart from a few typical characteristics of the rostrum that were already present in the earliest-diverging mysticetes (presence of lateral process of the maxilla, presence of multiple dorsal infraorbital foramina, thin lateral border of maxilla and presence of mesorostral groove), most of the other anatomical districts were unaffected by the transition so the earliest mysticetes show a number of archaeocete characters in the tympanic bulla, dentary and skull roof. The analysis of the whole dataset supported the hypothesis that the origin and evolution of mysticetes constituted a step-wise process and that the bauplan of the modern-day mysticetes was fully assembled at the level of the common ancestor of all Balaenomorpha.

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

“…Here we review and discuss several osteological correlates of feeding in Cetacea, of interest and utility to paleontologists and anatomists alike, that provide special relevance to important transitions in cetacean evolutionary history (Werth, 2000;Bisconti and Carnevale, 2022). These include the shift from predominantly terrestrial to aquatic prey capture and ingestion, the shift from typical mammalian mastication to swallowing prey entirely or almost entirely whole, and the complete separation of respiratory and digestive passages, such that the cetacean pharynx no longer has a free soft palate dividing naso-and oropharyngeal sections.…”

“…Jaw snapping and clapping probably play roles in displaying threat behavior, but possibly also play roles in debilitating prey (Norris and Møhl, 1983). The mysticete mandible is typically a thick, curved cylinder that lacks not only dentition but also enlarged processes and other prominent bony landmarks found in typical terrestrial mammals (Field et al, 2010;Bisconti and Carnevale, 2022). Baleen whale mandibles are specialized for rotational movements associated with bulk filtration and the subsequent need for expanded gape (Lambertsen, 1983;Lambertsen et al, 1995).…”

Convergent evolution of skim feeding in baleen whales

“…Reconciling these divergent interpretations has proved challenging, with nontraditional anatomical character interpretations that are consistent with the molecular data (e.g., Fordyce & Marx, 2013; Gol'din & Steeman, 2015; Marx & Fordyce, 2016) reputedly “gloss[ing] over the substantial issue of convergence posed by the many balaenid features of Caperea ” (Berta & Deméré, 2018, p. 73). As a result, many morphological analyses continue to recover—or regard— Caperea and right whales as sister taxa (Bisconti, 2015; Bisconti & Carnevale, 2022; Bisconti et al, 2017, 2021; Boessenecker & Fordyce, 2017; El Adli et al, 2014; Peredo et al, 2018), which in turn has shaped debates around baleen whale phylogeny, the placement of fossils, and the prevalence and role of convergent feeding evolution.…”

“…features of Caperea" (Berta & Deméré, 2018, p. 73). As a result, many morphological analyses continue to recoveror regard-Caperea and right whales as sister taxa (Bisconti, 2015;Bisconti & Carnevale, 2022;Bisconti et al, 2017Bisconti et al, , 2021Boessenecker & Fordyce, 2017;El Adli et al, 2014;Peredo et al, 2018), which in turn has shaped debates around baleen whale phylogeny, the placement of fossils, and the prevalence and role of convergent feeding evolution.…”