The Comparative Osteology of the Petrotympanic Complex (Ear Region) of Extant Baleen Whales (Cetacea: Mysticeti)

Anatomical comparisons of the ear region of baleen whales (Mysticeti) are provided through detailed osteological descriptions and high-resolution photographs of the petrotympanic complex (tympanic bulla and petrosal bone) of all extant species of mysticete cetaceans. Salient morphological features are illustrated and identified, including overall shape of the bulla, size of the conical process of the bulla, morphology of the promontorium, and the size and shape of the anterior process of the petrosal. We place our comparative osteological observations into a phylogenetic context in order to initiate an exploration into petrotympanic evolution within Mysticeti.The morphology of the petrotympanic complex is diagnostic for individual species of baleen whale (e.g., sigmoid and conical processes positioned at midline of bulla in Balaenoptera musculus; confluence of fenestra cochleae and perilymphatic foramen in Eschrichtius robustus), and several mysticete clades are united by derived characteristics. Balaenids and neobalaenids share derived features of the bulla, such as a rhomboid shape and a reduced anterior lobe (swelling) in ventral aspect, and eschrichtiids share derived morphologies of the petrosal with balaenopterids, including loss of a medial promontory groove and dorsomedial elongation of the promontorium. Monophyly of Balaenoidea (Balaenidae and Neobalaenidae) and Balaenopteroidea (Balaenopteridae and Eschrichtiidae) was recovered in phylogenetic analyses utilizing data exclusively from the petrotympanic complex.This study fills a major gap in our knowledge of the complex structures of the mysticete petrotympanic complex, which is an important anatomical region for the interpretation of the evolutionary history of mammals. In addition, we introduce a novel body of phylogenetically informative characters from the ear region of mysticetes. Our detailed anatomical descriptions, illustrations, and comparisons provide valuable data for current and future studies on the phylogenetic relationships, evolution, and auditory physiology of mysticetes and other cetaceans throughout Earth's history

Comparative Anatomy of the Bony Labyrinth (Inner Ear) of Placental Mammals.

BACKGROUND:Variation is a naturally occurring phenomenon that is observable at all levels of morphology, from anatomical variations of DNA molecules to gross variations between whole organisms. The structure of the otic region is no exception. The present paper documents the broad morphological diversity exhibited by the inner ear region of placental mammals using digital endocasts constructed from high-resolution X-ray computed tomography (CT). Descriptions cover the major placental clades, and linear, angular, and volumetric dimensions are reported. PRINCIPAL FINDINGS:The size of the labyrinth is correlated to the overall body mass of individuals, such that large bodied mammals have absolutely larger labyrinths. The ratio between the average arc radius of curvature of the three semicircular canals and body mass of aquatic species is substantially lower than the ratios of related terrestrial taxa, and the volume percentage of the vestibular apparatus of aquatic mammals tends to be less than that calculated for terrestrial species. Aspects of the bony labyrinth are phylogenetically informative, including vestibular reduction in Cetacea, a tall cochlear spiral in caviomorph rodents, a low position of the plane of the lateral semicircular canal compared to the posterior canal in Cetacea and Carnivora, and a low cochlear aspect ratio in Primatomorpha. SIGNIFICANCE:The morphological descriptions that are presented add a broad baseline of anatomy of the inner ear across many placental mammal clades, for many of which the structure of the bony labyrinth is largely unknown. The data included here complement the growing body of literature on the physiological and phylogenetic significance of bony labyrinth structures in mammals, and they serve as a source of data for future studies on the evolution and function of the vertebrate ear

Dimensions and orientations of the cochlea of placentals^a.

a<p>Measurements: Volume, total volume of cochlear canal (mm³); Coil, the total degrees completed by the cochlea; 2° Lamina, extension of secondary lamina through cochlea (°); Length, length of canal (mm); Aqueduct, length of cochlear aqueduct (mm); Ratio, aspect ratio calculated as height of spiral over width; Angle, formed between basal turn of cochlea and lateral semicircular canal (°). Values for extinct eutherians are averages <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066624#pone.0066624-Ekdale3" target="_blank">[83]</a>.</p

Bony labyrinth of<i>Orycteropus afer</i>.

<p><b>A</b>, stereopair and labeled line drawing of digital endocast in anterior view; <b>B</b>, stereopair and labeled line drawing of digital endocast in dorsal view; <b>C</b>, stereopair and labeled line drawing of digital endocast in lateral view; <b>D</b>, line drawing of cochlea viewed down axis of rotation to display degree of coiling; <b>E</b>, line drawing of cochlea in profile. Abbreviations listed at the end of the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0066624#s2" target="_blank">Materials and Methods</a> section.</p