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

Abstract

BackgroundVariation 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 FindingsThe 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.SignificanceThe 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.

Digital reconstruction of the inner ear of Leptictidium auderiense (Leptictida, Mammalia) and North American leptictids reveals new insight into leptictidan locomotor agility

“…Based on the study by Ekdale and Rowe (2011) and further recent works Luo et al 2012;Ekdale 2013Ekdale , 2015, we propose that thin and distinctly arcuate semicircular canals and the presence of a secondary crus commune, as present in all of our Leptictida, are plesiomorphic features of the leptictidan groundplan (in the sense of Hennig 1950). Mesozoic (Meng and Fox 1995;Ekdale and Rowe 2011;Ekdale 2015).…”

“…There is clearly no evidence from the bony labyrinth morphology for a close relationship of the investigated Leptictida and Macroscelidea (Benoit et al 2013;Ekdale 2013;pers. observ.…”

“…Furthermore, we compared our observations to the study by Ekdale (2013) and added the Leptictida under study to his data matrix on six bony labyrinth characters (see Table 2). …”

“…Absence of the secondary crus commune is a derived feature in Macroscelides, though the lateral and posterior semicircular canals can have a punctiform connection, if they touch each other. The cochlea of Macroscelides also has 2.25 turns, but its shape is very different in being much more conical, though it also has a high cochlear aspect ratio (Benoit et al 2013;Ekdale 2013;pers. obs).…”

“…The cochlear length was measured along the outer curvature from the anterior border of the fenestra vestibuli to the apex of the cochlea ). Measurements for the cochlea aspect ratio (cochlear height divided by cochlear width) and bony labyrinth volume were conducted according to Ekdale (2010Ekdale ( , 2013. The inner ear height (IEH) was calculated according to , and measured as the linear distance between the dorsal apex of the crus commune and the ventral apex of the cochlea.…”