The inner ear ofMegatheriumand the evolution of the vestibular system in sloths

Abstract

Extant tree sloths are uniquely slow mammals with a very specialized suspensory behavior. To improve our understanding of their peculiar evolution, we investigated the inner ear morphology of one of the largest and most popular fossil ground sloths, Megatherium americanum. We first address the predicted agility of this animal from the scaling of its semicircular canals (SC) relative to body mass, based on recent work that provided evidence that the size of the SC in mammals correlates with body mass and levels of agility. Our analyses predict intermediate levels of agility for Megatherium, contrasting with the extreme slowness of extant sloths. Secondly, we focus on the morphology of the SC at the inner ear scale and investigate the shape and proportions of these structures in Megatherium and in a large diversity of extant xenarthrans represented in our database. Our morphometric analyses demonstrate that the giant ground sloth clearly departs from the SC morphology of both extant sloth genera (Choloepus, Bradypus) and is in some aspects closer to that of armadillos and anteaters. Given the close phylogenetic relationships of Megatherium with the extant genus Choloepus, these results are evidence of substantial homoplasy of the SC anatomy in sloths. This homoplasy most likely corresponds to an outstanding convergent evolution between extant suspensory sloth genera.

Anatomical evidence for low frequency sensitivity in an archaeocete whale: comparison of the inner ear of Zygorhiza kochii with that of crown Mysticeti

“…Although the discussion of the inner ear focuses on the cochlea and structures associated with hearing sensitivity, a description of the vestibule and semicircular canals is included to allow comparisons between the vestibular systems of Zygorhiza and other mammals from the published literature (e.g. Spoor et al 2002Spoor et al , 2007Silcox et al 2009;Macrini et al 2010Macrini et al , 2013Billet et al 2012Billet et al , 2013Orliac et al 2012;Ekdale, 2013). Specific measurements of the vestibular system include the semicircular canal arc radii of curvature, lengths of the unampullated portions of the semicircular canals, and angles between the planes of the three semicircular canals (see Ekdale, 2010, 2013and Ekdale & Rowe, 2011 for further discussion of semicircular canal measurement methods).…”

Form and function of the mammalian inner ear

“…; Billet et al. ). For example, the arc of the anterior semicircular canal tends to be more elliptical in marine carnivorans such as pinnipeds than in terrestrial species such as canids (Fig.…”

“…The connection between the semicircular canals and rotational sensations has been known for nearly a century and a half (Dercum, 1879), and the connections between canal anatomy, orientation, sensitivity, behavior, and phylogeny is an active field of investigation in evolutionary biology. Several studies have used the size and shape of the semicircular canals to infer the agilities and locomotor abilities of numerous extinct and extant mammals, such as primates, whales, ungulates, and sloths (Spoor et al 2002(Spoor et al , 2007Silcox et al 2009;Macrini et al 2010Macrini et al , 2013Orliac et al 2012;Billet et al 2013). For example, the arc of the anterior semicircular canal tends to be more elliptical in marine carnivorans such as pinnipeds than in terrestrial species such as canids (Fig.…”

Bone cortical compactness in ‘tree sloths’ reflects convergent evolution

“…These include the musculoskeletal specializations allowing a suspensory posture and locomotion (Mendel, ; Nyakatura, ), an aberrant cervical count (Buchholtz & Stepien, ), the retention of water and growth of algae in the hair (Aiello, ), and small and variable semicircular canals (Billet et al. , ), with a weakly tilted lateral semicircular canal (Coutier et al. ).…”