Sensory anatomy of the most aquatic of carnivorans: the Antarctic Ross seal, and convergences with other mammals

Abstract

Transitions to and from aquatic life involve transformations in sensory systems. The Ross seal, , offers the chance to investigate the cranio-sensory anatomy in the most aquatic of all seals. The use of non-invasive computed tomography on specimens of this rare animal reveals, relative to other species of phocids, a reduction in the diameters of the semicircular canals and the parafloccular volume. These features are independent of size effects. These transformations parallel those recorded in cetaceans, but these do not extend to other morphological features such as the reduction in eye muscles and the length of the neck, emphasizing the independence of some traits in convergent evolution to aquatic life.

Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water

“…their canals to a lesser degree (22)(23)(24). Thus, distantly related radiations of marine reptiles independently evolved similar compact, robust, but not extremely atrophied cetaceanlike labyrinths, suggesting a general manner in which reptile vestibular sensory systems are altered for a pelagic lifestyle.…”

Evolutionary transformations of the malleus in pinnipeds, with emphasis on Southern Hemisphere taxa

“…In the inner ear, Loza et al (2017) reported how the Ross seal (Ommatophoca rossii), the most aquatic of pinnipeds, has reduced semicircular canals and proportionally reduced para flocculus relative to those of other phocids, in parallel with what is known for cetaceans. Our study discovered unique features of the Ross seal in the malleus that suggest a transformation similar to that of cetaceans expressed in the convergence of the articular facets of the malleus to each other(e.g.…”

The braincase and inner ear of ‘Metriorhynchus’ cf. ‘M.’ brachyrhynchus – implications for aquatic sensory adaptations in crocodylomorphs

“…Pelagic sauropterygians and sea turtles have more bulbous labyrinths with thicker 21 semicircular canals than other reptiles (Georgi and Sipla, 2008;Neenan et al, 2017;Evers et al, 2019), cetaceans miniaturized their labyrinth drastically compared to their body size (Spoor et al, 2002), and other aquatic mammals such as sirenians and seals also reduced their semicircular canals but to a lesser degree (Hyrtl, 1845;Loza et al, 2017). Hence it is clear that specific aspects of their labyrinth morphologies convergently evolved in independent pelagic lineages.…”