‘Ear stones’ in crocodylians: a cross-species comparative and ontogenetic survey of otolith structures

Abstract

The vestibular system of the inner ear is a crucial sensory organ, involved in the sensation of balance and equilibrium. It consists of three semicircular canals that sense angular rotations of the head and the vestibule that detects linear acceleration and gravity. The vestibule often contains structures, known as the otoliths or ‘ear stones’. Otoliths are present in many vertebrates and are particularly well known from the fossil record of fish, but surprisingly have not been described in detail in most tetrapods, living or extinct. Here, we present for the first time a survey of the otoliths of a broad sample of extant crocodylian species, based on computed tomography scans. We find that otoliths are present in numerous crocodylian species of different growth stages, and they continue to increase in size during ontogeny, with positive allometry compared to skull length. Our results confirm that otoliths are a common component of the crocodylian vestibular system, and suggest they play an important role in sensory detection. Otoliths are likely common, but overlooked, constituents of the inner ear in tetrapods, and a broader study of their size, shape and distribution promises insight into sensory abilities.

Neuroanatomy of the mekosuchine crocodylianTrilophosuchus rackhamiWillis, 1993

“…Aspects of crocodylomorph palaeoneurology were first investigated in the 19th century (Owen, 1842, 1850) and continued sporadically throughout the 20th century (e.g., Colbert, 1946a, 1946b; Edinger, 1938; Hopson, 1979; Yeh, 1958), though the frequency of palaeoneurological publications has increased drastically in the 21st century. With the advent of computed tomography and its increasing widespread availability to researchers, the past 14 years have witnessed a surge in papers that are dedicated either exclusively or partially on the palaeoneurology of crocodylomorphs (e.g., Blanco et al, 2015; Bona & Paulina Carabajal, 2013; Bona et al, 2013, 2017; Bowman et al, 2021; Brusatte, Muir, et al, 2016; Cowgill et al, 2021; Dumont Jr et al, 2020; Erb & Turner, 2021; Fernández et al, 2011; Fonseca et al, 2020; George & Holliday, 2013; Herrera et al, 2018; Herrera, 2015; Holliday & Gardner, 2012; Kley et al, 2010; Leardi et al, 2020; Melstrom et al, 2021; Pierce et al, 2017; Pochat‐Cottilloux et al, 2021; Puértolas‐Pascual et al, 2022; Ristevski et al, 2020a, 2021; Schwab et al, 2020; Schwab, Young, Herrera, et al, 2021, Schwab, Young, Walsh, et al, 2022; Schwab et al, 2022; Sereno & Larsson, 2009; Serrano‐Martínez et al, 2019a, 2019b, 2021; Sertich & O'Connor, 2014; Wilberg et al, 2021; Witmer & Ridgely, 2008; Witmer et al, 2008). Despite this, our understanding of crocodylomorph palaeoneurology is still in its relative infancy considering that the neuroanatomy for the vast majority of crocodylomorph taxa is yet to be investigated, including for the majority of Australian taxa.…”