Ontogenetic variation in the crocodylian vestibular system

Abstract

Crocodylians today live in tropical to subtropical environments, occupying mostly shallow waters. Their body size changes drastically during ontogeny, as do their skull dimensions and bite forces, which are associated with changes in prey preferences. Endocranial neurosensory structures have also shown to change ontogenetically, but less is known about the vestibular system of the inner ear. Here we use 30 high‐resolution computed tomography (CT) scans and three‐dimensional geometric morphometrics to investigate the size and shape changes of crocodylian endosseous labyrinths throughout ontogeny, across four stages (hatchling, juvenile, subadult and adult). We find two major patterns of ontogenetic change. First, the labyrinth increases in size during ontogeny, with negative allometry in relation to skull size. Second, labyrinth shape changes significantly, with hatchlings having shorter semicircular canal radii, with thicker diameters and an overall dorsoventrally shorter labyrinth than those of more mature individuals. We argue that the modification of the labyrinth during crocodylian ontogeny is related to constraints imposed by skull growth, due to fundamental changes in the crocodylian braincase during ontogeny (e.g. verticalisation of the basicranium), rather than changes in locomotion, diet, or other biological functions or behaviours.

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

“…This is consistent with the ontogenetic signal found in the vestibular system of crocodylians where a significant size and shape change occurs during growth [25]. However, otoliths grow with a slight positive allometry compared to skull length and labyrinth volume, which contrasts with the size of the labyrinth, which grows with negative allometry in relation to skull length [25]. It is unclear whether such allometric growth would be consistent with optimization for hearing the vocalizations of conspecifics, and a royalsocietypublishing.org/journal/rsos R. Soc.…”

“…Hatchling and juvenile specimens generally have smaller otoliths compared to subadults and adults ( figure 5 ) and if the otoliths simply maintained their size during ontogeny, or exhibited a proportional decrease in size, then their biological importance might seem to be less crucial. This is consistent with the ontogenetic signal found in the vestibular system of crocodylians where a significant size and shape change occurs during growth [ 25 ]. However, otoliths grow with a slight positive allometry compared to skull length and labyrinth volume, which contrasts with the size of the labyrinth, which grows with negative allometry in relation to skull length [ 25 ].…”

Ontogenetic variability of the intertympanic sinus distinguishes lineages within Crocodylia

“…Crocodylians undergo continuous growth during development, precluding a clear osteological assessment of ontogenetic stages and making it difficult to attribute precise size boundaries which would also differ for each species (Morris et al, 2019 ; Schwab et al, 2021 ). Therefore, extant specimens were classified into four size classes (hatchling, juvenile, sub‐adult and adult) based on their total SL and respective genus (see Table 1 , Table S1 ), in order to compare sinus shape between specimens of different sizes.…”

“…Crocodylians undergo continuous growth during development, precluding a clear osteological assessment of ontogenetic stages and making it difficult to attribute precise size boundaries which would also differ for each species (Morris et al, 2019;Schwab et al, 2021).…”

Neuroanatomy of the mekosuchine crocodylian Trilophosuchus rackhami Willis, 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.…”