Endothiodon cf. bathystoma (Synapsida: Dicynodontia) bony labyrinth anatomy, variation and body mass estimates

Abstract

The semicircular canal (SC) system of the inner ear detects head angular accelerations and is essential for navigation and spatial awareness in vertebrates. Because the bony labyrinth encloses the membranous labyrinth SCs, it can be used as a proxy for animal behavior. The bony labyrinth of dicynodonts, a clade of herbivorous non-mammalian synapsids, has only been described in a handful of individuals and remains particularly obscure. Here we describe the bony labyrinth anatomy of three Endothiodon cf. bathystoma specimens from Mozambique based on digital reconstructions from propagation phase-contrast synchrotron micro-computed tomography. We compare these findings with the bony labyrinth anatomy of their close relative Niassodon. The bony labyrinths of Endothiodon and Niassodon are relatively similar and show only differences in the shape of the horizontal SCs and the orientation of the vertical SCs. When compared to extant mammals, Endothiodon and Niassodon have highly eccentric SCs. In addition, the Endothiodon SCs are nearly orthogonal. An eccentric and orthogonal SC morphology is consistent with a specialization in rapid head movements, which are typical of foraging or feeding behaviors. Furthermore, we estimate the body mass of these Endothiodon specimens at ~116 to 182 kg, based on the average SC radii calculated using a linear regression model optimized by the Amemiya Prediction Criterion. Our findings provide novel insights into the paleobiology of Endothiodon which are consistent with the peculiar feeding mechanism among dicynodonts presumed from their multiple postcanine toothrows.

Novel Endocranial Data on the Early Therocephalian Lycosuchus vanderrieti Underpin High Character Variability in Early Theriodont Evolution

“…Different from what is usually the case in non-mammalian therapsids and mammals, in which the anterior semicircular is the longest (e.g., Olson, 1944;Sigogneau, 1974;Luo, 2001;Kielan-Jaworowska et al, 2004;Rodrigues et al, 2013;Laaß, 2016;Araújo et al, 2017Araújo et al, , 2018Benoit et al, 2017b,c;Bendel et al, 2018;Pusch et al, 2019), the lateral semicircular canal appears to be the longest of the three semicircular canals in Lycosuchus. Several studies have shown that the elongated morphology of the semicircular canals can be linked to the animals' lifestyle (e.g., Spoor, 2003;Spoor et al, 2007;Walsh et al, 2009;Wittmer and Ridgely, 2009;Cox and Jeffery, 2010;Laaß, 2015a;Araújo et al, 2018). The anomodont Kawingasaurus is the only other non-mammalian therapsid described with a lateral semicircular canal larger than the anterior and posterior one, but it was suggested that its elongated morphology is related to the inflated geometry of the vestibule rather than its locomotory behavior (Laaß, 2015a).…”

“…Among therapsids, a lateral semicircular canal larger than the anterior and posterior one has only previously been reported for the fossorial anomodont Kawingasaurus fossilis (Laaß, 2015a). The lateral and posterior canal share a secondary crus commune (Figure 11B), as in most non-mammalian therapsids (e.g., Olson, 1944;Sigogneau, 1974;Castanhinha et al, 2013;Rodrigues et al, 2013;Laaß, 2015aLaaß, , 2016Araújo et al, 2017Araújo et al, , 2018Benoit et al, 2017b,c;Bendel et al, 2018;Pusch et al, 2019). The curved vestibule has a length of 13.59 mm and is surrounded by the opisthotic and the prootic, of which the opisthotic constitutes its posterior and lateral border and the prootic its anterior border.…”

“…The lateral (horizontal) semicircular canal appears to be the longest of the three semicircular canals, with a height of 3.81 mm and a width of 10.88 mm ( Figures 10C,D, 11). This situation is unusual, because in most non-mammalian therapsids and mammals the anterior semicircular canal is the longest (e.g., Olson, 1944;Sigogneau, 1974;Luo, 2001;Kielan-Jaworowska et al, 2004;Rodrigues et al, 2013;Laaß, 2016;Araújo et al, 2017Araújo et al, , 2018Benoit et al, 2017b,c;Bendel et al, 2018;Pusch et al, 2019). Among therapsids, a lateral semicircular canal larger than the anterior and posterior one has only previously been reported for the fossorial anomodont Kawingasaurus fossilis (Laaß, 2015a).…”

“…The advent of widely-used computed tomographic (CT) imaging on fossil specimens has allowed paleontologists unprecedented access to the once-obscure internal features of vertebrate crania. In the context of mammalian origins, these novel endocranial data have been instrumental in elucidating patterns of brain and ear evolution, facial innervation, and the evolution of endothermy in non-mammalian therapsids (e.g., Rodrigues et al, 2013Rodrigues et al, , 2014Crompton et al, 2015Crompton et al, , 2017Laaß, 2015aLaaß, ,b, 2016Benoit et al, 2016aBenoit et al, ,b, 2017aBenoit et al, ,b,c, 2018Benoit et al, , 2019Araújo et al, 2017Araújo et al, , 2018Bendel et al, 2018;Pusch et al, 2019). As the therapsid clade ancestral to (and including) modern mammals, Cynodontia has been the primary focus for CTassisted morphological studies in Synapsida (e.g., Rowe et al, 1993;Rodrigues et al, 2013Rodrigues et al, , 2014Crompton et al, 2015Crompton et al, , 2017Benoit et al, 2016aBenoit et al, , 2019Pavanatto et al, 2019;Pusch et al, 2019).…”

Endocranial morphology of the Brazilian Permian dicynodont Rastodon procurvidens (Therapsida: Anomodontia)

“…The bony labyrinth of Endothiodon cf. E. bathystoma was reconstructed by Ara ujo et al (2018), and the work of Benoit et al (2018) compared the facial innervation of members of Anomodontia, the more inclusive clade containing dicynodonts and their closest relatives.…”