Synchrotron scanning reveals the palaeoneurology of the head-butting Moschops capensis (Therapsida, Dinocephalia)

Abstract

Dinocephalian therapsids are renowned for their massive, pachyostotic and ornamented skulls adapted for head-to-head fighting during intraspecific combat. Synchrotron scanning of the tapinocephalid Moschops capensis reveals, for the first time, numerous anatomical adaptations of the central nervous system related to this combative behaviour. Many neural structures (such as the brain, inner ear and ophthalmic branch of the trigeminal nerve) were completely enclosed and protected by bones, which is unusual for non-mammaliaform therapsids. The nearly complete ossification of the braincase enables precise determination of the brain cavity volume and encephalization quotient, which appears greater than expected for such a large and early herbivore. The practice of head butting is often associated with complex social behaviours and gregariousness in extant species, which are known to influence brain size evolution. Additionally, the plane of the lateral (horizontal) semicircular canal of the bony labyrinth is oriented nearly vertically if the skull is held horizontally, which suggests that the natural position of the head was inclined about 60–65°to the horizontal. This is consistent with the fighting position inferred from osteology, as well as ground-level browsing. Finally, the unusually large parietal tube may have been filled with thick conjunctive tissue to protect the delicate pineal eye from injury sustained during head butting.

Endocranial Casts of Pre-Mammalian Therapsids Reveal an Unexpected Neurological Diversity at the Deep Evolutionary Root of Mammals

“…8 b), a condition that is also present in other dinocephalians [Boonstra, 1968]. This strongly suggests that the brain was separated from the braincase walls by a thick layer of conjunctive, likely protective, tissues [Benoit et al, 2017b]. This is supported by the estimates of brain volume, which suggest that the brain occupies only 32-35% of the volume of the braincase in dinocephalians ( Table 1 ).…”

“…In Moschops this is due to the posteriorly tilted orientation of the braincase with respect to the skull ( Fig. 8 a) [Benoit et al, 2017b], whereas in biarmosuchians the endocast displays a very distinctive flexure, as quantified by the angle made by the fore-and hindbrain in the lateral view ( Fig. 3 b-7 b).…”

“…With their hypertrophied and deep hypophyseal fossa, the dinocephalians are very distinctive in this respect ( Fig. 8 b) [Boonstra, 1968;Benoit et al, 2017b]. The cerebellar flocculus of the endocast is prominent in all biarmosuchians ( Fig.…”

“…11 ). These very small values are thus better interpreted as secondary reduction, maybe driven by the evolution of a large body size [Hurlburt et al, 2013;Jirak and Janacek, 2017] and the necessity to protect the central nervous system [Benoit et al, 2017b]. Secondary simplification of brain anatomy and reduction of EQs have been documented in several clades, such as lissamphibians, chiropterans, and sirenians [O'Shea and Reep, 1990;Roth et al, 1997;Nieuwenhuys et al 1998;Safi et al, 2005;Benoit et al, 2013].…”

“…Since no postcranial material is associated with any of the specimens studied here, body mass was calculated using skull length following the method published and discussed in Benoit et al [2017b] for the rest of the comparative sample. This method used 3 different equations to estimate body mass using skull length (BM1, BM2, and BM3 in Table 1 ; discussed in Benoit et al [2017b]). An average body mass is calculated from the results of these 3 equations (average BM in Table 1 ).…”