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.
Highlights
Dinocephalia were middle Permian therapsids that lived some 260–265 million years ago, during the middle Permian (Rubidge & Sidor, 2001; Kemp, 2005)
It was preliminarily assigned to an anteosaurid prior to preparation (Modesto et al, 2001), but is considered to be a sub-adult specimen of Moschops capensis (Benoit et al, 2016a) based on the lack of a developed canine tooth, presence of talons and heels on all the teeth, broad intertemporal surface, massive postorbital bar, absence of naso-frontal boss, incomplete development of the supraorbital bosses and the unfused state of most cranial sutures, including the bones making up the fronto-parietal shield (FPS)
The bony labyrinth in Moschops is very unusual amongst non-mammalian therapsids (NMT) because compared to other NMT in which all semicircular canals look similar, the bony labyrinth of Moschops displays a distinctly larger anterior semicircular canal, a character commonly encountered in archosaurs (Hopson, 1979; Witmer et al, 2003; Witmer et al, 2008; Walsh, Luo & Barrett, 2013) and mammals as well as in the sister taxon of Mammaliaformes, Brasilitherium (Olson, 1944; Kemp, 1969; Kemp, 1979; Kemp, 2009; Hopson, 1979; Gow, 1986; Luo, 2001; Kielan-Jaworowska, Cifelli & Luo, 2004; Castanhinha et al, 2013; Rodrigues, Ruf & Schultz, 2013b; Laaß, 2015; Ekdale, 2013)
Summary
Dinocephalia were middle Permian therapsids that lived some 260–265 million years ago, during the middle Permian (Rubidge & Sidor, 2001; Kemp, 2005). Cranial fighting surfaces made of compact bone are encountered in odontocetes only, in the maxillary crests of the bottlenose whale (Hyperoodon ampullatus) (Gowans & Rendell, 1999; Lambert, Buffrénil & Muizon, 2011; Bianucci et al, 2013). This has resulted in the lack of clear extant analogues to the cranial specialisations of dinocephalians
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