CT images of the maxillary and mandibular bosses of <i>Choerosaurus dejageri</i>.

<p>A-F, the maxillary boss in transverse sections. G-I, the maxillary boss in longitudinal sections. J-L, the mandibular boss in longitudinal sections. Abbreviations: C, Canine; Canc, Cancellous bone; Dent, Dentary; Inc, Incisor; MdBoss, Mandibular boss; Mx, Maxilla; MxBoss, Maxillary boss; MxCan, Maxillary canal; Nvsc, Neurovascular canal; Pit, Pitted surface of the mandibular boss; Pmx, Premaxilla; PreC, Precanine tooth on the maxilla; Radvsc, Radial vasculature; Spl, Splenial. Scale bar: 2 mm.</p

Different types of fighting hypothesized in <i>Choerosaurus dejageri</i>.

<p>A, lateral head pushing; B, lateral head butting; C, lateral flank butting. The arrows represent head movements. The stars represent the points of impact.</p

CT images of the braincase of <i>Moschops capensis</i> AM6556.

<p>A, longitudinal section. B-D, transverse sections. Abbreviations: Ept, epipterygoid; CanFr, cancellous frontal bone; FrParSh, frontoparietal shield; Ju, jugal; Mes, mesethmoid; Nvsc, neurovascular canal; Oc?, unidentified occipital bone; Ocd, occipital condyle; Orb, orbite; Orbsp, orbitosphenoid; Par, Parietal; Pin, pineal tube; PostFr, postfrontal; PostOrb, postorbital; PrOt, prootic; Sq, squamosal; TempFs, temporal fenestra. Scale bar: 20mm.</p

Digital 3D rendering of the skull of <i>Choerosaurus dejageri</i>.

<p>A, lateral right view; B, lateral left view; C, dorsal view; D, ventral view. The matrix appears in shades of dark grey and the preserved bone is in light grey.</p

Cranial Bosses of <i>Choerosaurus dejageri</i> (Therapsida, Therocephalia): Earliest Evidence of Cranial Display Structures in Eutheriodonts

<div><p><i>Choerosaurus dejageri</i>, a non-mammalian eutheriodont therapsid from the South African late Permian (~259 Ma), has conspicuous hemispheric cranial bosses on the maxilla and the mandible. These bosses, the earliest of this nature in a eutheriodont, potentially make <i>C</i>. <i>dejageri</i> a key species for understanding the evolutionary origins of sexually selective behaviours (intraspecific competition, ritualized sexual and intimidation displays) associated with cranial outgrowths at the root of the clade that eventually led to extant mammals. Comparison with the tapinocephalid dinocephalian <i>Moschops capensis</i>, a therapsid in which head butting is strongly supported, shows that the delicate structure of the cranial bosses and the gracile structure of the skull of <i>Choerosaurus</i> would be more suitable for display and low energy combat than vigorous head butting. Thus, despite the fact that <i>Choerosaurus</i> is represented by only one skull (which makes it impossible to address the question of sexual dimorphism), its cranial bosses are better interpreted as structures involved in intraspecific selection, i.e. low-energy fighting or display. Display structures, such as enlarged canines and cranial bosses, are widespread among basal therapsid clades and are also present in the putative basal therapsid <i>Tetraceratops insignis</i>. This suggests that sexual selection may have played a more important role in the distant origin and evolution of mammals earlier than previously thought. Sexual selection may explain the subsequent independent evolution of cranial outgrowths and pachyostosis in different therapsid lineages (Biarmosuchia, Dinocephalia, Gorgonopsia and Dicynodontia).</p></div

Reappraisal of the envenoming capacity of <i>Euchambersia mirabilis</i> (Therapsida, Therocephalia) using μCT-scanning techniques

<div><p><i>Euchambersia mirabilis</i> is an iconic species of Permo-Triassic therapsid because of its unusually large external maxillary fossa linked through a sulcus to a ridged canine. This anatomy led to the commonly accepted conclusion that the large fossa accommodated a venom gland. However, this hypothesis remains untested so far. Here, we conducted a μCT scan assisted reappraisal of the envenoming capacity of <i>Euchambersia</i>, with a special focus on the anatomy of the maxillary fossa and canines. This study shows that the fossa, presumably for the venom-producing gland, is directly linked to the maxillary canal, which carries the trigeminal nerve (responsible for the sensitivity of the face). The peculiar anatomy of the maxillary canal suggests important reorganisation in the somatosensory system and that a ganglion could possibly have been present in the maxillary fossa instead of a venom gland. Nevertheless, the venom gland hypothesis is still preferred since we describe, for the first time, the complete crown morphology of the incisiform teeth of <i>Euchambersia</i>, which strongly suggests that the complete dentition was ridged. Therefore <i>Euchambersia</i> manifests evidence of all characteristics of venomous animals: a venom gland (in the maxillary fossa), a mechanism to deliver the venom (the maxillary canal and/or the sulcus located ventrally to the fossa); and an apparatus with which to inflict a wound for venom delivery (the ridged dentition).</p></div

The skull and braincase of <i>Moschops capensis</i>.

<p>A, the skull of <i>Moschops</i> in lateral view. B-C, 3D rendering from CT data, B, the endocranial cast (green) and emissary vein (red) of <i>Moschops</i> in frontal view. C, the endocranial cast (green) and emissary vein (red) of <i>Moschops</i> in lateral view. D, virtual transverse section through the braincase of <i>Moschops</i>. E-F, the pathway of the shockwave (red arrows) thourgh the braincase of <i>Moschops</i> illustrated on the lateral view (E) and the transverse section (F). Abbreviations: Bas, basicranium; BrC, Braincase; EmV, emissary veins; Ept, Epipterygoid; Fmg, foramen magnum; FrParSh, Fronto-parietal shield; Md, mandible; Orb, orbit; Pin, pineal tube; Pit, pituitary fossa; PrOt, prootic; SkR, skull roof. Scale bar: 50mm.</p