First CT-assisted study of the palate and postcrania of Diarthrognathus broomi (Cynodontia, Probainognathia).

Diarthrognathus broomi is a transitional taxon between non-mammaliaform cynodonts and Mammaliaformes that occurred during the Late Triassic to Early Jurassic. All known specimens of Diarthrognathus represent juveniles, and the postcrania have not been thoroughly described. The palatal, basicranial and postcranial elements of the referred specimen NMQR 1535 are described here for the first time using 3D reconstructions generated from X-ray micro-computed tomography (µCT) data. The presence of a large interpterygoid vacuity, open medial suture between the vomers and medially unossified secondary palate all support the interpretation that NMQR 1535 is a juvenile. In addition, Diarthrognathus uniquely possesses "suborbital" vacuities, which distinguishes it from every other known cynodont. The presence of an ossified olecranon process, among other features, suggests that Diarthrognathus may have been a scratch-digger. The postcranial skeleton of Diarthrognathus appears to be more plesiomorphic than tritylodontids, Brasilodon and other tritheledontids as, among other traits, it retains amphicoelous vertebrae. However, this taxon also displays synapomorphies with the more derived cynodonts, such as the mammalian pattern of neurocentral ossification and possible absence of an ectepicondylar foramen.

The maxillary canal of the titanosuchid Jonkeria (Synapsida, Dinocephalia).

The maxillary canal of the titanosuchid dinocephalian Jonkeria is described based on digitised serial sections. We highlight that its morphology is more like that of the tapinocephalid Moschognathus than that of Anteosaurus. This is unexpected given the similarities between the dentition of Jonkeria and Anteosaurus (i.e., presence of a canine) and the fact that the branching pattern of the maxillary canal in synapsids usually co-varies with dentition. Hypotheses to account for similarities between Jonkeria and Moschognathus (common ancestry, function in social signalling or underwater sensing) are discussed. It is likely that the maxillary canal carries a strong phylogenetic signal, here supporting the clade Tapinocephalia.

Craniodental anatomy in Permian-Jurassic Cynodontia and Mammaliaformes (Synapsida, Therapsida) as a gateway to defining mammalian soft tissue and behavioural traits.

Mammals are diagnosed by more than 30 osteological characters (e.g. squamosal-dentary jaw joint, three inner ear ossicles, etc.) that are readily preserved in the fossil record. However, it is the suite of physiological, soft tissue and behavioural characters (e.g. endothermy, hair, lactation, isocortex and parental care), the evolutionary origins of which have eluded scholars for decades, that most prominently distinguishes living mammals from other amniotes. Here, we review recent works that illustrate how evolutionary changes concentrated in the cranial and dental morphology of mammalian ancestors, the Permian-Jurassic Cynodontia and Mammaliaformes, can potentially be used to document the origin of some of the most crucial defining features of mammals. We discuss how these soft tissue and behavioural traits are highly integrated, and how their evolution is intermingled with that of craniodental traits, thus enabling the tracing of their previously out-of-reach phylogenetic history. Most of these osteological and dental proxies, such as the maxillary canal, bony labyrinth and dental replacement only recently became more easily accessible-thanks, in large part, to the widespread use of X-ray microtomography scanning in palaeontology-because they are linked to internal cranial characters. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Tooth replacement patterns in the Early Triassic epicynodont Galesaurus planiceps (Therapsida, Cynodontia).

Sixteen specimens of the Early Triassic cynodont Galesaurus planiceps (including eight that were scanned using micro-computed tomography) representing different ontogenetic stages were assembled to study the dental replacement in the species. The growth series shows that the incisors and postcanines continue to develop and replace, even in the largest (presumably oldest) specimen. In contrast, replacement of the canines ceased with the attainment of skeletal maturity, at a basal skull length of ~90 mm, suggesting that Galesaurus had a finite number of canine replacement cycles. Additionally, the functional canine root morphology of these larger specimens showed a tendency to be open-rooted, a condition not previously reported in Mesozoic theriodonts. An alternating pattern of tooth replacement was documented in the maxillary and mandibular postcanine series. Both postcanine series increased in tooth number as the skull lengthened, with the mandibular postcanine series containing more teeth than the maxillary series. In the maxilla, the first postcanine is consistently the smallest tooth, showing a proportional reduction in size as skull length increased. The longer retention of a tooth in this first locus is a key difference between Galesaurus and Thrinaxodon, in which the mesial-most postcanines are lost after replacement. This difference has contributed to the lengthening of the postcanine series in Galesaurus, as teeth continued to be added to the distal end of the tooth row through ontogeny. Overall, there are considerable differences between Galesaurus and Thrinaxodon relating to the replacement and development of their teeth.

Reappraisal of the envenoming capacity of Euchambersia mirabilis (Therapsida, Therocephalia) using µCT-scanning techniques.

Euchambersia mirabilis 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 Euchambersia, 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 Euchambersia, which strongly suggests that the complete dentition was ridged. Therefore Euchambersia 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).