Evolution of facial innervation in anomodont therapsids (Synapsida): Insights from X‐ray computerized microtomography

Abstract

Anomodontia was the most successful herbivorous clade of the mammalian stem lineage (non-mammalian synapsids) during the late Permian and Early Triassic. Among anomodonts, Dicynodontia stands apart because of the presence of an osseous beak that shows evidence of the insertion of a cornified sheath, the ramphotheca. In this study, fourteen anomodont specimens were microCT-scanned and their trigeminal canals reconstructed digitally to understand the origin and evolution of trigeminal nerve innervation of the ramphotheca. We show that the pattern of innervation of the anomodont "beak" is more similar to that in chelonians (the nasopalatine branch is enlarged and innervates the premaxillary part of the ramphotheca) than in birds (where the nasopalatine and maxillary branches play minor roles). The nasopalatine branch is noticeably enlarged in the beak-less basal anomodont Patranomodon, suggesting that this could be an anomodont or chainosaur synapomorphy. Our analyses suggest that the presence or absence of tusks and postcanine teeth are often accompanied by corresponding variations of the rami innervating the caniniform process and the alveolar region, respectively. The degree of ossification of the canal for the nasal ramus of the ophthalmic branch also appears to correlate with the presence of a nasal boss. The nasopalatine canal is absent from the premaxilla in the Bidentalia as they uniquely show a large plexus formed by the internal nasal branch of the maxillary canal instead. The elongated shape of this plexus in Lystrosaurus supports the hypothesis that the rostrum evolved as an elongation of the subnarial region of the snout. Finally, the atrophied and variable aspect of the trigeminal canals in Myosaurus supports the hypothesis that this genus had a reduced upper ramphotheca.

A redescription of Orovenator mayorum (Sauropsida, Diapsida) using high‐resolution μCT, and the consequences for early amniote phylogeny

“…Many amniotes possess a group of similar‐sized supralabial foramina on the lateral surface of the maxilla (Laurin & Reisz ; Benoit et al . ). An anterior maxillary foramen noticeably larger than the other foramina, and often associated with an anteriorly orientated groove as seen in Orovenator , can be seen in many sauropsids.…”

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

“…However, the EQs of R. procurvidens are still lower than those of the cynodont Brasilitherium (0.14 AE 0.05 and 0.24 AE 0.09; Rodrigues et al 2013), and shows markedly less encephalization than Mammaliaformes, as seen in Hadrocodium (0.24 AE 0.08, 0.51 AE 0.19) and Morganucodon (0.20 AE 0.07, 0.35 AE 0.13) (Rowe et al 2011) (Table S1). This demonstrates the progressive increase of EQs approaching the early evolution of mammaliaforms in comparison with early Therapsida (Benoit et al 2017b(Benoit et al , 2018 (Fig. 9).…”

“…Finally, R. procurvidens preserves non-compressed maxillary recesses. The plesiomorphic condition in Dicynodontia is compressed or reduced maxillary recesses due to the oversized tusks displayed by early dicynodonts, as noted recently by Benoit et al (2018). Those authors also pointed out that the enlargement or uncompression of the recesses (or sinuses) is linked to the loss of the caniniform tusks or expansion of the caniniform process, seen in dicynodonts such as Abajudon, Myosaurus, Oudenodon, cistecephalids, and most bidentalians.…”

“…Variation of encephalization within Therapsida. Based on dataset compiled by Benoit et al (), see Table . NPT, no pineal tube.…”

“…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.…”