Convergent Evolution of a Eusuchian-Type Secondary Palate within Shartegosuchidae

Abstract

Shartegosuchids are a poorly known, early-branching group of Asian and North American crocodylomorphs. Shartegosuchids have been hypothesized to have eusuchian-type secondary palates, but a paucity of described material makes assessing this difficult. Our fieldwork in western Mongolia recovered cranial material of a new Shartegosuchus specimen from the Ulan Malgait Formation, which we CT-scanned and digitally reconstructed to investigate its palatal morphology. We then incorporated this new anatomical information into a revised phylogenetic dataset to assess its affinities. Our study confirms that Shartegosuchus has a posteriorly placed choana that is fully enclosed by the pterygoids, but differs from Eusuchia in possessing a secondary palatal fenestra and reduced palatine bones. Shartegosuchus, together with Adzhosuchus, Fruitachampsa, and Nominosuchus, forms the monophyletic group Shartegosuchidae. Shartegosuchidae is nested within a larger clade Shartegosuchoidea, and this clade is an earlierdiverging lineage than Eusuchia, showing that a eusuchian-type secondary palate evolved multiple times in crocodylomorphs, including very early in the group's evolutionary history. The co-occurrence of Nominosuchus in the Ulan Malgait Formation and the Shishigou Formation allows us to assign an early Oxfordian age to Shartegosuchus. The independent evolution of a eusuchian-type secondary palate in an oreinorostral group suggests that the link between platyrostry and a closed secondary palate has been overstated.

The craniomandibular anatomy of the early archosauriformEuparkeria capensisand the dawn of the archosaur skull

“…The palatine contacts the vomer anteromedially, the pterygoid posteriorly and posteromedially (no palatine–pterygoid fenestra is present, unlike in ornithosuchids [ 22 , 76 ]), and the maxilla laterally. The palatine does not contact the ectopterygoid posteromedially ( contra Ewer [ 54 ]); this contact is also absent in other stem and crown archosaurs ( Proterosuchus fergusi [ 126 ], Prolacerta [ 125 ], Chanaresuchus [ 80 ], Garjainia prima [ 127 ], Herrerasaurus ischigualastensis [ 77 ], crocodylomorphs [ 128 ]). The anterior margin of the palatine is gently anteriorly concavely curved to form the posterior margin of the choana.…”

“…Open Sci. 7: 200116 this contact is also absent in other stem and crown archosaurs (Proterosuchus fergusi [126], Prolacerta [125], Chanaresuchus [80], Garjainia prima [127], Herrerasaurus ischigualastensis [77], crocodylomorphs [128]). The anterior margin of the palatine is gently anteriorly concavely curved to form the posterior margin of the choana.…”

Crocodylomorph cranial shape evolution and its relationship with body size and ecology

“…Apart from notosuchians, other crocodylomorph subgroups contribute to the higher disparity of terrestrial forms, mainly non-mesoeucrocodylian crocodylomorphs (such as protosuchids, gobiosuchids and shartegosuchoids; Pol & Norell, 2004; Clark, 2011; Irmis et al ., 2013; Buscalioni, 2017; Dollman et al ., 2018), for which a series of cranial specialisations have been previously reported (Buscalioni, 2017; Dollman et al ., 2018). Among these, modifications related to brachycephaly (e.g.…”

Crocodylomorph cranial shape evolution and its relationship with body size and ecology

“…Apart from notosuchians, other crocodylomorph subgroups contribute to the higher disparity of terrestrial forms, mainly non-mesoeucrocodylian crocodylomorphs (such as protosuchids, gobiosuchids and shartegosuchoids; Pol & Norell, 2004;Clark, 2011;Irmis et al, 2013;Buscalioni, 2017;Dollman, Clark, Norell, Xing, & Choiniere, 2018), for which a series of cranial specializations have been previously reported (Buscalioni, 2017;Dollman et al, 2018). Among these, modifications related to brachycephaly (e.g.…”