Convergent Evolution of a Eusuchian-Type Secondary Palate within Shartegosuchidae

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 earlier-diverging 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.