Palate evolution in early-branching crocodylomorphs: Implications for homology, systematics, and ecomorphology

Abstract

Living crocodylomorphs have an ossified secondary palate with a posteriorly positioned choana that enables their semi-aquatic, predatory ecology. In contrast, the earliest branching members of Crocodylomorpha have an open palate with anteriorly positioned choanae. The evolution of an ossified secondary palate and a posteriorly positioned choana features strongly in hypotheses of broad-scale phylogenetic relationships within Crocodylomorpha. Renewed investigations into palatal morphology among extinct members of the clade show surprising variability in the anatomy of the palate, with at least one and potentially a second independent occurrence of "eusuchian-type" palate outside of Eusuchia. Understanding the trajectory of crocodylomorph palatal evolution is, therefore, a key to inferring crocodylomorph interrelationships and ecomorphology. To document early-branching crocodylomorph palatal anatomy, we developed an anatomical comparative dataset using computed tomography scan data and literature, comprising 12 early-branching crocodylomorph taxa. To understand discrete phenotypic changes in palatal structure, we compiled a phylogenetically broadly sampled character-taxon matrix from the existing literature, and revised its palatal characters, adding 10 new palatal characters. Our comparative anatomical investigations allow us to propose an adapted hypothesis for the closure of the palate and the posterior migration of the choana. Our phylogenetic findings corroborate previous research showing that non-crocodyliform crocodylomorphs ("sphenosuchians") are paraphyletic, with the exclusion of the clade Hallopodidae. Non-mesoeucrocodylian crocodyliforms ("protosuchians") are paraphyletic, but form three monophyletic clades: Notochampsoidea, Shartegosuchoidea, and Gobiosuchidae. We find a potential association between secondary palate development and dietary shifts, particularly with regard to hypothesized origins of herbivory.

Keywords: crocodylian; phylogenetics; secondary palate.

FIGURE 1

Palatal view of the three‐dimensional (3D) visualizations of the palates of “sphenosuchians” (a) Dibothrosuchus, (b) Litargosuchus, (c) Sphenosuchus. Ch r, choanal ridge; ect, ectopterygoid; j, jugal; max, maxilla; pal, palatine; pal d, palatine depression; pm, premaxilla; pt, pterygoid; v, vomer

FIGURE 2

Interpreted anatomical reconstruction of the palatal view of the secondary palates of ‘sphenosuchians’ (a) Dibothrosuchus, (b) Litargosuchus, (c) Sphenosuchus, and (d) Terrestrisuchus. Missing or incomplete bones are in a lighter shade with a dashed line. Scale bar is 1 cm. Ch, choana; ch r, choanal ridge; i for, incisive foramen; pal d, palatine depression; sub fen, suborbital fenestra

FIGURE 3

Palatal view of the three‐dimensional (3D) visualizations of the secondary palates of “protosuchians” (a) Protosuchus, (b) Orthosuchus, (c) Nominosuchus, and (d) Shartegosuchus. Ch g, choanal groove; ch r, choanal ridge; ect, ectopterygoid, j, jugal, max, maxilla, pal, palatine, pm, premaxilla, pt, pterygoid, v, vomer

FIGURE 4

Interpreted anatomical reconstruction of the palatal view of the secondary palate “protosuchians” (a) Orthosuchus, (b) Protosuchus, (c) UCMP 97638, (d) Gobiosuchus, (e) Sichuanosuchus, (f) Nominosuchus, (g) Shantungosuchus, (h) Fruitachampsa, and (i) Shartegosuchus. Missing or incomplete bones are in a lighter shade with a dashed line. Scale bar is 1 cm. Ant pal fen, anterior palatal fenestra; ch, choana; i for, incisive foramen; naso phy, ventrally open nasopharyngeal duct; sub fen, suborbital fenestra

FIGURE 5

Time calibrated strict consensus of the six MPTs (length = 1,263, CI = 0.446, and RI = 0.654)

FIGURE 6

Major palatal configurations in palate evolution of crocodylomorphs represented by 19 skulls in palatal view. Palatal configurations were reconstructed based on CT scan data, or published data of known crocodylomorphs. Asterisks indicate crocodylomorph taxa not sampled in this analysis. Areas of the palate not preserved or not observable are illustrated with outlines used dashed lines and transparent coloring. Notable changes in anatomical palatal structures are bullet‐pointed, and the evolutionary stages are separated by black dashed lines. Palatal bones are colored, ectopterygoid, purple; jugal, green; maxilla, red; palatine, pink; premaxilla, light blue; pterygoid, brown