Dinosaur disparity differently: contrasting pattern of cranial variation and macroevolution across dinosaurs and crocodylomorphs

FELICE, R N; WATANABE, A; CUFF, A R; POL, D

Congreso; 79° Annual Meeting of the Society of Vertebrate Paleontology; 2019

Archosaurs exhibit tremendous variation in cranial structure and function, generated through distinct developmental programmes. Moreover, archosaur clades exhibit heterogenous patterns of ecological and morphological disparity. What factors have driven these divergences across archosaurs? Using a high-dimensional surface geometric morphometric approach, we quantified 3-D skull shape evolution and integration (trait correlations stemming from genetic, developmental, or functional interactions) in avians (n=353), non-avian dinosaurs (n=37), and extinct and extant crocodylomorphs (n=45) with 757-1291 landmarks and sliding semilandmarks. Maximum likelihood and covariance ratio analysis demonstrates that all archosaurs exhibit high integration in the occipital region, but crocodylomorphs show higher integration among facial elements than dinosaurs, including birds. In contrast to non-avian dinosaurs, birds exhibit high pterygoid-quadrate integration, presumably related to cranial kinesis. Crocodylomorphs and birds similarly exhibit high disparity (quantified as Procrustes variance) in the face and low disparity in the neurocranium. The highest rates of evolution occur in the anterior rostrum of birds, but in the nasals of crocodylomorphs. Disparity and rates are largely correlated in birds and non-avian dinosaurs, but decoupled in crocodylomorphs. In particular, the crocodylomorph pterygoid displays high disparity, but low rates of evolution, suggesting that disparity in the secondary palate in this clade accumulated slowly. Non-avian dinosaurs exhibit different patterns, with elevated disparity and rates in the parietal, quadratojugal, and jaw joint and low disparity in the anterior face. This distribution is driven partially by the prevalence of cranial ornaments (e.g., ceratopsians) and variation in the temporal fenestrae. With cranial variation primarily localized to the posterior skull, non-avian dinosaurs are unique among archosaurs. This pattern likely reflects diverse ?chewing? mechanisms and display structures in Ornithischia, compared to birds that are constrained by high encephalization to have limited variation in the adductor chamber. Crocodylomorphs show rapid disparification in the face, similar to birds, and slower disparification in the pterygoid related to jaw articulation and extension of the secondary palate. Combined, these results emphasize the dominance of divergent feeding mechanisms in driving disparate patterns of cranial evolution across archosaurs.