Comparative cranial biomechanics reveal macroevolutionary trends in theropod dinosaurs, with emphasis on Tyrannosauroidea

Tyrannosaurus is viewed as a model organism in vertebrate paleontology, with numerous studies analyzing its feeding biomechanics. Nonetheless, the evolution of this feeding performance has been under-addressed in Tyrannosauroidea, especially in basal tyrannosauroids. Here we used muscle-force reconstruction and finite element analysis (FEA) to quantify the cranial performance of tyrannosauroids and outgroup theropod clades. 2D (planar) cranial models, set to standardized skull lengths and jaw adductor forces, were used to analyze the evolution of feeding behavior in a large sample size of Tyrannosauroidea and other theropods. Sampled stresses matched well between planar and 3D analyses of three disparately shaped crania, suggesting valid interpretations from 2D models along the lateral sides of theropod crania if symmetrical bite loadings are assumed. We traced cranial evolution by sampling stresses at homologous points of theropod crania and input their average stress values into a maximum likelihood ancestral character state estimation. Our results show tyrannosauroids having moderate-to-low cranial stresses. We further tested whether the average stress value correlates with head size through phylogenetic generalized least square regressions. We found that 2D FEA provides significant information on the evolution of feeding performance in a major dinosaur clade. Along internal branches of Tyrannosauroidea, hypothetical common ancestors exhibit low cranial stress values owing to a combination of robust skulls and cranial protuberances. These traits may have been passed down to later tyrannosauroids, enabling them to handle high forces. Our results additionally demonstrate a possible correlation between cranial shape (brevirostrine versus longirostrine) and inferred cranial performance in non-tyrannosauroid clades.