Evan Johnson‐Ransom

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.

Members of the dinosaur clade Spinosauridae had numerous traits attributed to feeding in or around water, and their feeding apparatus has often been considered analogous to modern crocodylians. Here we quantify the craniodental morphology of Spinosauridae and compare it to modern Crocodylia. We measured from spinosaurid and crocodylian skeletal material the area of alveoli as a proxy for tooth size to determine size‐heterodonty. Geometric morphometrics were also conducted on tooth crowns and tooth bearing regions of the skull. Spinosaurids overall had relatively large alveoli, and both they, and crocodylians, had isolated regions of enlarged alveoli. Spinosaurines also had enlarged alveoli along the caudal dentary that baryonychines lacked, which instead had numerous additional caudal tooth positions. Size‐heterodonty was positively allometric, and spinosaurids overlapped with generalist/macro‐generalist crocodylians of similar sizes. Spinosaurid crown shape morphologies overlapped with certain slender‐longirostrine crocodylians, yet lacked molariform distal crowns typical of most crocodylians. Spinosaurid rostra and mandibles were relatively deep with undulating margins correlating with local tooth sizes, which may indicate a developmental constraint. Spinosaurines had a particularly long concavity caudal to their rosette of anterior cranial teeth, with a corresponding bulbous rostral dentary. The spinosaurid feeding apparatus was well suited for quickly striking and creating deep punctures, but not cutting flesh or durophagy. The jaws interlocked to secure prey and move it deeper into the mouth. The baryonychines probably did little oral processing, yet spinosaurines could have processed relatively large vertebrates. Overall, there is no indication that spinosaurids were restricted to fish or small aquatic prey.

Tyrannosaurus has been an exemplar organism in feeding biomechanical analyses. An adult Tyrannosaurus could exert a bone‐splintering bite force, through expanded jaw muscles and a robust skull and teeth. While feeding function of adult Tyrannosaurus has been thoroughly studied, such analyses have yet to expand to other tyrannosauroids, especially early‐diverging tyrannosauroids (Dilong, Proceratosaurus, and Yutyrannus). In our analysis, we broadly assessed the cranial and feeding performance of tyrannosauroids at varying body sizes. Our sample size included small (Proceratosaurus and Dilong), medium‐sized (Teratophoneus), and large (Tarbosaurus, Daspletosaurus, Gorgosaurus, and Yutyrannus) tyrannosauroids, and incorporation of tyrannosaurines at different ontogenetic stages (small juvenile Tarbosaurus, Raptorex, and mid‐sized juvenile Tyrannosaurus). We used jaw muscle force calculations and finite element analysis to comprehend the cranial performance of our tyrannosauroids. Scaled subtemporal fenestrae areas and calculated jaw muscle forces show that broad‐skulled tyrannosaurines (Tyrannosaurus, Daspletosaurus, juvenile Tyrannosaurus, and Raptorex) exhibited higher jaw muscle forces than other similarly sized tyrannosauroids (Gorgosaurus, Yutyrannus, and Proceratosaurus). The large proceratosaurid Yutyrannus exhibited lower cranial stress than most adult tyrannosaurids. This suggests that cranial structural adaptations of large tyrannosaurids maintained adequate safety factors at greater bite force, but their robust crania did not notably decrease bone stress. Similarly, juvenile tyrannosaurines experienced greater cranial stress than similarly‐sized earlier tyrannosauroids, consistent with greater adductor muscle forces in the juveniles, and with crania no more robust than in their small adult predecessors. As adult tyrannosauroid body size increased, so too did relative jaw muscle forces manifested even in juveniles of giant adults.