Morphometric Changes In Semicircular Canal Shape Within Theropoda (Dinosauria: Saurischia) And Their Dietary Implications
Authors:   , King James 
Published: February 2023

heropods were a suborder of dinosaurs that displayed a large variety of dietary preferences throughout the Mesozoic and into the Cenozoic as modern birds. Being ancestrally carnivorous, many of the large-bodied early theropods were hypercarnivorous; however, members of Theropoda diversified their diets into omnivory and herbivory. Modern vertebrates with different dietary preferences have different spatial sensitivities to changes in head and body movement. In order to test if theropod diet plays a major role in the rostral (RSC), caudal (CSC), and lateral (LSC) semicircular canal shape, therizinosaurs, tyrannosaurids, ratites, an allosaurid, an ornithomimid, and a phorusrhacid were analyzed via 2D Geometric Morphometrics to see if their cross-sectional semicircular canal shapes differed based on the respective diets of each taxa. Each canal sensed the pitch (RSC), roll (CSC), and yaw (LSC) movements of the head and would allow for head and body to compensate for the movement in order to maintain balance. This study applied a Principal Components Analysis (PCA) and Analysis of Variance (ANOVA) to test for shape change among the semicircular canals of carnivorous, herbivorous, and omnivorous dinosaurs and bird canals. Neither the LSCs nor the CSCs showed patterns that could be interpreted as diet-based groupings among all of the species tested. The RSC graphs, however, clustered the taxa into separate groups based on their trophic level. The PCA demonstrated that the cross-sectional shapes of dinosaurs, ratites, and phorusrhacids are based off of diet (PC1) and the angularity of each shape (PC2). Grouping the taxa by diet and shape angularity implies that there is a spatial sensitivity difference among the dataset based around the diet/foraging strategy of each dinosaur and bird. The ANOVA attempted to assess the amount of variation between the carnivores, herbivores, and omnivores; however, the herbivores failed tests for normality and equality of variance. This indicates that variation among the levels of diet could not be measured. A normality and variance failure implies that the shapes of the herbivores RSCs were statistically different from the rest of the taxa sampled; however, a larger dataset should be retested to confirm that the failure did not come from sampling bias. The clustering of the carnivores show a difference between dinosaurs that are thought to be predaceous carnivores (Alioramus, Allosaurus, and Gorgosaurus) and those that are thought to be scavengers or opportunistically carnivorous (Tyrannosaurus). Llallawavis, a phorusrhacid, plotted near the omnivores even though it is assumed to be a carnivore. One interpretation of this result is that Llallawavis was more of an opportunistic carnivore than an active predator. The omnivores (ostrich, emu, Falcarius, and Struthiomimus) grouped together in both axes of the RSC. Falcarius fell out closer to the carnivores in both axes while still maintaining a close proximity to the other omnivores. This pattern is interpreted as being an evolutionary holdover from Falcarius’ carnivorous ancestry and not an indication of a carnivorous basal therizinosaur; dentition and postcranial anatomy support this interpretation based on the denticle density and size as well as the pubis in the pelvic girdle. The herbivores (cassowary, Nothronychus, and Erlikosaurus) grouped separately from the rest of the specimens in the dataset. The cassowary plotted closer to the omnivores along the y-axis; however, this was expected due to it supplementing its frugivorous diet with insects and arthropods. Nothronychus and the cassowary plotted next to each other supporting a specialized diet for Nothronychus; however, no other interpretations for Nothronychus could be made outside of herbivory. Erlikosaurus grouped further away from Nothronychus and the cassowary by itself implying that it may be closest to a true herbivore out of all of the organisms in the study.