Morphological evolution and functional consequences of giantism in tyrannosauroid dinosaurs
By: Andre J. Rowe, Emily J. Rayfield
Summarized by: This article was summarized by Gillian Rohde, who is a senior biology major at Binghamton University. She is on a pre-physician assistant track. Outside of school, she enjoys reading, hiking, and drawing.
Data: Seven different skulls across six species within the family Tyrannosauroidae were scanned for this study: Raptorex kriegsteini, Bistahieversor sealeyi, Albertosaurus sarcophagus, Alioramus altai, Daspletosaurus torosus, and two Tyrannosaurus rex (one juvenile and one adult). The family Tyrannosauroidae is defined by shared characteristics such as broad skulls, small forelimbs (arms), long legs, and a meat-based diet. However, species of this family vary in size, skull morphology, and ecological niche. The researchers used complete skull specimens from different museums and research institutions. R. kriegsteini is a species defined by a single specimen thought to be a juvenile that measures ten feet long. B. sealeyi as an adult measured 30 feet long and had many unique features including an extra opening above the eyes to lighten the skull. A. sarcophagus measured 30 feet long and are thought to have been apex predators of their environment. A. altai is estimated to have measured 20 feet long and is defined by a low and long skull decorated with crests above the nose. D. torosus measured about 30 feet long and are defined by their fused cranial bones and orbital crests. Finally, T. rex, the most well-known species of the family, was an apex predator that measured over 40 feet long.
Hypotheses: Researchers posed two hypotheses. The researchers first proposed that larger tyrannosaurid species experienced higher stresses within their skulls during feeding because of their large size. Researchers thought these larger species would also be able to absorb these higher stresses without breaking. The researchers then proposed an alternative hypothesis that stated smaller-bodied tyrannosaurid species would experience higher feeding stress than larger species, because larger species have more robust skull morphology: wider-set jaws and a deeper upper skull.
Methods: The researchers scanned the skulls through surface scanning or CT scanning to create 3D models. Surface scanning uses light to measure the surface of an object, and the data collected creates a digital model on a computer. CT scanning uses X-rays to create a 3D model. The scientists used the 3D models from each specimen, used computer programs to perform simulations that would mimic the feeding stresses on the 3D models, and calculated those different stresses. The researchers measured von Mises stresses, which are equivalent values made up of tensile, compressive, and shear stresses. Tensile stress is the force per unit area that a material experiences due to pulling. This measures how much a material (in this case, the skull bones) resists being pulled apart. Compressive stress is the force per unit area that a material experiences due to a force trying to shorten or compress a material. This measures how much a material resists being compacted. Finally, shear stress is the force from per unit area a material experiences due to another material sliding against it, which causes tearing. A larger stress value means that a material has lower resistance to breaking. These stresses all occur during feeding from the upper and lower jaw grinding against each other and the food during chewing across all jawed animals.
The study also measured principal strain, which is the maximum and minimum amounts of stress a material experiences under load. Load is an external force or pressure applied to an object, causing it to experience stress. The scientists calculated these feeding stresses for the different Tyrannosauroidae skulls at both their actual size and at a size corrected scale. The size-corrected models made it so all the skulls were all the same size, which allowed them to compare the relative stress based solely on skull shape. The actual size model is based on both skull morphology and the size of the skull, which makes it biased towards larger specimens.
Results: The study found that higher von Mises stresses (i.e., combined stress from chewing) were experienced in midsized tyrannosaurids when measuring skulls at their actual size. This is because larger tyrannosaurids ate larger prey, which required an increased bite force. However, to make up for this increased bite force, large tyrannosaurids had increased resistance to stress due to their wider-set jaws and thicker skull bones. This increased resistance is shown in the “size corrected” scale in Figure 1. When researchers examined the stresses for the size corrected models, the smaller species that had narrower jaws experienced higher stress and thus were more likely to experience breakage during feeding. This lack of stress resistance is why smaller tyrannosaurids ate smaller prey. Max principal strain (from jaws grinding against food) was also the highest in the smaller tyrannosaurid species. These findings support hypothesis one, where larger tyrannosaurids experience higher feeding stress due to the size of their prey, but to make up for this, they have greater stress resistance than smaller species.
Importance of the Study: This study is important for learning more about how members of the Tyrannosauridae family fed and what niche each species occupied. A niche is the role that an organism plays in its environment. Smaller species of Tyrannosauridae did not share the same apex predator role that larger species had. Smaller species of Tyrannosauridae would have hunted smaller prey and thus not competed with larger species. The high stress resistance in larger tyrannosaurids may explain their evolutionary success as apex predators.
Broader Implications Beyond This Study: The methods from this study can be applied to other groups of carnivorous organisms and thus be used to figure out the niches of other species. The feeding stress and stress resistance can be used to infer the size of prey. Larger predators who hunted larger prey would have increased stress resistance.
Citation: Rowe, A. J., & Rayfield, E. J. (2024). Morphological evolution and functional consequences of giantism in tyrannosauroid dinosaurs. IScience, 27(9). https://doi.org/10.1016/j.isci.2024.110679