The forelimbs of theropods (bipedal, predatory dinosaurs) have become greatly reduced in several different lineages, each time with vastly different results. The function, development, and evolutionary history of reduced forelimbs remains a mystery. In this project, detailed analysis of the shape and bony features (osteology) and the musculature of the arms of key theropod species will allow for identification of the most important features in the evolution of highly reduced forelimbs and how reduced forelimbs were used over time. This information will be used to examine the trends in the osteology, musculature, and overall size of the forelimb across the entire theropod family tree.
This study provides an engaging backdrop for educating the public about evolution in the context of vestigialization and the transitional stages of limb reduction (something that has happened in numerous vertebrate groups). Moreover, this project will advance understanding of the fundamental functional structure of the theropod forelimb and allows for further studies of forelimb function, such as grasping, predation, and the origin (and loss) of flight. New insights into the evolution and function of dinosaur forelimbs generated by this study will be further used for reconstructions of theropods for museum displays and educational television programs.
One of the great remaining mysteries of the evolution and behavior of predatory dinosaurs (theropods) consists of the function of their forelimbs. These dinosaurs were bipedal and eventually evolved into birds. Before their arms became wings, they were many different shapes and sizes and possibly capable of accomplishing many different goals. There are a few groups of theropods in which the forelimbs are very small compared to the rest of their body, and it is often assumed that these forelimbs could not have been of use to the animal. This project sought to thoroughly investigate the anatomy, evolution, and possible functions of the theropod forelimb with a focus on those theropods with relatively small forelimbs. The research focused on figuring out what the muscles of the theropod forelimb would have looked like and how those muscles evolved along the evolutionary family tree. Additionally, the project looked for any evolutionary trends in the relationship of forelimb size to overall body size in theropods. The results of this project established the baseline arrangement of muscles in a primitive theropod dinosaur, then used this starting point to investigate changes in the musculature in two groups that show reduction in forelimb size: abelisaurids and tyrannosaurids. It was found that even though the forelimbs of these animals, which include Tyrannosaurus rex, were small, there is substantial evidence that they still were used by their owners. Evidence of large muscles in the forelimbs suggest that they may have used their forelimbs during predator-prey interactions or when interacting with other members of their species. During the course of this project, a new technique was developed that brings statistical rigor to soft tissue reconstruction in extinct animals. This technique will be of great use to future researchers who want to learn about soft tissues that are not usually preserved in the fossil record, such as muscles or organs. This project also found that the evolution of forelimb size in theropods is more complex than previously thought. By using statistical techniques that take into account the shared evolutionary history of theropods, it was shown that no one trend can account for forelimb evolution within the group, and that different families have their own unique trajectory. Furthermore, optimal ratios of forelimb size to body size were under active selection in different groups of theropods, so that while tyrannosaurs were undergoing forelimb reduction, those dinosaurs that were on the evolutionary line to birds were undergoing forelimb elongation as their arms became more and more wing-like. These results greatly expand our understanding about how the forelimbs of theropods may have been evolving over their entire lineage, and what factors may have influence their evolution. They also provide important information about theropod forelimbs that is vital to future research in the field, including basic knowledge of the musculature and the overall evolutionary patterns. Additionally, the new information about forelimb function in theropods can be used to improve museum displays, educational materials, and even films in the realistic portrayal of dinosaurs and how they may have behaved.