Austin Andrus, Dr. Brooks Britt, Department of Geological Sciences
I recently discovered a new genus of reptile from the Triassic period (roughly 225 million years ago). Reptiles like it are called “drepanosaurs,” and share many unique features such as large claws, unusual, humped shoulder blades, opposable fingers, and prehensile tails. Many of these structures appear to be well-suited for climbing, and the scientific community generally considers drepanosaurs to be arboreal. My new specimen, however, is more complete than previous discoveries have been, and it includes skeletal features that could indicate a digging lifestyle. Digging and climbing are on opposite ends of the behavioral spectrum, so my study seeks to determine which lifestyle is the best match for my animal. In order to do this, I used both subjective and objective approaches. Subjectively, I compared the anatomy of my new drepanosaur to the anatomy of modern animals to which have been used as metaphors for drepanosaur movement. Objectively, I collected skeletal measurements from modern digging and climbing animal populations in order to statistically determine whether my drepanosaur has the adaptations that predict digging and climbing. My study was restricted to the features of the drepanosaur forelimb, leaving other skeletal regions as potential candidates for future research.
Methods: Comparative Anatomy
Previous researchers have often suggested that drepanosaurs may have climbed, pointing to chameleons and pigmy anteaters as models for their arboreal behavior. I proposed a third model, which compares my drepanosaur to modern digging animals such as moles. I assessed similarities and differences between the functional anatomy of my drepanosaur and of the modern animals in these three models. Each model has its strengths and weaknesses. For example, the chameleon shares a unique shoulder orientation with my drepanosaur, which it uses to keep its arms underneath it and grasp branches. However, chameleons also possess very flexible forelimbs for reaching between perches, whereas the forelimbs of the drepanosaur are very rigid and would prevent this activity. To enhance my study of drepanosaur functional anatomy, I worked closely with a paleoartist and an animator. We sculpted and animated a digital drepanosaur skeleton in order to estimate the range of motion that it could move through, and which locomotion strategies were most realistic for the creature.
Methods: Statistical Analysis
Other studies have suggested that there are specific skeletal adaptations that appear in modern digging and climbing animals to help them in their specialized tasks. Diggers have thick arm bones with large muscle attachments to increase mechanical advantage when shoveling earth. Climbers, on the other hand, typically have highly-curved claws which aid in gripping tree bark. I gathered my own measurements from modern animal populations to confirm the validity of these trends, and compared them to the proportions of drepanosaur bones.
To confirm that the arm bones of digging mammals develop differently than those of climbers, I measured the proportions bones from 109 mammals. I chose to use mammals instead of reptiles because digging reptiles usually do not use their forelimbs, as the drepanosaur appears to have done. The difference between climber proportions and digger proportions was highly statistically significant, and the proportions of my drepanosaur skeletons were very similar to those of the digging population.
Measuring claw proportions was less straightforward. In the past, several methods of measuring curvature have been proposed, with varying results. I photographed the claws of 59 reptile species, and categorized each species as either terrestrial or arboreal. I created digital outlines of the claws and used nine different strategies to measure their proportions. This allowed me to compare the strengths and weaknesses of each measurement strategy, and determine which best predict climbing behavior.
Among the three models that I considered for comparative anatomy, the digging model provided the best explanation of the anatomy of the drepanosaur forelimb, and the chameleon model provided the least explanation. The pygmy anteater model did not explain drepanosaur adaptations as comprehensively as did the digging model, but it offered useful insight into how an animal with many digging adaptations inherited from its ancestors may still live in the trees.
Statistical analysis of digging adaptations showed that the forelimb of my new drepanosaur is well-adapted to a digging lifestyle, while the results of the claw curvature study were varied. On average, climbing lizards have short, thick, curved claws unlike the long flat claws of the drepanosaur. Still, curvature varies greatly between species, and there is considerable overlap in shape between ground-dwellers and tree-climbers.
Discussion and Conclusions
Both qualitative and quantitative analysis of the forelimb of the new drepanosaur suggest that its features are best explained by a digging lifestyle. Still, it cannot be assumed that it did not climb at all. Adaptations in the neck, hind-feet, and tail (not thoroughly considered in this study) still suggest the possibility of climbing habits. It is probable that the new BYU drepanosaur may have existed in both biomes—digging for food and climbing for protection, or vice versa.
This study was only commented on the behavior of one drepanosaur, but its results may have implications for others. Perhaps other drepanosaurs should not be simply assumed to be arboreal. This study provides a model for thorough consideration of the behavioral potential in any extinct animal’s skeleton.