Forelimb orientation and locomotion of sauropod dinosaurs: insights from the ?Middle Jurassic Tafaytour tracksites (Argana Basin, Morocco)

ABSTRACT

Forelimb posture in sauropod dinosaurs is still poorly understood. Although a laterally directed (semisupinated) manus is the plesiomorphic condition in sauropodomorphs, the sauropod track record prevailingly shows anterolateral to anterior manus orientations, suggesting a high degree of manus pronation. The ?Middle Jurassic Tafaytour tracksites described herein preserve at least 10 trackways, nine of which show laterally oriented, and in two examples even posterolaterally oriented, manual impressions. Located in the Argana Basin of the Western High Atlas, Morocco, the tracksite yields hundreds of footprints on a highly bioturbated surface, including examples with well-preserved digit and claw impressions. Footprint morphology and trackway configuration vary greatly between trackways. A literature review indicates that laterally directed manual impressions are restricted to small- and medium-sized trackmakers. Statistical analysis was performed on a larger sample (n = 79) of small sauropod trackways from around the world, indicating that lateral manual orientations are correlated with low locomotion speeds and narrow trackway gauges. Manus pronation in sauropods is hypothesized to occur when the forelimb is actively contributing to the propulsion of the animal, which would be the case at faster speeds or at wider trackway gauges where the center of mass is located more anteriorly. We present new approaches to the quantitative analysis of trackway data. Mean configuration plots allow for direct graphical comparisons of different trackways. Two types of trackway asymmetries are defined and quantified. The apparent glenoacetabular distance (GAD) represents a feasible proxy for body size, at least for the smaller sauropod trackmakers analyzed herein.

ACKNOWLEDGMENTS

We are grateful to S. Läbe and P. M. Sander for fruitful discussions and comments on an early version of the manuscript. J. Knüppe is thanked for contributing artwork. We furthermore thank all participants of the field trip of the 2015 First International Congress on Continental Ichnology (ICCI) for their support. In particular, we thank D. Falk for his help in the course of taking the photogrammetric photographs, and F. Scholze, A. Uchman, H. Klein, M. Masrour, and F. Pérez-Lorente for contributing additional photographs. Last but not least, we thank editor S. Salisbury as well as D. Castanera and J. Nair for their constructive reviews, which improved the manuscript. The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. Volkswagen Stiftung grant no. 85 882 in the grant initiative ‘Research in Museums’ supported this research.