Estimating sauropod body mass and gait

Abstract

This dissertation presents an analysis of recent and fossil tracks with quantitative and in-terdisciplinary methods for estimating the body mass and locomotion of a sauropod trackmaker. By employing methods from natural and engineering sciences, this research demonstrates that interdisciplinary research on tetrapod tracks can provide insights beyond conventional paleontological research. The novelty in this dissertation is that it brings as-pects from traditional vertebrate ichnology together with modern methods and considera-tions from biomechanics and soil mechanics to gain additional information about sauropod paleobiology.<br /> Track and trackways are structures left behind by an animal. Their formation is dependent on the substrate (i.e., soil or sediment) that contains the tracks, as well as the anatomy and locomotion of the trackmaker. Fossil tracks can provide a great deal of information about the extinct trackmaker, such as type, size, speed, behavior, and even pathologies. Although, it would be intuitive to think that the body mass of the trackmaker can also be determined from tracks, this has not been done before. Particularly, since body mass, which, for example, can reach record-breaking values in the case of the sauropod dinosaurs, is one of the fundamental attributes of any animal. Common mass estimation methods require body fossils for reconstructing density/volume or to make scaling relationships from long bones. However, body fossils are usually not available in most tracksites due to preservation conditions. Thus, estimating the weight of the trackmaker from its tracks, both extant and extinct, is the object of research in this dissertation.<br /> For determining the exact geometry and dimensions of both recent and fossil tracks, pre-cise documentation is required. Photogrammetry is a method from geodesy that has proved to be very useful for vertebrate ichnology. It uses digital images to generate three-dimensional (3D) models. The interpretability of these 3D models is improved by the geo-logical method of vertical exaggeration, which stretches the vertical axis of a model to vis-ualize previously unseen structures in the tracks. Applying vertical exaggeration is novel to vertebrate ichnology and reveals important information of the trackmaker from its tracks, such as travel direction and anatomical details of the hands and feet.<br /> To test if estimating the weight from fossil tracks is feasible, studying recent trackmakers for calibration is necessary. Elephants are the largest living land animals and often used as living analogs to the extinct sauropod dinosaurs. Elephant footprints are digitized and used as the basis of a numerical simulation (finite element analysis), constrained by the substrate properties. The load required to generate these footprints was reconstructed and the ele-phant’s weight was back calculated. Although, weight estimation for a recent trackmaker is possible with an error of about 15%, careful assessment of the influence of the trackmak-er’s locomotion is also important. For fossil trackmakers, precise evaluation of the locomo-tion, let alone the gait, is difficult to ascertain from tracks. The main gaits, such as walk, trot, pace, and gallop, are determined by studying horses, which make them a prime exam-ple for understanding locomotion from tracks. Together with basic estimations of the trackmaker’s size, it is possible to estimate the gait from tracks. Different gaits also mean varying distribution of the mass among the limbs during locomotion, which is of particular interest for any mass estimation approach on tracks. For instance, the fraction of the weight distributed on the hindlimbs is high when the center of mass of the animal is positioned posteriorly and low when the position of the center of mass is positioned anteriorly.<br /> Considering all these influencing factors, mass estimation and reconstruction of movement are possible from recent, as well from fossil tracks. For example, in the case of a 150 million year old sauropod trackway, the mass of the trackmaker was estimated to be about 16 tonnes, which is in good agreement with other mass estimates from body fossils. With these results, this research intends to provide a foundation for future applications of the gait and mass estimation approaches based on tracks, and hopes to inspire others to employ interdisciplinary methods to tetrapod tracks to exploit their, often underestimated, high information content.