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Making use of a track-trackmaker association: locomotor inference of an early amniote with help of "fossilized behavior"
Abstract
A deeper understanding of an extinct species’ paleobiology is a common goal of studies into the functioning of the musculo-skeletal system which is only fragmentarily preserved in fossil tetrapod remains. The combination of a body fossil and ichnofossils stemming from the same species offers the chance to link fossilized anatomical features with “fossilized behavior”. In a recent project, a unique combination of an articulated complete specimen of Orobates pabsti (Diadectidae) and Ichnotherium sphaerodactylum tracks to which O. pabsti has previously been assigned as the trackmaker was exploited for an in-depth reconstruction of the locomotion of this species. Phylogenetic analyses place O. pabsti close to the crown group node of amniotes and often recover the diadectids as the fossil sister taxon to modern amniotes. Early amniotes became increasingly independent of aquatic habitats and this key evolutionary transition is reflected in the reconstructed locomotor behavior of O. pabsti. Research into the fossil’s anatomy, the fossil’s potential joint mobility and potential movements within the I. sphaerodactylum tracks, a comparative analysis of extant tetrapod locomotor mechanics, and finally into a fossil-inspired walking machine (OroBOT) will be summarized.
... Moreover, we used the animated digital skeleton from a previous study . Please refer to our previous publications for specifics of how the digital specimen was generated and the rationale for animation properties (Nyakatura et al., 2015Nyakatura, 2017Nyakatura, , 2019. In brief, the holotype specimen was microCT-scanned and individual bone fragments were segmented and thus freed from the surrounding rock matrix. ...
... For the animated step cycle, fossil trackways assigned to Orobates (cf. Voigt et al., 2007) were analyzed and used as a hard constraint, i.e., each manus and pes was forced to match the trackways (Nyakatura, 2019;. Limb and spine kinematics were animated in accordance with general biomechanical principles of sprawling tetrapod locomotion identified in a previous study and were anatomically plausible in terms of joint mobility . ...
The stem amniote Orobates pabsti has been reconstructed to be capable of relatively erect, balanced, and mechanically power-saving terrestrial locomotion. This suggested that the evolution of such advanced locomotor capabilities preceded the origin of crown-group amniotes. We here further investigate plausible body postures and locomotion of Orobates by taking soft tissues into account. Freely available animation software BLENDER is used to first reconstruct the lines of action of hindlimb adductors and retractors for Orobates and then estimate the muscle strain of these muscles. We experimentally varied different body heights in modeled hindlimb stride cycles of Orobates to find the posture that maximizes optimal strains over the course of a stride cycle. To validate our method, we used Caiman crocodilus. We replicated the identical workflow used for the analysis of Orobates and compared the locomotor posture predicted for Caiman based on muscle strain analysis with this species’ actual postural data known from a previously published X-ray motion analysis. Since this validation experiment demonstrated a close match between the modeled posture that maximizes optimal adductor and retractor muscle strain and the in vivo posture employed by Caiman, using the same method for Orobates was justified. Generally, the use of muscle strain analysis for the reconstruction of posture in quadrupedal vertebrate fossils thus appears a promising approach. Nevertheless, results for Orobates remained inconclusive as several postures resulted in similar muscle strains and none of the postures could be entirely excluded. These results are not in conflict with the previously inferred moderately erect locomotor posture of Orobates and suggest considerable variability of posture during locomotion.
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