Modeling Sprawling Locomotion of the Stem Amniote Orobates: An Examination of Hindlimb Muscle Strains and Validation Using Extant Caiman

Abstract

The stem amnioteOrobates pabstihas 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 ofOrobatesby taking soft tissues into account. Freely available animation software BLENDERis used to first reconstruct the lines of action of hindlimb adductors and retractors forOrobatesand then estimate the muscle strain of these muscles. We experimentally varied different body heights in modeled hindlimb stride cycles ofOrobatesto find the posture that maximizes optimal strains over the course of a stride cycle. To validate our method, we usedCaiman crocodilus. We replicated the identical workflow used for the analysis ofOrobatesand compared the locomotor posture predicted forCaimanbased 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 thein vivoposture employed byCaiman, using the same method forOrobateswas justified. Generally, the use of muscle strain analysis for the reconstruction of posture in quadrupedal vertebrate fossils thus appears a promising approach. Nevertheless, results forOrobatesremained 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 ofOrobatesand suggest considerable variability of posture during locomotion.