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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 B LENDER  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.

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