Pelvic and hindlimb muscular reconstruction of the paravian theropod Buitreraptor gonzalezorum and its palaeobiological implications

Soft tissue reconstructions of fossil vertebrates provide valuable data and support for functional interpretations. Here, we present the pelvic and hindlimb muscular reconstruction of Buitreraptor gonzalezorum, a unenlagiine dromaeosaurid theropod. Dromaeosaurids employed their hindlimbs for locomotion and predation, with a specialised pedal digit II. Using the 'extant phylogenetic bracket' methodology, we reconstruct 29 muscles out of the 37 typically found in extant archosaur hindlimbs. Some characteristics resemble those of specialised cursorial tetrapods. For example, M. iliotibialis shows an increased hip abduction moment arm, while primary protractors exhibit an increased hip flexion moment arm, and main retractors display a decreased hip extension moment arm. However, positive allometric growth of limb inertia relative to body mass and the gradual transition to a 'knee-based' locomotion observed in extant birds may have influenced muscle moment arms. Regarding predatory functions, the muscular features and length proportions of the metatarsus and pedal digit II phalanges suggest faster movements compared to derived dromaeosaurids. Also, a robust flexor muscle, likely M. flexor perforans et perforatus digiti II, was inserted on the proximoventral heel of the second phalanx of pedal digit II. These functional interpretations suggest the presence of specialised behaviours different from those observed in more derived dromaeosaurids.