MUSCULAR RECONSTRUCTION OF THE HINDLIMB SUPPORTS A SPECIALIZED CURSORIAL LOCOMOTORY HABIT IN BUITRERAPTOR GONZALEZORUM (THEROPODA, PARAVES)

FEDERICO A. GIANECHINI; LUCIANO COLLI

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2022

Buitreraptor gonzalezorum is a non-avian theropod from the Upper Cretaceous (Cenomanian) ofnorthern Patagonia, Argentina. Most phylogenetic analyses recovered it as a member of theUnenlagiinae, a subfamily of Dromaeosauridae. Non-avian theropods were terrestrial bipedal animalsalthough some taxa present a morphology related to a specialized cursorial habit (e.g.,arctometatarsal condition). Previous studies identified morphological characters that suggest thistype of specialization in B. gonzalezorum. In order to provide morphofunctional information related tolocomotion, a reconstruction of the hindlimb musculature of this taxon was made. The study wasbased on the specimens MPCA 245 (holotype), MPCA 238, and MPCN-PV-598, which include thepelvic girdle, the femur, tibia, fibula, metatarsals, and pedal phalanges, many of them well-preserved.In B. gonzalezorum, the dorsal border of the ilium is laterally everted and consequently, the origin ofthe M. iliotibialis is laterally displaced, likely increasing the hip abduction moment arm of this muscle.This would have counteracted an over-adduction generated by the reaction force of the ground at themoment of foot support, an effect observed in bipedal cursorial animals, such as the ostrich. Thepreacetabular iliac blade is anteroposteriorly elongated, indicating that the iliac protractor muscles,such as the M. iliofemoralis, originated more anteriorly with respect to the hip joint, increasing theirmoment arms. Instead, the postacetabular iliac lamina and ischium are short, indicating that retractormuscles originate closer to the hip joint (e.g., M. iliofibularis) or have a more proximal insertion (e.g., M.caudofemoralis). These features possibly reduce the moment arm of the retractors. Finally, the M.tibialis anterior, one of the main ankle flexors, inserts proximally on the metatarsus, decreasing itsmoment arm. Generally, in cursorial tetrapods, main limb protractors have relatively larger momentarms than main limb retractors, and muscles implied in locomotion tend to insert more proximally,especially those that extend the limb. Moreover, the proximal insertion of the M. tibialis anterior in B.gonzalezorum could indicate that this animal flexed the ankle with a high angular speed, a necessaryfeature for the large range of motion in which distal limb segments and joints move duringlocomotion. These results provide additional evidence supporting a specialized cursorial habit for thisspecies. This proposed habit can be corroborated in the future with a tridimensional digitalmusculoskeletal modeling of the hindlimb. However, this study demonstrates the importance ofmuscular reconstructions for functional morphology studies in fossil vertebrates.