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

ABSTRACT

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.

KEYWORDS:

• Muscles
• appendicular skeleton
• functional morphology
• Unenlagiinae
• Buitreraptor

Acknowledgments

Thanks to Mirta Ortiz, Andrea Arcucci, and Ana Ochoa Porcel for donating the bird specimens used for dissections. The dissections were performed with the assistance of Anabel Di Carlantonio, from the Área de Zoología, UNSL. The authors also thank curators of different institutions where fossil and osteological specimens were examined: Pablo Tubaro, Darío, Lijtmaer, and Yolanda Davies, from the División de Ornitología, MACN, Buenos Aires, Argentina; Sergio Bogan, from the Fundación de Historia Natural ‘Félix de Azara’, Universidad Maimónides, Buenos Aires, Argentina; Carlos Muñoz, from the MPCA, Cipolletti, Río Negro, Argentina; Pablo Chafrat, from the MPCN, General Roca, Río Negro, Argentina; Daniel Brinkman, from the YPM, Connecticut, USA; and Carl Mehling, from the AMNH, New York, USA. Thanks to Alejandro Otero and Daisuke Suzuki for providing us with bibliographical material and for their valuable comments. The comments and suggestions of Mattia Baiano, Jorge Meso, and an anonymous reviewer have notably improved the quality of this work.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Fondo para la Investigación Científica y Tecnológica from Argentina [PICT 2014-1449]; and the Secretaría de Ciencia y Tecnología UNSL [PROICO 2-0618].