Osteohistology of two phorusrhacids reveals uninterrupted growth strategy.

Phorusrhacidae were apex predators that primarily dominated South America ecosystems for at least 40 million years with their imposing size and predatory lifestyle-yet some aspects of their biology remain poorly understood. Osteohistology is a tool for understanding growth dynamics and biomechanical adaptations. Despite their ecological significance, histological analyses of these birds were non-existent until the recent publication of a sampled metatarsus. Here, we present the first osteohistological data of a phorusrhacid femur together with other hindlimb elements representing two taxa, Andrewsornis abbotti and Physornis fortis, from the late Oligocene of Argentina. Ground-section microscopy reveals highly vascularized fibrolamellar bone tissue, indicating rapid, sustained growth with lines of arrested growth appearing only in the outer circumferential layer. Extensive secondary remodeling, particularly at muscle attachment sites, highlights high mechanical stress, consistent with active cursorial locomotion. Body mass estimates (60-89 kg for Andrewsornis abbotti) and high relative cortical thickness in both specimens (bone compactness = 0.55-0.78) provide additional data for comparative studies. Our findings confirm the uninterrupted growth strategy of phorusrhacids, consistent with other large-bodied flightless neognaths but in contrast to most giant flightless palaeognath birds.