Reassessing hindlimb morphology in greater and lesser rheas (Aves, Palaeognathae, Rheidae): New insights for fossil rheid identification

Abstract

Correct identification of Rhea americana and Rhea pennata in the fossil record has important paleoenvironmental implications; however, the strong similarity between their postcranial skeletons complicates taxonomic assignments. This study analyzes hindlimb morphology in both species to identify diagnostic characters useful for fossil and archaeological material and to reassess previously reported remains. Femora, tibiotarsi, and tarsometatarsi of adult and juvenile R. americana and R. pennata , comprising a total of 37 specimens, were comparatively analyzed using qualitative morphology and linear measurements. Several osteological differences were identified, particularly in distal tibiotarsal and tarsometatarsal morphology, including trochlear configuration and divergence patterns. Some differences are maintained in juvenile specimens, although ontogenetic variation may complicate identification in certain cases. Reevaluation of fossil and archaeological specimens supported revised taxonomic interpretations for several specimens, whereas poorly preserved or morphologically ambiguous materials were conservatively assigned to Rheidae indet. Although hindlimb bones of R. americana and R. pennata are broadly similar, distal tibiotarsal and especially tarsometatarsal morphology provide useful diagnostic characters. Trochlear configuration and divergence appear to be among the most consistent features, whereas other traits are affected by ontogenetic, intraspecific, and preservational variation. These results contribute to a more robust framework for identifying fossil rheids and highlight the importance of evaluating multiple characters when dealing with fragmentary material.