Meeting Abstract
It is commonly assumed that “muscle scars” on bones correspond to the cross-sectional areas of muscle-tendon units. This issue is vital both for understanding morphology itself (e.g. musculoskeletal integration) and for reconstructing musculoskeletal form in extinct taxa. Archosaurian reptiles famously evolved disparate skeletal forms with differences in body size, posture, gait and other aspects of locomotion reflected by variations of the muscle attachments. We tested how well hindlimb muscle sizes can be predicted from skeletal evidence in archosaurs. With a high-precision manual digitizer (±0.01mm), we digitized the bony attachment areas (AA) of all major hindlimb muscles in five juvenile Nile crocodiles and five Elegant-crested tinamous. Additional, lower-precision (~±1mm) older digitized data from an adult ostrich, emu, turkey and chicken were added to our avian dataset for comparison. We measured the physiological cross-sectional areas (PCSAs) of the same muscles for all specimens via dissection. Our prediction was that fleshier attachments would give more consistent estimates of muscle PCSA from AA, whereas more tendinous attachments would have more variable PCSA estimates from AA within and across taxa. Examining how well homologous muscles in Crocodylia and Aves preserve consistent PCSA:AA ratios, we found that only a few muscles do. Finally, we show examples how these ratios can be analysed in a phylogenetic context across Archosauria, applying them to fossil specimens; vs. other methods from the literature.
