HUTCHINSON, J.R.: The role of fossils in reconstructing the evolution of tendons on the line to birds.

I integrate lines of evidence from 1) tendon and muscle anatomy and function in extant birds and their outgroups with 2) surface and histological features of fossil bones and 3) biomechanical computer simulations of hindlimb musculoskeletal functions. This evidence allows me to reconstruct the pattern of muscle and tendon evolution on the lineage leading to crown group birds. Fossils reveal qualitative and quantitative changes of muscle/tendon lines of action, moment arms, cross-sectional areas, and mineralization. These inferences reveal remarkable details about the evolution of terrestrial locomotor function before the diversification of crown group birds. For example, fossils show how the relative sizes and moment arms of key knee extensor muscles expanded in early dinosaurs, facilitating bipedalism long before birds evolved. Birds closely related to crown group birds evolved new anatomical mechanisms for maintaining large extensor moment arms about the knee and ankle, including ossifications of tendons, sesamoids, and cartilages, such as the cranial cnemial crest, patella, and hypotarsus. The ossification of many lower limb tendons, particularly prominent in extensors, is another recent novelty of birds. Many of these features signal the increased importance of muscles that develop large forces, tendon stresses, and joint moments in order to stabilize or rotate the strongly flexed hip and knee joints of birds during standing and moving. This material is based on work supported by the National Science Foundation under a grant awarded in 2001.