ERICKSON, Gregory M., Museum of Paleontology, Museum of Vertebrate Zoology, and Dept. of Integrative Biology, University of California, Berkeley, CA 94720-4780
Long bones are the major skeletal elements of vertebrate appendages (e.g., the femur, tibia, and humerus). The radiations of many vertebrate clades were facilitated by locomotory innovations involving long bones, thus it is necessary to understand how these elements changed biomechanically to understand more fully how vertebrates evolved as a whole. Such evolutionary understanding has not been possible because the biomechanical attributes of long bones are unknown for most vertebrate taxa, except for birds and mammals. Consequently, I conducted biomechanical testing on long bones from a diversity of vertebrates that possess locomotory attributes present in ancestral taxa and analyzed the data using phylogenetic character analysis. Biomechanical tests included: (1) three-point bend experiments to determine material properties: ultimate strength, Young's modulus, and yield strains; (2) in vivo strain gauge analyses to determine how long bones are loaded during locomotion; and (3) accommodation assessments to determine if long bones show phenotypic plasticity in response to loading changes. The results of these studies indicate that: (1) the range of values for the material properties of long bones was established approximately 475 million years before present (mybp) in the first long bone homologs and these values have remained within their respective ranges throughout evolution; (2) the physiological strain regimes established approximately 475 mybp have remained unchanged throughout evolution except in the lineages leading to the Aves and Mammalia; (3) accommodation responses to overloaded conditions occurred 475 mybp and this capacity has persisted throughout evolution; (4) the capacity to accommodate to understrained conditions is a derived trait that independently evolved in the avian and mammalian lineages, perhaps in association with the evolution of endothermy; (5) the thresholds for loading accommodation probably increased in the avian and mammalian lineages in concert with the evolution of erect posture and selection for efficient and rapid locomotion; (6) long bone size, shape, and loading changes occurred with nearly all cladogenic events involving locomotory changes; (7) histological changes during evolution may reflect both growth rate and loading conditions during ontogeny.