75/125 YEARS

An Integrative Study of the Evolution of Size and Postural Support in Theropod Dinosaurs-Part I: The Musculoskeletal Apparatus

HUTCHINSON, John R., Museum of Paleontology and Dept. of Integrative Biology, Univ. of California, Berkeley, CA 94720-4780

I focus on the evolutionary morphology of the hindquarters of theropod dinosaurs, analyzing characteristics of the musculoskeletal apparatus within a phylogenetic context, using other saurians as outgroups. I analyze musculoskeletal data from extant and extinct saurian reptiles to develop a broad picture of the evolution of the theropod hindquarters. My data support the hypothesis that the evolution of the terrestrial locomotory module seen in extant birds was a pattern of sequential assembly and modification, with numerous independent specializations and instances of convergence. Fossils are important for reconstructing polarity of musculoskeletal changes. Many instances of musculoskeletal specialization, plesiomorphy, and homoplasy in extant clades cannot be identfied without a broad phylogenetic framework including fossils.

I also compare musculoskeletal reconstructions generated by my analysis with attempts by previous researchers. The results sound a note of caution for scientists (and artists) engaging in soft tissue reconstruction with extinct vertebrates. Such reconstructions must adopt a more rigorous methodology that uses an integrative approach to homology. A major concern is the absence or equivocality of a large amount of musculoskeletal data, especially in taxa without very similar extant relatives. Muscle scars and other osteological correlates must be carefully interpreted with reference to myological data from extant relatives. A phylogenetic methodological basis allows falsifiable hypotheses to be formulated and tested against a phylogenetic framework.

The incompleteness of the data urges additional caution in making detailed functional assertions based on these data, unless these assertions incorporate sensitivity analyses that consider the incomplete data. This matter is compounded by the disparity between extant birds and more basal theropods. Significant musculoskeletal transformations in the pelvis, hindlimb, and tail throughout theropod evolution argue against hypotheses of functional homogeneity in theropod terrestrial locomotion, despite seemingly contrary evidence from trackways.

The historical approach to studying the evolution of size and postural support in theropod dinosaurs is incomplete unless integrated with data from an ahistorical approach that tests functional hypotheses. The second phase of my study (in progress) uses biomechanical computer simulations and kinematic data from extant taxa to test functional hypotheses. When integrated with the evolutionary morphology, consilience among these data should reveal patterns in the evolution of size and postural support in theropod dinosaurs. This will help me to test my primary hypothesis, that extinct theropods followed the apparent mammalian trend of adopting a more columnar limb posture at a larger body size. In combination with case studies of other clades, this integrative approach should illuminate more general trends in the evolution of size and support in terrestrial vertebrates.

75/125 YEARS