Modeling the evolution of substrate use in the hands and feet of primates, birds, and non-avian theropod dinosaurs
Stiegler, Josef Barrett
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The hands and feet of non-avian theropods have historically been characterized as structures adapted for grasping prey items and for cursorial locomotion, respectively. The purpose of this study was to challenge those assumptions in light of observations on the intradigital proportions of the hands and feet of theropods compared to modern taxa. Linear measurements of elements from the hands of mammals and squamates, the feet of birds, and both the hands and feet of non-avian theropods were collected to observe clustering of taxa in morphospace. The evolution of each linear character was modeled for primates, birds, and non-avian theropods using Ornstein-Uhlenbeck models in the "OUCH!" software package. Brownian motion models were nearly universally rejected in favor of single and multi-optimum selection based models. Model testing indicated directional selection for the most proximal and distal non-ungual elements in each digital ray, and either stabilizing selection or brownian motion processes for intermediate elements. The results for models applied to non-avian theropods suggest selection for metacarpal proportions convergent with arboreal mammals, and proportions of the penultimate phalanx similar to the feet of birds that use clawed adhesion for vertical substrate use and predation. We propose that "clinging" as opposed to "grasping" is a more apt hypothesis for behavior leading to elongation of the penultimate phalanx, as is the case in most non-avian theropods. According to modeling results, microraptorine dromaeosaurids evolved proportionally long metacarpals relative to their manual phalanges convergently with birds, and were comparable to other non-avian theropods in pedal proportions. These results contradict previous hypotheses in the literature regarding arboreal substrate use within microraptorinae. The scansoriopterygid Epidendrosaurus was a slight outlier in both manual and pedal proportions from other non avian maniraptorans, but adaptation for an arboreal lifestyle in this taxon is not supported by this analysis. The "Raptor Prey Restraint" model for predatory behavior in deinonychosaurian theropods was rejected by this analysis, though it is likely that the evaluated traits fail to capture the necessary anatomical variation to more fully test this hypothesis.