Finger hyperextension in alligators and dinosaurs

What is it about?

Here we test whether alligators hyperextend their fingers when they walk. We then use our results to reinterpret adaptations traditionally assumed to magnify and accommodate finger hyperextension in dinosaurs and other fossil archosaurs, by comparing alligator adaptations with those of a basal archosaur with contested forelimb posture, the rauisuchid Postosuchus.

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Why is it important?

To the best of our knowledge, this is the first study to investigate the effects of soft tissues on finger joint range of motion in an archosaur (American alligator) and then use the results to interpret finger joint range of motion in extinct archosaurs. Our results continue to provide broad support for the conclusions of the range of motion studies of the shoulder, elbow and wrist joints in these alligators; soft tissues extrinsic to joints may guide and restrict range of motion, while the articular cartilage may guide and increase range of motion. However, we note that additional studies with larger sample sizes and from a broader array of joints could help test this conclusion, as well as to establish which joints do not fit this general pattern. Additionally, our results provide support for Hutson's (2015) modification of Weems' (2006) hypothesis, namely that some dinosaurs abandoned use of their finger joints. We extend this hypothesis to the rauisuchid Postosuchus and other extinct quadrupedal archosaurs that possess similar hyperextension characters. If our conclusions are supported by further studies, this interpretation indicates that quadrupedal archosaurs trended towards metacarpogrady (walking on the tips of their vertically-held palms with hyperextended fingers) and that this stilt-like hand posture simultaneously helped alleviate the limb disparity (shorter fore- than hindlimbs) representative of many groups of archosaurs.

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