What is it about?
Here we take the first step in investigating whether wrist range of motion in dinosaurs can be extrapolated from manipulations of fossil bones. Our results allow us to explore why crocodiles evolved a wrist that unfolds automatically like those of birds. Additionally, we briefly review the convoluted literature surrounding conflicting claims of how automatic wrist mechanisms are initiated, i.e., via impaction between bones or via soft tissue linkages such as tendons and ligaments.
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Why is it important?
This paper is important because: 1) it gathers together and summarizes previous literature on automatic wrist folding/unfolding mechanisms in various tetrapods (four-legged animals) for the first time since 1897. Our literature review verifies that the history of research on this topic is typified by repeated losses and rediscoveries of how the automatic mechanism of birds supposedly operates. We say "supposedly" because the only focused test (Vazquez, 1994) of the widely held assumption of how birds automatically fold/unfold their wrists has either been overlooked or misunderstood in all recent research investigating whether an automatic wrist mechanism existed in dinosaurs. Our experiment verifies that (for alligators and ostriches) Vazquez (1994) was correct in concluding that soft tissues spanning both the elbow and wrist joints provide the main means via which elbow joint flexion/extension causes automatic wrist adduction (chop the hand down)/abduction(chop the hand up), respectively, not impaction between the radius (lateral bone of forearm) and the humerus at the elbow joint (upper arm bone) as the elbow flexes. 2) This paper is also important because it concludes that alligators and their relatives may have evolved an automatic wrist mechanism to keep the wrist stiffly unfolded (straightened) when standing upright upon it and pushing off while walking. 3) Finally, this paper provides supportive evidence for Hutson's (2015) proposal that quadrupedal archosaurs such as dinosaurs and crocodilians converged upon adaptations to abandon locomotor use of their semi-pronated (mechanically misaligned with the elbow joint) wrist and finger joints. Specifically, the automatic wrist unfolding in crocodilians is suggested as an adaptation to help turn the wrist and palm of crocodilians into a stilt-like extension of the forearm.
Perspectives
The process of researching, designing and carrying out this experiment was quite different from the elbow and shoulder joint experiments carried out in the same alligator and ostrich specimens. It was different because we approached this experiment with powerful preconceived notions of how we expected the automatic wrist mechanisms of alligators and birds to operate that were based on words that biased our thinking. Namely, these automatic wrist mechanisms are commonly called "wrist folding," which immediately biased our thinking to expect that folding of the wrist was the more important action and function of this automatic mechanism. Additionally, we discovered that there is a difference between reading previous literature and actually understanding it. For example, Vaquez's (1994) test of how birds actually fold/unfold their wrists automatically is commonly cited, but these citations (including ourselves before this study) seemed unaware that his test built upon previous literature to falsify the common assumption that bird wrists fold/unfold due to impaction between the forearm and upper arm. These discoveries gave us our first lesson in how science is not always a steady progression of knowledge building upon previous literature; in this case it seems to have stalled since the first focused study by Bergmann (1839). Discussions with other authors conducting wrist studies and reviews of literature published after this experiment have not given us confidence that this situation is going to change soon. Hopefully, more tests of Vazquez's (1994) work, using tetrapods other than birds (such as here with alligators), will be forthcoming.
Joel David Hutson
DePaul University
Read the Original
This page is a summary of: A repeated-measures analysis of the effects of soft tissues on wrist range of motion in the extant phylogenetic bracket of dinosaurs: Implications for the functional origins of an automatic wrist folding mechanism in crocodilia, The Anatomical Record, March 2014, Wiley,
DOI: 10.1002/ar.22903.
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