What is it about?

This paper reports on the first major finding of my project on the structural and functional origins of forearm pronation (twisting forwards and inwards): all tetrapods (four-legged animals) except chameleons and therians (marsupial and placental mammals) have forearms, wrist joints and finger joints stuck permanently in a semi-pronated karate-chop position. Semi-pronation orients flexion of the wrist and finger joints 90 degrees inwards relative to the plane of flexion/extension at the elbow joint. Semi-pronation was present in the first tetrapods, and may have evolved to help direct the hand anteriorly in a sprawling forelimb with a laterally directed elbow. In an animal with sprawling forelimbs this joint misalignment is not a mechanical problem, because sprawling animals rotate their upper arm like an axle to walk. However, if animals (such as dinosaurs) evolved elbows tucked in posteriorly, closer to the body, the 90 degree difference in joint flexion between the elbow and wrist/finger joints causes the latter joints to flex inwards instead of backwards. This publication tested the hypothesis that quadrupedal dinosaurs alleviated this mechanical constraint, but found a surprising result: the two forearm bones of quadrupedal dinosaurs tended to collapse in cross section, but stayed in a semi-pronated orientation overall.

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Why is it important?

This report on the evidence that dinosaurs retained the misaligned elbow and wrist/finger joints evolved by earlier, more sprawling ancestors, is important because it shows that the forearms of dinosaurs did not evolve the extra pronation necessary to align their wrist/finger joints with their elbow joints (as therian mammals and chameleons did). This information has been recognized only indirectly by previous studies, a situation which has long hampered progress in our understanding of the postural and locomotor evolution of dinosaurs and other archosaurs, such as crocodiles. Here, I build upon related work that analyzed the automatic wrist mechanism of crocodiles and their peculiar, stilted wrist and palmar stance (Hutson & Hutson, 2014, 2015a), to propose a modification and extension of Weems' (2006) hypothesis that some dinosaurs walked on the tips of their palmar bones with fingers hyperextended out of the way: quadrupedal archosaurs trended towards abandonment of their semi-pronated wrist and finger joints. This trend manifested itself in widespread convergence upon adaptations for turning the wrist and palm into a vertical, stiffened extension of the forearm. The useless fingers were hyperextended laterally. This modification of Weems' (2006) hypothesis to include the retention of semi-pronation as a mechanical constraint provides the first testable hypothesis for investigating why the postural and locomotor evolution of the archosaurian forelimb diverged from that of therian and chameleon forelimbs.

Perspectives

The evidence that dinosaurs retained semi-pronation is not intuitively obvious (it took me 2.5 years for me to visually recognize this configuration because my mind was not ready to comprehend what I was looking at). This information has been overlooked, despite dedicated searches by many capable researchers, for several reasons. First, it is rarely recognized in research written in the English language that semi-pronation is the plesiomorphic (primitive) condition for tetrapod forearms. Knowledge of this condition in non-English publications is restricted to older European literature, but this knowledge was not used in European research on the evolution of dinosaurs. Thus, research on changes in posture and gait in dinosaurs over evolutionary time have been missing a critical piece of information that greatly affects forelimb posture and gait. Second, the fact that the existence of semi-pronation has not been recognized in any major studies of dinosaurian evolution reveals that the very language used to describe the structure and function of forearm bones suffers from biased language learned in human anatomy courses, or comparative anatomy courses using language learned from studies of human and therian mammal anatomy. Thus, misconceptions and preconceived notions have surrounded studies of pronation in dinosaurs. This bias helps prevent researchers (including myself initially) from recognizing why dinosaurian forelimb joints do not look or operate like those of therian mammals. Third, the lack of awareness that the forearms of dinosaurs did not evolve the extra pronation necessary to align their wrist/finger joints with their elbow joints (as therians mammals and chameleons did) means that no one has yet asked why only therians and chameleons evolved fully pronated wrist/finger joints. In this paper I suggest that the different evolutionary histories of these three groups of animals may provide an answer to this question, namely by investigating any differences in the selection pressures that dominated the postural and locomotor evolution of their forelimbs.

Joel David Hutson
DePaul University

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This page is a summary of: Quadrupedal Dinosaurs did not evolve fully pronated Forearms: New Evidence from the Ulna, Acta Palaeontologica Polonica, January 2014, Polska Akademia Nauk Instytut Paleobiologii (Institute of Paleobiology, Polish Academy of Sciences),
DOI: 10.4202/app.00063.2014.
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