What is it about?
This experiment investigated why fleshed alligator forearms appear to be able to pronate/supinate when physically forced, but their bare forearm bones cannot rotate on their long axes. Our examinations of alligators and ostriches indicate that alligators and most other tetrapods (four-legged animals) passively cross/uncross their forearm bones while walking in the same manner recently described in lizards, which is different from how therian mammals (marsupials and placentals) cross/uncross their forearm bones. Additionally, we bring together scattered reports on how many tetrapods, such as birds and quadrupedal dinosaurs, evolved ways to immobilize their forearm bones for stability. We describe two of these adaptations in functional terms.
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Why is it important?
Previous authors have described how many types of tetrapods with forelimbs subjected to increased limb bone loads (e.g., swimming, digging, increased weight support, running, and gliding/flapping flight) evolve forearm bones with stiffened, reinforced articulations. However, it was only recently that Johann M. F. Landsmeer described and illustrated (using lizards) the functional morphology of the forearm bone articulations during a quadrupedal step cycle. This study is the first to apply Landsmeer's findings to other tetrapods and to describe how various tetrapods have converged upon two similar ways of restricting movement between the forearm bones. Our results, when combined with previous studies, show that most bipedal dinosaurs retained the adaptations to passively allow their forearm bones to cross/uncross, which provides further support for the consensus that bipedal dinosaurs could not have actively pronated/supinated their forearms like humans can. More importantly though, these findings call attention to overlooked reports that the first bird (Archaeopteryx) had evolved a forearm bone configuration commonly found in tetrapods that reduce or inhibit forearm bone movements, in this case most likely in response to the stresses of gliding/flapping. Closely related dromaeosaurs (e.g., Velociraptor) also had an avian forearm bone configuration, lending osteological support for the hypothesis that some maniraptoran dinosaurs were secondarily flightless. Thus, this work is important in that it may provide osteological evidence that can help pinpoint when flight evolved in birds and pterosaurs (the other aerial archosaurs), as well as if any dinosaurs such as deinonychosaurs were descended from gliding/flapping ancestors, analogous to how the ostrich is descended from ancient flying birds, and therefore retains the adaptations for immobilizing its forearm bones.
Perspectives
The findings of this paper include the second major conclusion of my (JDH) research on the structural and functional origins of forearm pronation (twisting inwards and forwards): except for therian mammals, tetrapods cannot actively pronate/supinate (rotate) their forearm bones and hands. Instead, all nontherian tetrapod forearms appear to have retained the plesiomorphic (primitive), passive mechanism named independent flexion/extension by Johann Landsmeer (1983, 1984). Despite several attempts to describe how nontherian forearm bones displace against each other, before Landsmeer (1983, 1984) these movements had been confused with therian mammal pronation/supination, because they look the same when one physically forces any tetrapod forearm to twist. Thus, many researchers have claimed, and continue to claim, that nontherian tetrapod forearms are capable of active pronation/supination, because that's what the radius (lateral forearm bone), and pronator/supinator musculature names given to all tetrapods were named after, but in humans. What made Landsmeer different was that he gradually realized that the two mechanisms function differently, and by 1990 had firmly described how independent flexion/extension passively operates in lizards. He did not, however, apply his fundamental breakthrough to other tetrapods, but our paper ties previous works together to argue that Landsmeer successfully described the plesiomorphic mechanism via which tetrapod forearm bones evolved to alleviate the torsion produced by twisting the forearm bones over an immobile, firmly placed hand. This proposal is supported by evidence that forearm bone articulations have been widely noted to be morphologically similar and highly conserved (ancient and unchanging) across most groups of terrestrial tetrapods. In combination with Hutson (2015), which attempted clear up the misconceptions surrounding the plesiomorphic position of pronation in nontherian tetrapods, this information on the plesiomorphic mobility of the forearm bones can now be applied to many other problems surrounding research on the postural and locomotor evolution of the forelimb in various tetrapod groups.
Joel David Hutson
DePaul University
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This page is a summary of: An examination of forearm bone mobility in Alligator mississippiensis (Daudin, 1802) and Struthio camelus Linnaeus, 1758 reveals that Archaeopteryx and dromaeosaurs shared an adaptation for gliding and/or flapping , Geodiversitas, September 2015, Museum National d'Histoire Naturelle, Paris, France,
DOI: 10.5252/g2015n3a3.
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