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Dinosaur forearm construction and posture (resend)



From: Ben Creisler
bcreisler@gmail.com

I am really stumped as to why this posting as not gone through to the
DML today. This is the 6th time I have tried to send it, changing the
subject line and other small details. My other postings have worked
fine. I don't know if some embedded feature or combination of
characters or words is blocking the content as email.


Joel D. Hutson (2014)
Quadrupedal dinosaurs did not evolve fully pronated forearms: new
evidence from the ulna.
Acta Palaeontologica Polonica (in press)
doi:  http://dx.doi.org/10.4202/app.00063.2014
http://app.pan.pl/article/item/app000632014.html



Therians (marsupials and placentals), archosaurs, and chameleons are
remarkable in that they evolved postures and gaits with inturned
forelimbs. However, recent studies have indirectly recognized that,
unlike fully pronated therian and chameleon forearms, dinosaur
forearms were mechanically constrained by semi-pronated (misaligned)
joints. This has led to the hypothesis that quadrupedal dinosaurs
mitigated this constraint via proximal migration of the radius,
indirectly forming a more pronated, tubular manus distally. To test
this hypothesis, a standardized pose was used to examine the forearm
pronation of ornithischian dinosaurs that were obligatory quadrupeds
and facultative bipeds. Results show that only restructuring of the
distal, not the proximal radius, causes additional pronation of the
pre-axial edge of the carpus, but also unexpectedly reveal that the
ulna may help form a tubular manus by supinating the post-axial edge.
Thus, relative to the plane of the elbow joint the wrist and finger
joints remain wholly semi-pronated. These findings do not support the
hypothesis that a tubular cross-section evolved in dinosaurs to
pronate the manus further to allow the finger joints to participate in
locomotion. Instead these results indicate that quadrupedal dinosaurs
tended to abandon propulsive use of their wrist and finger joints by
converting their carpus + metacarpus into a vertical stilt-like
extension of the forearm. Prior studies have overlooked that this
divergent path to parasagittal forelimb kinematics had its
phylogenetic basis in the retention of the semi-pronated forearm joint
alignment that is plesiomorphic to tetrapods. Thus, this test provides
the first functional explanation for the convergent responses of
quadrupedal archosaurs to their misaligned forearm joints, and
provides a foundation for elucidating why the quadrupedal evolution of
archosaur forelimbs diverged from those of therians and chameleons.