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Quadrupedal dinosaur wrist motion insights from alligator and ostrich wrists



From: Ben Creisler
bcreisler@gmail.com

A new online paper:

Joel David Hutson and Kelda Nadine Hutson (2014)
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 (advance online publication)
DOI: 10.1002/ar.22903
http://onlinelibrary.wiley.com/doi/10.1002/ar.22903/abstract

A recent study hypothesized that avian-like wrist folding in
quadrupedal dinosaurs could have aided their distinctive style of
locomotion with semi-pronated and therefore medially facing palms.
However, soft tissues that automatically guide avian wrist folding
rarely fossilize, and automatic wrist folding of unknown function in
extant crocodilians has not been used to test this hypothesis.
Therefore, an investigation of the relative contributions of soft
tissues to wrist range of motion (ROM) in the extant phylogenetic
bracket of dinosaurs, and the quadrupedal function of crocodilian
wrist folding, could inform these questions. Here, we repeatedly
measured wrist ROM in degrees through fully fleshed, skinned, minus
muscles/tendons, minus ligaments, and skeletonized stages in the
American alligator Alligator mississippiensis and the ostrich Struthio
camelus. The effects of dissection treatment and observer were
statistically significant for alligator wrist folding and ostrich
wrist flexion, but not ostrich wrist folding. Final skeletonized wrist
folding ROM was higher than (ostrich) or equivalent to (alligator)
initial fully fleshed ROM, while final ROM was lower than initial ROM
for ostrich wrist flexion. These findings suggest that, unlike the
hinge/ball and socket-type elbow and shoulder joints in these
archosaurs, ROM within gliding/planar diarthrotic joints is more
restricted to the extent of articular surfaces. The alligator data
indicate that the crocodilian wrist mechanism functions to
automatically lock their semi-pronated palms into a rigid column,
which supports the hypothesis that this palmar orientation
necessitated soft tissue stiffening mechanisms in certain dinosaurs,
although ROM-restricted articulations argue against the presence of an
extensive automatic mechanism.