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Dinosaur shoulder joint mobility

A new paper in the Journal of Experimental Biology:

Hutson JD, Hutson Kn 2012 Using the American alligator and a
repeated-measures design to place constraints on in vivo shoulder
joint range of motion in dinosaurs and other fossil archosaurs. J Exp
Biol 216(2) 275--84

Using the extant phylogenetic bracket of dinosaurs (crocodylians and
birds), recent work has reported that elbow joint range of motion
(ROM) studies of fossil dinosaur forearms may be providing
conservative underestimates of fully fleshed _in vivo_ ROM. As humeral
ROM occupies a more central role in forelimb movements, the placement
of quantitative constraints on shoulder joint ROM could improve fossil
reconstructions. Here, we investigated whether soft tissues affect the
more mobile shoulder joint in the same manner in which they affect
elbow joint ROM in an extant archosaur. This test involved separately
and repeatedly measuring humeral ROM in _Alligator mississippiensis_
as soft tissues were dissected away in stages to bare bone. Our data
show that the ROMs of humeral flexion and extension, as well as
abduction and adduction, both show a statistically significant
increase as flesh is removed, but then decrease when the bones must be
physically articulated and moved until they separate from one another
and/or visible joint surfaces. A similar ROM pattern is inferred for
humeral pronation and supination. All final skeletonized ROMs were
less than initial fully fleshed ROMs. These results are consistent
with previously reported elbow joint ROM patterns from the extant
phylogenetic bracket of dinosaurs. Thus, studies that avoid separation
of complementary articular surfaces may be providing fossil shoulder
joint ROMs that underestimate _in vivo_ ROM in dinosaurs, as well as
other fossil archosaurs.

David Černý