Bird wrists evolved first in dinosaurs

[Dan Chure <danchure@easilink.com>]

The following is available free on line at 
http://rspb.royalsocietypublishing.org/content/early/2010/02/24/rspb.2009.2281.full.pdf+html

Corwin Sullivan, David W. E. Hone, Xing Xu and Fucheng Zhang 2010 The 
asymmetry of the carpal joint and the evolution of wing folding in 
maniraptoran theropod dinosaurs.
Proceedings Royal Society B on-line

Abstract
In extant birds, the hand is permanently abducted towards the ulna, and 
the wrist joint can bend extensively in this direction to fold the wing 
when not in use. Anatomically, this asymmetric mobility of the wrist 
results from the wedge-like shape of one carpal bone, the radiale, and 
from the well-developed convexity of the trochlea at the proximal end of 
the carpometacarpus. Among the theropod precursors of birds, a strongly 
convex trochlea is characteristic of Coelurosauria, a clade including 
the highly derived Maniraptora in addition to tyrannosaurs and 
compsognathids. The shape of the radiale can be quantified using a 
‘radiale angle’ between the proximal and distal articular surfaces. 
Measurement of the radiale angle and reconstruction of ancestral states 
using squared-change parsimony shows that the angle was small (15°) in 
primitive coelurosaurs but considerably larger (25°) in primitive 
maniraptorans, indicating that the radiale was more wedge-shaped and the 
carpal joint more asymmetric. The radiale angle progressively increased 
still further within Maniraptora, with concurrent elongation of the 
forelimb feathers and the forelimb itself. Carpal asymmetry would have 
permitted avian-like folding of the forelimb in order to protect the 
plumage, an early advantage of the flexible, asymmetric wrist inherited 
by birds.