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Theropod evolution of crouched-limb posture in birds

From: Ben Creisler

A new online paper in Nature:

Vivian Allen, Karl T. Bates, Zhiheng Li & John R. Hutchinson (2013)
Linking the evolution of body shape and locomotor biomechanics in
bird-line archosaurs.
Nature (advance online publication)

Locomotion in living birds (Neornithes) has two remarkable features:
feather-assisted flight, and the use of unusually crouched hindlimbs
for bipedal support and movement. When and how these defining
functional traits evolved remains controversial. However, the advent
of computer modelling approaches and the discoveries of exceptionally
preserved key specimens now make it possible to use quantitative data
on whole-body morphology to address the biomechanics underlying this
issue. Here we use digital body reconstructions to quantify
evolutionary trends in locomotor biomechanics (whole-body proportions
and centre-of-mass position) across the clade Archosauria. We use
three-dimensional digital reconstruction to estimate body shape from
skeletal dimensions for 17 archosaurs along the ancestral bird line,
including the exceptionally preserved, feathered taxa Microraptor,
Archaeopteryx, Pengornis and Yixianornis, which represent key stages
in the evolution of the avian body plan. Rather than a discrete
transition from more-upright postures in the basal-most birds
(Avialae) and their immediate outgroup deinonychosauria, our results
support hypotheses of a gradual, stepwise acquisition of more-crouched
limb postures across much of theropod evolution, although we find
evidence of an accelerated change within the clade Maniraptora (birds
and their closest relatives, such as deinonychosaurs). In addition,
whereas reduction of the tail is widely accepted to be the primary
morphological factor correlated with centre-of-mass position and,
hence, evolution of hindlimb posture, we instead find that enlargement
of the pectoral limb and several associated trends have a much
stronger influence. Intriguingly, our support for the onset of
accelerated morpho-functional trends within Maniraptora is closely
correlated with the evolution of flight. Because we find that the
evolution of enlarged forelimbs is strongly linked, via whole-body
centre of mass, to hindlimb function during terrestrial locomotion, we
suggest that the evolution of avian flight is linked to anatomical
novelties in the pelvic limb as well as the pectoral.