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Ornithischian dinosaur quadrupedality

From: Ben Creisler

A new article in Acta Palaeontologica Polonica:

Susannah C.R. Maidment and Paul M. Barrett (2012)
Osteological correlates for quadrupedality in ornithischian dinosaurs.
Acta Palaeontologica Polonica (in press)
doi: http://dx.doi.org/10.4202/app.2012.0065

The evolution of quadrupedality from bipedal ancestors is an
exceptionally rare transition in tetrapod evolution, but it has
occurred several times within the herbivorous dinosaur clade
Ornithischia. Stegosauria, Ankylosauria and Ceratopsidae are all
uncontroversially quadrupedal, while basal ornithischians and basal
ornithopods are uncontroversially bipedal. However, stance in
iguanodontian ornithopods, including the hadrosaurs, and in
nonceratopsid ceratopsians is debated because robust osteological
correlates of quadrupedality have not been identified. We examine a
suite of characteristics that have been previously proposed as
osteological correlates for bipedality or quadrupedality in dinosaurs.
These include both discrete anatomical features, which we assess as
correlates for quadrupedality using character optimization onto a
composite cladogram, and proportional ratios, which we assess as
correlates by reconstructing nodal ancestral states using
squared-change parsimony, followed by optimization. We also examine
the correlation of these features with body size. An anterolateral
process on the proximal ulna, hoof-shaped manual unguals, a
transversely broadened ilium, a reduced fourth trochanter and a femur
longer than the tibia are found to be robust correlates of
quadrupedality in ornithischian dinosaurs. Along the ceratopsid ‘stem’
lineage, quadrupedal characters were acquired in a stepwise fashion,
with forelimb characters developing prior to changes in the hind limb.
In contrast, iguanodontid ornithopods display a mosaic of character
states, indicating varying degrees of facultative quadrupedality that
probably arose for a variety of different reasons. Hadrosaurs are
found to possess all character states associated with quadrupedality
and were probably predominantly quadrupedal. In general, quadrupedal
ornithischians do not appear to have been constrained by their bipedal
ancestry to a particular order of character acquisition.