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Quadrupedal Ornithischian Dinosaur Stance and Gait
From: Ben Creisler
New in PLoS ONE:
Susannah C. R. Maidment, Deborah H. Linton, Paul Upchurch & Paul M.
Limb-Bone Scaling Indicates Diverse Stance and Gait in Quadrupedal
PLoS ONE 7(5): e36904.
most primitive ornithischian dinosaurs were small bipeds, but
quadrupedality evolved three times independently in the clade. The
transition to quadrupedality from bipedal ancestors is rare in the
history of terrestrial vertebrate evolution, and extant analogues do
not exist. Constraints imposed on quadrupedal ornithischians by their
ancestral bipedal bauplan remain unexplored, and consequently, debate
continues about their stance and gait. For example, it has been
proposed that some ornithischians could run, while others consider
that none were cursorial.
Drawing on biomechanical concepts of limb bone scaling and locomotor
theory developed for extant taxa, we use the largest dataset of
ornithischian postcranial measurements so far compiled to examine
stance and gait in quadrupedal ornithischians. Differences in femoral
midshaft eccentricity in hadrosaurs and ceratopsids may indicate that
hadrosaurs placed their feet on the midline during locomotion, while
ceratopsids placed their feet more laterally, under the hips. More
robust humeri in the largest ceratopsids relative to smaller taxa may
be due to positive allometry in skull size with body mass in
ceratopsids, while slender humeri in the largest stegosaurs may be the
result of differences in dermal armor distribution within the clade.
Hadrosaurs are found to display the most cursorial morphologies of the
quadrupedal ornithischian cades, indicating higher locomotor
performance than in ceratopsids and thyreophorans.
Limb bone scaling indicates that a previously unrealised diversity of
stances and gaits were employed by quadrupedal ornithischians despite
apparent convergence in limb morphology. Grouping quadrupedal
ornithischians together as a single functional group hides this
disparity. Differences in limb proportions and scaling are likely due
to the possession of display structures such as horns, frills and
dermal armor that may have affected the center of mass of the animal,
and differences in locomotor behaviour such as migration, predator
escape or home range size.