The exceptionally rare transition to quadrupedalism
from bipedal ancestors occurred on three independent occasions
in ornithischian dinosaurs. The possible driving forces
behind these transitions remain elusive, but several hypotheses—including
the development of dermal armour and the
expansion of head size and cranial ornamentation—have been
proposed to account for this major shift in stance. We
modelled the position of the centre of mass (CoM) in several
exemplar ornithischian taxa and demonstrate that the anterior
shifts in CoM position associated with the development of an
enlarged skull ornamented with horns and frills for display/
defence may have been one of the drivers promoting ceratopsian
quadrupedality. A posterior shift in CoM position coincident
with the development of extensive dermal armour in
thyreophorans demonstrates this cannot have been a primary
causative mechanism for quadrupedality in this clade.
Quadrupedalism developed in response to different selective
pressures in each ornithischian lineage, indicating different
evolutionary pathways to convergent quadrupedal
morphology.