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Nonplantigrade Foot Posture: A Constraint on Dinosaur Body Size

Ben Creisler

New in PLoS ONE:

Tai Kubo & Mugino O. Kubo (2016)
Nonplantigrade Foot Posture: A Constraint on Dinosaur Body Size.
PLoS ONE 11(1): e0145716.

Dinosaurs had functionally digitigrade or sub-unguligrade foot
postures. With their immediate ancestors, dinosaurs were the only
terrestrial nonplantigrades during the Mesozoic. Extant terrestrial
mammals have different optimal body sizes according to their foot
posture (plantigrade, digitigrade, and unguligrade), yet the
relationship of nonplantigrade foot posture with dinosaur body size
has never been investigated, even though the body size of dinosaurs
has been studied intensively. According to a large dataset presented
in this study, the body sizes of all nonplantigrades (including
nonvolant dinosaurs, nonvolant terrestrial birds, extant mammals, and
extinct Nearctic mammals) are above 500 g, except for macroscelid
mammals (i.e., elephant shrew), a few alvarezsauroid dinosaurs, and
nondinosaur ornithodirans (i.e., the immediate ancestors of
dinosaurs). When nonplantigrade tetrapods evolved from plantigrade
ancestors, lineages with nonplantigrade foot posture exhibited a
steady increase in body size following Cope’s rule. In contrast,
contemporaneous plantigrade lineages exhibited no trend in body size
evolution and were largely constrained to small body sizes. This
evolutionary pattern of body size specific to foot posture occurred
repeatedly during both the Mesozoic and the Cenozoic eras. Although
disturbed by the end-Cretaceous extinction, species of mid to large
body size have predominantly been nonplantigrade animals from the
Jurassic until the present; conversely, species with small body size
have been exclusively composed of plantigrades in the nonvolant
terrestrial tetrapod fauna.