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Gigantism in eutherians and saurischians: cartilage and sub-articular bone shape

From: Ben Creisler

New in PLoS ONE:

Matthew F. Bonnan,  D. Ray Wilhite, Simon L. Masters, Adam M. Yates,
Christine K. Gardner & Adam Aguiar (2013)
What Lies Beneath: Sub-Articular Long Bone Shape Scaling in Eutherian
Mammals and Saurischian Dinosaurs Suggests Different Locomotor
Adaptations for Gigantism.
PLoS ONE 8(10): e75216.

Eutherian mammals and saurischian dinosaurs both evolved lineages of
huge terrestrial herbivores. Although significantly more saurischian
dinosaurs were giants than eutherians, the long bones of both taxa
scale similarly and suggest that locomotion was dynamically similar.
However, articular cartilage is thin in eutherian mammals but thick in
saurischian dinosaurs, differences that could have contributed to, or
limited, how frequently gigantism evolved. Therefore, we tested the
hypothesis that sub-articular bone, which supports the articular
cartilage, changes shape in different ways between terrestrial mammals
and dinosaurs with increasing size. Our sample consisted of giant
mammal and reptile taxa (i.e., elephants, rhinos, sauropods) plus
erect and non-erect outgroups with thin and thick articular cartilage.
Our results show that eutherian mammal sub-articular shape becomes
narrow with well-defined surface features as size increases. In
contrast, this region in saurischian dinosaurs expands and remains
gently convex with increasing size. Similar trends were observed in
non-erect outgroup taxa (monotremes, alligators), showing that the
trends we report are posture-independent. These differences support
our hypothesis that sub-articular shape scales differently between
eutherian mammals and saurischian dinosaurs. Our results show that
articular cartilage thickness and sub-articular shape are correlated.
In mammals, joints become ever more congruent and thinner with
increasing size, whereas archosaur joints remained both congruent and
thick, especially in sauropods. We suggest that gigantism occurs less
frequently in mammals, in part, because joints composed of thin
articular cartilage can only become so congruent before stress cannot
be effectively alleviated. In contrast, frequent gigantism in
saurischian dinosaurs may be explained, in part, by joints with thick
articular cartilage that can deform across large areas with increasing