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Casea and evolution of sacrum in non-mammalian synapsids

Ben Creisler

New in PLoS ONE:

Aaron R. H. LeBlanc & Robert R. Reisz (2014)
New Postcranial Material of the Early Caseid Casea broilii Williston,
1910 (Synapsida: Caseidae) with a Review of the Evolution of the
Sacrum in Paleozoic Non-Mammalian Synapsids.
PLoS ONE 9(12): e115734.

Here we use the description of a new specimen of the small caseid
synapsid Casea broilii that preserves the sacral, pelvic and hind limb
regions in great detail and in three dimensions, as a unique
opportunity to reevaluate the early stages in the evolution of the
sacrum in the lineage that led to mammals. We place this new material
in the context of sacral evolution in early caseid synapsids and
conclude that the transition from two to three sacral vertebrae
occurred in small-bodied species, suggesting that it was not an
adaptation to heavy weight bearing. Furthermore, we compare
descriptions of sacral anatomy among known early synapsids, including
caseids, ophiacodontids, edaphosaurids, varanopids, and
sphenacodontians and review sacral evolution in early synapsids. Based
on the descriptions of new species of caseids, edaphosaurids, and
varanopids over the past several decades, it is clear that a sacrum
consisting of three vertebrae evolved independently at least four
times in synapsids during the Late Carboniferous and Early Permian.
Furthermore, similarities in the morphologies of the sacral vertebrae
and ribs of these early synapsids lead us to conclude that an anterior
caudal vertebra had been incorporated into the sacral series
convergently in these groups. Given the repeated acquisition of a
three-vertebra sacrum in early synapsids and no apparent link to body
size, we argue that this sacral anatomy was related to more efficient
terrestrial locomotion than to increased weight bearing.