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Pachycephalosaur Caudal Basket in PLoS ONE



From: Ben Creisler
bcreisler@gmail.com

New in PLoS ONE:


Brown, C.M. & Russell, A.P. (2012)
Homology and Architecture of the Caudal Basket of Pachycephalosauria
(Dinosauria: Ornithischia): The First Occurrence of Myorhabdoi in
Tetrapoda.
PLoS ONE 7(1): e30212.
doi:10.1371/journal.pone.0030212
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0030212



Background

Associated postcranial skeletons of pachycephalosaurids, most notably
those of Stegoceras and Homalocephale, reveal enigmatic osseous
structures not present in other tetrapod clades. The homology and
functional significance of these structures have remained elusive as
they were originally interpreted to be abdominal ribs or gastralia,
and more recently have been interpreted as de novo structures in the
tail.

Principal Findings

Analysis of these structures in nearly all pachycephalosaurid
skeletons has facilitated a complete description of their
architecture, and the establishment of patterns consistent with those
of myorhabdoid ossifications — ossifications of the myoseptal tendons
associated with myomeres. The presence and structure of myorhabdoid
ossifications are well established for teleost fish, but this marks
their first recognition within Tetrapoda. These elements are both
structurally and histologically distinct from the deep, paraxial
ossified tendon bundles of other ornithischian clades, although they
may have performed a similar function in the stiffening of the tail.

Conclusions/Significance

These myorhabdoi are not de novo structures, but are instead
ossifications (and therefore more amenable to fossilization) of the
normally unossified plesiomorphic caudal myosepta of vertebrates. The
ubiquitous ossification of these structures in pachycephalosaurids
(all specimens preserving the tail also exhibit myorhabdoid
ossifications) suggests it is a likely synapomorphic condition for
Pachycephalosauria.