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Intercostal plates in ornithischian dinosaurs

From: Ben Creisler

A new online paper:

Clint A. Boyd, Timothy P. Cleland and Fernando Novas (2011)
Osteogenesis, homology, and function of the intercostal plates in
ornithischian dinosaurs (Tetrapoda, Sauropsida). 
Zoomorphology (advance online publication)
DOI: 10.1007/s00435-011-0136-x

Intercostal plates are bony structures positioned lateral to the anterior
dorsal ribs in some ornithischian dinosaurs. Some propose these plates are
homologous, or functionally analogous, with the uncinate processes of
extant avian dinosaurs that assist in breathing, while others suggest they
served a defensive function. To elucidate their osteogenesis, homology, and
function, a histological survey of intercostal plates from three taxa
(Hypsilophodon, Talenkauen, and Thescelosaurus) was undertaken. This study
reveals that osteogenesis of intercostal plates closely resembles that of
secondary centers of ossification in endochondral bone, typically present
in the epiphyses of mammalian long bones. In contrast, ossification of
avian uncinate processes begins at a primary ossification center via the
development of a bony collar around a cartilaginous model. Based on these
data, intercostal plates and avian uncinate processes are likely not
evolutionary homologs. Dense packets of obliquely oriented Sharpey?s fibers
within the parallel-fibered bone of somatically mature intercostal plates
indicate these plates were positioned medial to at least a portion of the
hypaxial musculature, which does not support their use as bony armor.
Rather, we propose that intercostal plates performed some biomechanical
function, either assisting in breathing in a way analogous to avian
uncinate processes, or working together with the sternal ribs and sternal
plates of these ornithischian taxa to provide increased rigidity to the
anterior portion of the ribcage. 

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