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Secondary Cartilage in the Post-Hatching Skull of Hypacrosaurus stebingeri in PLoS ONE

From: Ben Creisler

New in PLoS ONE:

Bailleul, A.M., Hall, B.K., & Horner, J.R. (2012)
First Evidence of Dinosaurian Secondary Cartilage in the Post-Hatching
Skull of Hypacrosaurus stebingeri (Dinosauria, Ornithischia).
PLoS ONE 7(4): e36112.

Bone and calcified cartilage can be fossilized and preserved for
hundreds of millions of years. While primary cartilage is fairly well
studied in extant and fossilized organisms, nothing is known about
secondary cartilage in fossils. In extant birds, secondary cartilage
arises after bone formation during embryonic life at articulations,
sutures and muscular attachments in order to accommodate mechanical
stress. Considering the phylogenetic inclusion of birds within the
Dinosauria, we hypothesized a dinosaurian origin for this “avian”
tissue. Therefore, histological thin sectioning was used to
investigate secondary chondrogenesis in disarticulated craniofacial
elements of several post-hatching specimens of the non-avian dinosaur
Hypacrosaurus stebingeri (Ornithischia, Lambeosaurinae). Secondary
cartilage was found on three membrane bones directly involved with
masticatory function: (1) as nodules on the dorso-caudal face of a
surangular; and (2) on the bucco-caudal face of a maxilla; and (3)
between teeth as islets in the alveolar processes of a dentary.
Secondary chondrogenesis at these sites is consistent with the
locations of secondary cartilage in extant birds and with the
induction of the cartilage by different mechanical factors - stress
generated by the articulation of the quadrate, stress of a ligamentous
or muscular insertion, and stress of tooth formation. Thus, our study
reveals the first evidence of “avian” secondary cartilage in a
non-avian dinosaur. It pushes the origin of this “avian” tissue deep
into dinosaurian ancestry, suggesting the creation of the more
appropriate term “dinosaurian” secondary cartilage.