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Hypacrosaurus embryo with secondary cartilage

From: Ben Creisler

New in PLoS ONE:

Alida M. Bailleul, Brian K. Hall & John R. Horner (2013)
Secondary Cartilage Revealed in a Non-Avian Dinosaur Embryo.
PLoS ONE 8(2): e56937

The skull and jaws of extant birds possess secondary cartilage, a
tissue that arises after bone formation during embryonic development
at articulations, ligamentous and muscular insertions. Using
histological analysis, we discovered secondary cartilage in a
non-avian dinosaur embryo, Hypacrosaurus stebingeri (Ornithischia,
Lambeosaurinae). This finding extends our previous report of secondary
cartilage in post-hatching specimens of the same dinosaur species. It
provides the first information on the ontogeny of avian and
dinosaurian secondary cartilages, and further stresses their
developmental similarities. Secondary cartilage was found in an
embryonic dentary within a tooth socket where it is hypothesized to
have arisen due to mechanical stresses generated during tooth
formation. Two patterns were discerned: secondary cartilage is more
restricted in location in this Hypacrosaurus embryo, than it is in
Hypacrosaurus post-hatchlings; secondary cartilage occurs at far more
sites in bird embryos and nestlings than in Hypacrosaurus. This
suggests an increase in the number of sites of secondary cartilage
during the evolution of birds. We hypothesize that secondary cartilage
provided advantages in the fine manipulation of food and was selected
over other types of tissues/articulations during the evolution of the
highly specialized avian beak from the jaws of their dinosaurian