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Chondroid bone in hadrosaur embryos and nestlings

Ben Creisler

A new online paper:

Alida M. Bailleul, Catherine Nyssen-Behets, Benoît Lengelé, Brian K.
Hall & John R. Horner (2015)
Chondroid bone in dinosaur embryos and nestlings (Ornithischia:
Hadrosauridae): Insights into the growth of the skull and the
evolution of skeletal tissues.
Comptes Rendus Palevol (advance online publication)

In histology textbooks, the vertebrate skeleton is represented as
almost entirely made of bone and cartilage. This is a false dichotomy
and in fact, a continuum of intermediate tissues between bone and
cartilage exists. Chondroid bone ([CB] or chondroid tissue), one of
the most well-known intermediate tissues, has been reported in
mammals, birds and crocodilians. It accommodates (1) rapid growth of
the skull and (2) the development of craniofacial sutures. Since CB is
present in the extant phylogenetic bracket of the Dinosauria, we
hypothesized that it was also present in non-avian dinosaurs. By means
of paleohistological examination and microradiography, we report for
the first time the presence of CB in non-avian dinosaur embryos and
nestlings (Ornithischia: Hadrosauridae). It was found in five
locations: (1) scattered within the bone trabeculae of an embryonic
surangular; (2) and (3) in the coronoid process and in the alveolar
processes of an embryonic dentary; (4) in the mandibular symphyses of
an embryonic and a post-hatching dentary; (5) at the
fronto-postorbital suture of an embryo. In these areas, CB was present
in large amounts, suggesting that it played an important role in the
rapid growth of the hadrosaurian skull during embryonic development.
Moreover, the CB present in the sutural borders of a Hypacrosaurus
frontal suggests that it was also involved in sutural growth, as it
has been reported to be in mammalian and avian sutures. This is the
first step taken to document and understand dinosaurian sutures from a
histological perspective and it sheds light on an old problem by
reporting the presence of CB in an additional clade within the
Vertebrata. It is parsimonious to propose that CB in the chick embryo,
Gallus gallus, the American alligator, Alligator mississippiensis and
the hadrosaurs of the present study are homologous and that CB arose
once and was inherited from their common ancestor.