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Re: Draco-Stygi-Pachy paper


 Horner JR, Goodwin MB (2009) Extreme Cranial Ontogeny in the Upper
 Cretaceous Dinosaur Pachycephalosaurus. PLoS ONE 4(10): e7626.

 Abstract [...]
 Coronal histological sections of the frontoparietal
 dome of an adult Pachycephalosaurus wyomingensis reveal a dense
 structure composed of metaplastic bone with a variety of extremely
 fibrous and acellular tissue. [...] These dinosaurs employed
 metaplasia to rapidly grow and change the size and shape of their
 horns, cranial ornaments and frontoparietal domes, resulting in
 extreme cranial alterations during late stages of growth.

And here's what that means:

John Horner & Ellen Lamm (2009): Metaplasia provided dinosaur skulls extreme morphological plasticity during ontogeny, online-only supplement to JVP 29(3), 117A

Arguably, one of the most interesting characteristics of the Dinosauria is the incredible morphological variety of their skulls. Numerous species in both the Ornithischia and Saurischia sport some sort of cranial display feature on one or more of their cranial elements. In addition, these features undergo considerable alteration during ontogeny, so much so that ontogenetic stages of some species have been described as different species. In each case these major alterations appear to have occurred very rapidly during late stages of ontogeny. In an attempt to determine how such changes could occur in short periods of time, numerous dinosaur skull bones were histologically examined. Surprisingly, every ornamented bone revealed tissues produced by metaplasia. Metaplastic bone is the result of fibrous tissues having been transformed directly into bone without the presence of a periosteum or osteoblasts. The metaplastic tissues observed in this study revealed a wide variety of tissue types, most often extremely fibrous, acellular and/or avascular, or sometimes highly cellular (fibrocytes rather than osteocytes) and highly vascularized. Mature metaplastic tissues are much denser than ordinary bone. Because these tissue types do not contain cells with canaliculi their initial thickness is limited, although at depth they are often remodeled by secondary reconstruction. In many of the bones sectioned there was evidence of deposition, erosion, and subsequent deposition indicative of continuous shape change. Metaplastic tissues found in the skulls of extant avian taxa form and alter very quickly, a characteristic
apparently acquired from their dinosaurian ancestors.

Bone formed without osteoblasts!?! Wow.

Unfortunately I missed the talk.

Metaplasia has already been described for *Majungasaurus* (SVP Memoir 8).