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Rauisuchian osteoderms: structure and formation

From: Ben Creisler

A new advance online paper:

Palaeohistology and external microanatomy of rauisuchian osteoderms
(Archosauria: Pseudosuchia).
Palaeontology (advance online publication)
DOI: 10.1111/j.1475-4983.2011.01098.x

The presence of postcranial dermal armour is plesiomorphic for Archosauria.
Here, we survey the external microanatomy and histology of postcranial
osteoderms (i.e. dorsal paramedian and caudal osteoderms) of rauisuchians,
a widely distributed assemblage of extinct predatory pseudosuchians from
the Triassic. The osteoderms of eight rauisuchian taxa were found to be
rather compact bones, which usually lack significant bone remodelling or
large areas of cancellous bone. The presence of highly vascularized woven
or fibrolamellar bone tissue deposited in the core areas indicates higher
growth rates during earlier life stages, whereas a more compact
parallel-fibred bone matrix indicates reduced growth rates in later
development. This pattern of change corroborates earlier studies on long
bone histology. With the exception of a bone tissue found in the sample of
Batrachotomus kupferzellensis, which might be the result of metaplastic
ossification, the general mode of skeletogenesis is comparable with
intramembraneous ossification. The lack of cancellous bone tissue and
remodelling processes associated with bone ornamentation, as well as the
predominantly intramembraneous mode of ossification, indicates that
rauisuchian osteoderm formation differs profoundly from that of the
osteoderms of the only extant pseudosuchian lineage, the crocodylians.

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