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Re: Rauisuchian osteoderms: structure and formation

Interesting. According to the authors, the mode of osteoderm formation in 
rauisuchians appears to be different from those of extant crocodylians 
(intramembranous vs. a radiating mass of calcified spicules). This seems to 
suggest that osteoderm formation occurred multiple time in the Pseudosuchia, 
and might not have been a plesiomorphic trait for archosaurs (though the 
potential may have been). A followup study on some crocodyliforme osteoderms 
would be a great complement to this study.



"I am impressed by the fact that we know less about many modern [reptile] types 
than we do of many fossil groups." - Alfred S. Romer

----- Original Message -----
> From: "bh480@scn.org" <bh480@scn.org>
> To: dinosaur@usc.edu
> Cc: 
> Sent: Thursday, 29 September 2011 1:01 PM
> Subject: Rauisuchian osteoderms: structure and formation
> From: Ben Creisler
> bh480@scn.org
> 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
> http://onlinelibrary.wiley.com/doi/10.1111/j.1475-4983.2011.01098.x/abstract
> 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 e
gy. 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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