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Re: Diplodocoid neural spine bifurcation influenced by ontogenetic growth

If it's not been mentioned on the DML, it has been discussed
*extensively* on SV-POW!:


-- Mike.

On 4 April 2012 18:04, Ben Creisler <bcreisler@gmail.com> wrote:
> From: Ben Creisler
> bcreisler@gmail.com
> A new online paper not yet mentioned on the DML:
> D. Cary Woodruff & Denver W. Fowler (2012)
> Ontogenetic influence on neural spine bifurcation in diplodocoidea
> (dinosauria: Sauropoda): A critical phylogenetic character.
> Journal of Morphology (advance online publication)
> DOI: 10.1002/jmor.20021
> http://onlinelibrary.wiley.com/doi/10.1002/jmor.20021/abstract
> Within Diplodocoidea (Dinosauria: Sauropoda), phylogenetic position of
> the three subclades Rebbachisauridae, Dicraeosauridae, and
> Diplodocidae is strongly influenced by a relatively small number of
> characters. Neural spine bifurcation, especially within the cervical
> vertebrae, is considered to be a derived character, with taxa that
> lack this feature regarded as relatively basal. Our analysis of dorsal
> and cervical vertebrae from small-sized diplodocoids (representing at
> least 18 individuals) reveals that neural spine bifurcation is less
> well developed or absent in smaller specimens. New preparation of the
> roughly 200-cm long diplodocid juvenile Sauriermuseum Aathal 0009
> reveals simple nonbifurcated cervical neural spines, strongly
> reminiscent of more basal sauropods such as Omeisaurus. An identical
> pattern of ontogenetically linked bifurcation has also been observed
> in several specimens of the basal macronarian Camarasaurus, suggesting
> that this is characteristic of several clades of Sauropoda. We suggest
> that neural spine bifurcation performs a biomechanical function
> related to horizontal positioning of the neck that may become
> significant only at the onset of a larger body size, hence, its
> apparent absence or weaker development in smaller specimens. These
> results have significant implications for the taxonomy and
> phylogenetic position of taxa described from specimens of small body
> size. On the basis of shallow bifurcation of its cervical and dorsal
> neural spines, the small diplodocid Suuwassea is more parsimoniously
> interpreted as an immature specimen of an already recognized
> diplodocid taxon. Our findings emphasize the view that nonmature
> dinosaurs often exhibit morphologies more similar to their ancestral
> state and may therefore occupy a more basal position in phylogenetic
> analyses than would mature specimens of the same species. In light of
> this, we stress the need for phylogenetic reanalysis of sauropod
> clades where vital characters may be ontogenetically variable,
> particularly when data is derived from small individuals.