Re: Fw: Spinosaurus again

["Jaime A. Headden" <qilongia@yahoo.com>]

I don't really understand the issues about relating the bones to one another as
a problem, though it is possible. Theoretically.

The cervicals and dorsals all possess elongated spindle-like centra with a
flared caudal rims, and a large cranial "ball" condylus articulating into the
deep posterior cotylus of the preceding centrum. The primary features that make
*S. aegyptiacus* vertebrae so distinct are the absence of distinct central
laminae in most of the dorsals, or at least their great reduction in size in
the most cervical-like of them. Not all of the neural spines were found
articulated to centra, and only one vertebrae with an open neurocentral suture
was found even closely associated, as Stromer noted (which allowed him to place
these into a single element), whereas the other vertebrae were more or less a
hodgepodge, so even the series as known was likely quite different from
Stromer's arrangement, although I'll grant him he did do a good job of laying
the evidence for the articulationm even if I disagree.

  None of the cervicals have complete neural spines and the anterior and
posterior margins appear to be abrupt and distinct from the neural arch itself,
such that one might expect tall and robust spines compared to, say,
*Afrovenator* or *Monolophosaurus*, both representative tetanurans. This
matches what we have of *Baryonyx* and even *Acrocanthosaurus*, but also
*Allosaurus*.

  There is no evidence the caudal vertebrae is not theropodan, but rather its
shape, being extremely anteroposteriorly short, has been likened to some
ornithischians (say, a hadrosaur or stegosaur tail might have a
similarly-shaped centrum). However, the centrum, as acknowledged by Stromer,
was also hollow. It is not so enlarged as to prevent articulation if we assume
the sacrum (as known) was not complete and may, in fact, be reversed so that
the complete sacral face available was articulated to a dorsal or indeed an
unknown free sacral.

  It is also clear to me that Stromer had his vertebral sequence in error,
based largely on his attempt to coalesce centra and neural spines and
articulate the spines into a dorsally-arrayed arch. Problematically, this
avoids some problems, such as the absence of laminae around the transverse
processes, extending from the arch to the centrum, and from the spine to the
processes and the zygapophyses, as well as the size and orientation of the
zygapophyses themselves. Indeed, some of the spines could be CAUDAL, rather
than DORSAL, and the order of the centra, when arrayed in the typical tetanuran
arrangement of gradual flattening of the central faces and shortening of the
centra (approaching a ratio of 1) between length and posterior height, one
would arrive at a wholly dissimilar arrangement of spines. I used various
criteria to arrange the centra and spines, comparing them with other tetanurans
for consistency (not just *Baryonyx*), including the shape and presence of the
basal "bulge" of the spines, laminae, centrum "flattening", length:height ratio
of the centrum, spine distal morphology (where available), and the length of
the neural spine base (articulation with the centrum). I find myself
distrustful of using the plates in Stromer's work since as Smith et al.
describe, there are some differences between bone and drawing, and I don't
trust the renderings of the centra/spine articulation, though I do not accuse
the artist of distortion, either. This reorganization seems to shorten the
spine, somewhat, as well.

  Cheers,

Jaime A. Headden
http://bitestuff.blogspot.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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