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Re: dinosaur humps (was Re: A Whole Bunch Of Questions)



Jaime Headden wrote-

> The first criticism on Bailey's work and as yet unchallenged (as on the
> list) is that the neural spines of the dinosaurs offered as "humped" do
> not bear rugose textures and nor are they thickened mediolaterally or at
> least as mediolaterally broad as they are craniocaudally long, as occurs
> in most "withered" ungulates, especially bovids.

>From the available illustrations, the neural spines of Ouranosaurus and
Spinosaurus look just as rugose in texture as those of Bison or
Gigantocamelus.  Indeed, it would be hard to disprove rugosity in
Spinosaurus, given the state of the material.  One of Bailey's main points
was that humped artiodactyls have transversely narrow neural spines (which I
can confirm), not "at least as mediolaterally broad as they are
craniocaudally long" (pg. 1126), so that is not a valid criticism.

> On the other hand, the
> neural spines of *Acrocanthosaurus*, contra those of *Spinosaurus*, _do_
> have rugose texturing that implies muscular development. But the lack of a
> "withers" as in mammals implies these are related, as in shorter-spined
> theropods, to the epaxial, parasagittal vertebral musculature.

Bailey specifically argued such neural spines were for the attachment of
"large epaxial ligaments and muscles", so you are not contradicting him
here.

> A fatty hump has no skeletal
> analogue and cannot be indicted or contradicted on osteological material.

And no one's arguing for its presence, so it's useless to mention.

> Such a rugose neural spine anatomy occurs to a limitied extent in
> *Suchomimus* above the hips. The neural spines of *Spinosaurus* and
> *Baryonyx* as well as *Suchomimus*, do not have distal mediolateral
> expansions, or broader distal ends than the mid-shaft of the spine.

Do all tall-spined artiodactyls?  Bison seems not to have very much
transverse expansion.

> In
> *Acrocanthosaurus* (rugose) and *Ouranosaurus* (non-rugose) the neural
> spines have broadened "tables" (but this is also seen in the short-spined
> *Tarascosaurus*, and has been linked in thyreophorans to osteoderm
> support, however unlikely in the various unarmored theropods with the
> neural "tables". While the expanded distal ends occur in mammals with
> withers, they are to a more extreme degree in ungulates than in
> tall-spined dinosaurs.

Based on the condition in Bison
(http://land.sfo.ru/eng/images/Animals/bizon1b.jpg), I would argue the spine
tables are comparable in extent to Acrocanthosaurus (Harris 1998, fig. 20,
21).  Even when they are not, why is it necessary for the development to be
equal in dinosaurs and mammals?  Sure such expansion increases the area for
epaxial attachment, but organisms are not as well adapted anatomically as
would be possible.

> Another tall-spined dinosaur, *Amargasaurus* (as
> well as its relative *Dicraeosaurus*) lacks expanded distal ends, but
> taper instead with lamina indicating extensive epaxial musculature as in
> other, shorter-spined sauropods (which, as in *Suchomimus*, also are
> tallest above the dorsals (with the exception of *Amargasaurus*)) but no
> "hump" or narrow sail.

So again, you're saying distal transverse expansion is for the attachment of
"large epaxial ligaments and muscles", which is exactly what Bailey argues.
Whether it's connected to a spine table or laminae is of no consequence, as
they are functionally analogous.  Such vertebrae as found in dicraeosaurids
would have undoubtedly made a hump instead of a sail, given their great
transverse diameter.

>   It would be nice to see a published rebuttal to Bailey's theory
> detailing these things. They would appear to contradict his conclusions.

At the best, they agree with his conclusions, or state that the dinosaurian
condition was less developed than the mammalian one (but not demonstrably
different).  At the worst, they are inaccurate or inapplicable to the
debate.

Mickey Mortimer