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



Jaime Headden wrote-

>   In comparison, *Bison* and *Spinosaurus* are not comparable. No extant
> ungulate has a thoracic neural spine as narrow as that seen in
> *Spinosaurus* or *Ouranosaurus*, though they do near the condition in
> *Acrocanthosaurus*. Rather, a few extinct ungulates do bear neural spines
> in ther thoracic region that have proportions near to that of
> *Ouranosaurus*; they stiull have distal transverse expansions of the
> spines and are relatively broader than in *Spinosaurus* whose spines,
> contra the above observations at least to this observer, give or take the
> spines that are deformed and quite wavy in transverse aspect, are very
> smooth, lack lamina except near the base, where there is a basal
> craniocaudal expansion that likely braced the strong epaxial muscles that
> moved the spine itself.

Spinosaurus' neural spines are ~30% of central width, compared to Bison's
~25%.  I see no evidence Spinosaurus has transversely narrower spines than
Bison.  You agree below that some Bison have non-rugose neural spines ("Here
[in Bison], at least, a rugose texture as seen in Acrocanthosaurus is
missing,"), so this is obviously not necessary for a hump.  The immaturity
of Spinosaurus' holotype could be an additional reason this is not seen in
that taxon, as we might expect only older animals to develop such texturing.
Bison shows no neural spine laminae either, so this is not a valid argument
against Spinosaurus' hump.  I agree Spinosaurus lacks spine tables, and is
in this way unlike Bison.  But you have yet to explain why this is necessary
for a hump.  Indeed, as (as you argue below) Spinosaurus' head was
comparatively so much smaller than Bison's, it might not have needed such
extreme transverse development of the neural spines to attach such large
ligaments and muscles to.

> Otherwise, and all ungulates (including several
> powerfully spined brontotheres which have the record for spine to centrum
> ratio), the neural spines lack distal transverse expansion, bear a single
> longitudinal channel on both cranial and caudal margins, and taper or are
> semi-circular in profile rather than being rectangular in aspect distally.

This is confusing.  You're saying ungualates LACK spine tables now?
Spinosaurus also has interspinous ligament grooves posteriorly (Stromer
1915- "Its posterior edge, which far ventrally is double edged, is then
rounded and here rather often ornamented with a shallow channel"), and Bison
has no such grooves anteriorly, so is not different from Spinosaurus in this
respect.  Spinosaurus' neural spines have a similar distal profile to
cervical 7 and thoracic 1 of Bison.

> Mickey
> cites rugose texturing, and observes a poor preservation, but this is
> wrong: the material was very well preserved, with smooth and perfectly
> articulated spines when recovered -- McGraf was a very competant field
> worker, and though the material of *Paralititan* is not perfect in
> preservation style, this was no meter to the perservation the type of
> *Spinosaurus* was in: the plates available from Stromer show no doctoring
> -- other material from Baharija show various styles of preservation,
> including some as in *Paralititan*, some worse off, some better off, and
> the condition of the last matches that of *Spinosaurus*, so I am given to
> think this is inadequate a refutation. If there are further similarities,
> then I would be willing to hear them.

I cited the absence of rugose texturing more developed than that in Bison
(which we agree on, see above).  I never "observed a poor preservation".  I
said "Indeed, it would be hard to disprove rugosity in
Spinosaurus, given the state of the material."  The state of the material is
"destroyed", and Stromer's figures (while excellent) are not at the scale
needed to determine rugosity levels.  It's irrelevent now however.

>   As for humped animals, no -- I repeat NO -- animal has a hump which is
> correlated to bones. The humps of zebu cattle and camels are above the
> bones, and in camels occur caudal to the shoulders.

This is NOT, and I repeat NOT, applicable to the argument at hand.

> The elongation of the
> neural spines between the shoulder blades occurs as a brace to the nuchal
> ligaments for supporting a massive head at the end of a short neck lacking
> neural spines. This is hardly a problem in theropods or iguanodontians
> which have relatively robust cervicals fully capable, via the "S-curve"
> design, of supporting a head without neaural spine bracing. The point of
> the withers hump in ungulates is entirely unneccessary in theropods at
> least, and for obligate bipeds such as the slender-armed *Ouranosaurus*,
> would have been pointless given the robust neck and small skull compared
> to a relatively short-spined ungulate such as the camel.

Perhaps this is related to the reduced "S-curve" in spinosaurids then
(Charig and Milner, 1997).  As for Ouranosaurus, it is thought of as
facultatively quadrupedal last time I checked (Rasmussen, 1998).  As
ABSRDists need to learn, the absence of a known use for a structure should
not override the morphological evidence for one.

>   Bailey's flaw is that he fails to correlate the spine height and breadth
> at the distal end with a functional relationship to the short neck and
> large skull being supported. In the reptiles to which the "hump" theory
> was applied, this is hardly a problem, as the neck is far more robust and
> the skull hardly as massive.

Bailey actually addresses this-
"Although it was formerly thought that humps are related to support of big
horns or heavy heads (Koch, 1932), there is no significant correlation
(Guthrie, 1990); high humps are evidently an adaptation for sustained long
distance mobility across long and open home ranges marked by forage that is
both short, sparse, and low in nutrition (Guthrie, 1990)."

> I had written:
>
> <<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.>
>
>   Not when someone's arguing for humps in dinosaurs, such as Bailey.

Surely you are not so dense as to not realize Bailey has a broader
definition of humps than you do, and such qualms are merely semantic and do
not help the debate move forward.  We are discussing muscular humps, not
fatty humps.  No need to mention the latter.

> The
> muscular component of the *Spinosaurus* dorsal series is lost without the
> longitudinal lateral ridges marking the boundaries between the vertebral
> musculature from vertebra to vertebra (and not the interspinals which are
> marked by the longitudinal cranial and caudal channels). The complex of
> intervertebrals in *Spinosaurus* appear to have been limited to the
> expanded base of the spine.

The muscular component is lost?  Sorry, you'll have to restate that.
Neither Spinosaurus nor Bison have longitudinal lateral ridges on their
neural spines in any case.

>   http://lamar.colostate.edu/~lctodd/coding.htm offers a link to photos
> of the a modern *Bison bison*. Here, at least, a rugose texture as seen in
> *Acrocanthosaurus* is missing, whereas the bones preserve similar laminae
> on the lateral surfaces of the cranial and caudal margins, also seen in
> *Acrocanthosaurus* but absent in *Ouranosaurus* or *Spinosaurus*.

This is quite useful, especially
http://lamar.colostate.edu/~lctodd/thoracic.htm .  Still, no laminae are in
evidence.  The anterior view shows a thin anterior margin, like Spinosaurus.
The posterior margin is indistinct in lateral view.  It could be a thin
margin, or a bifurcated margin.  The latter would not be laminae as such.
Instead, they would be expanded and centrally depressed areas for ligament
attachment (as seen in both Spinosaurus and Cervus [pers. obs.]).

> In fact, given the anatomy, aside from the lack of a
> circular or ovate cross-section, *Spinosaurus* spines resemble those of
> purported thermodynamic *Dimetrodon* spines, given their apparent lack of
> much muscular support.

To the contrary, aside from the lack of spine tables, Spinosaurus' spines
resemble those of Bison.

> <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.>
>
>   The degree of mammal spine development gives us a resounding correlate
> between bone and muscle. To assume otherwise for dinosaurs, so that
> simialr muscles would be on the spines without similar anchors to mark
> their place, would invariably fall flat in the face of comparative myology
> and the evidences given for fossil forms.

I'm saying Spinosaurus didn't necessarily have expaxial ligaments and
muscles as powerfully developed as mammals do.  Thus, the osteological
correlates wouldn't be expected to be the same.

> For the most part, in hadrosaurs and
> iguanodontids, the neural spines are tall and still have a similar form to
> ungulates. Where else to draw the line, when one form departs from this? I
> see no reason to give *Spinosaurus* a hump versus a sail.
> The dinosaurian condition is
> in fact similar to ungulates. The problem is in *Spinosaurus*
> applicability, since it, of all dinosaurs with tall neural spines, is
> distinctly a departure in form.

And here we get to an important consideration.  All tall-spined dinosaurs
did not have similar neural spine morphology.  We might expect a continuum
of dorsal structures, from the hump-like structure in Acrocanthosaurus, to
the sail-like one of Amargasaurus cervicals.  Maybe iguanodonts had
something in between, but much closer to a hump.  And maybe Spinosaurus had
a hump that was not as well muscled as iguanodonts or bovids, so was
narrower and more sail-like.  I think of a sail as being primarily bone and
skin, so Spinosaurus' structure does not fit this paradigm in my mind.  The
interspinous ligament grooves show flesh between the spines was substantial.
The amount of flesh lateral to the spines is less certain, considering the
lack of obvious osteological correlates in Bison.  Perhaps a word like
"ridge" would be more accurate than "hump" in Spinosaurus' case, considering
the narrow peak.  Bailey's emphasis on muscular structures versus skin-based
ones is still accurate however.

Mickey Mortimer