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Re: Ouranosaurus - how many species? Other "spino"-Igaunodontids?
In addition, to distinguishing tall sail-like structures from humps, one could
presumably further distinguish humps depending on the ratio of fat to muscle
content. I'm not arguing that these features were all structurally identical
or that they all addressed the exact same combination of selective pressures
(eg. in cases where a sail may have had an additional role in communication, in
addition to serving for heat dissipation of food storage).
However, the fact that *all* of these humped / sailed species existed during
the early cretaceous would suggest that almost all of these adaptations
represent cases of convergent evolution in response to some environmental
pressure. It would have to be an environmental pressure that was specific to
the early cretaceous and had similar consequences for all of these widely
distributed and distantly related taxa. This would also nicely explain the
surprising relative lack of humped or sailed species in other time periods.
Considering this conjecture it seems plausible that the unusual morphology of
Tenontosaurus may have been a response to the same pressure. Tenontosaurs
probably had the deepest, as well as one of the longest, tails, relative to
body size, of any ornithopods.
So, I'd tentatively argue that Tenontosaurs should be included in lists of
humped or sailed species.
- Jonas Weselake-George
P.S. Modification of the dorsal side of the animal or the tail is less likely
to interfere with locomotory or feeding adaptations. So, both areas may be
analogously good candidates for such elongated structures to develop. In fact,
it could further be ventured that the different locomotory behaviour of
Tenontosaurs (eg. clearly visible in the structure of the feet) and habitat
requirements either associated with different locomotory behaviour or a need to
pass under obstacles, may have been deciding factors in why Tenontosaurs would
develop an elongate/deep tail instead of a hump.
On Tue, 27 Oct 2009 15:26:00 -0700 (PDT)
Tim Williams <email@example.com> wrote:
> Jonas Weselake-George <firstname.lastname@example.org> wrote:
> > If it wasn't for the odd phylogenetic placement, one could
> > also argue that Tenontosaurus fits this category as well. Of
> > course, in this case the equivalent of the "spine" takes the
> > form of a much enlarged and relatively laterally compressed
> > tail, not a sail/whithers. However, it is likely that what
> > ever drove the unique sail/whithers development in other
> > ornithopods and large theropods during this period is behind
> > this oddity as well.
> Elongation of neural spines has been tied to two very different anatomical
> structures: (a) sail, (b) ridge or hump. The extremely elongated neural
> spines of taxa such as _Ouranosaurus_, _Amargasaurus_ (paired), and
> _Spinosaurus_ have usually always been interpreted as supporting a membranous
> sail. But for taxa such as _Acrocanthosaurus_ and _Lanzhousaurus_, which
> have "moderately" tall spines, the spines might have been buried in a
> muscular ridge or hump. So the latter would be more like the hump of a bison
> (mostly muscle) than the hump of a camel (fat), where the spines are not
> elongated for this purpose. For _Tenontosaurus_, the enlarged caudal neural
> spines were probably enclosed in flesh, to produce a strong, muscular tail.
Jonas Weselake-George <email@example.com>