Re: Spinosaurs as what?

[Tim Williams <twilliams_alpha@hotmail.com>]

Renato Santos wrote:

> *Spinosaurus* has as far as we know piscivorous adaptations; I'm just 
> proposing a modus operandi for the creature's feeding habits based on 
> extant creatures that approximate its niche (if there are any). In fact, 
> *Spinosaurus* might as well had brightly colored feet as some herons do to 
> scare off fish as it waded ;-)

Before you go too much further with this, I would caution that not everybody 
is so keen on the idea that spinosaurids spent most of their time in water 
catching fish.

Sues et al. (2002) point out that the long narrow snouts of spinosaurids and 
piscivorous crocodilians (e.g., gavials) are actually rather different.  
Spinosaurids have flat-sided snouts that are convex on top, whereas the 
snouts of fish-eating crocs are more tubular.  Sues et al. (2002) suggest 
that this morphology, like the extensive secondary palate, rendered the 
spinosaurid snout less susceptible to bending stresses, and was not a 
piscivorous adaptation per se.  They also suggest that the shape of 
spinosaurid teeth (conical or only weakly recurved) are "consistent with 
their use for impaling and holding prey".  All in all, they suggest that the 
jaws of spinosaurids were adapted for seizing small prey.  The authors 
conclude:

"The skull of _Irritator_ does not appear to be well-suited for catching and 
processing large, resistant prey. Its structure differs from that in other 
large theropod dinosaurs such as _Allosaurus_ (Rayfield et al., 2001) and 
_Tyrannosaurus_ (Erickson et al., 1996), presumably reflecting different 
modes of feeding. Most likely spinosaurid theropods rapidly and forcefully 
seized smaller prey, which was then processed by dorsoventral motion of the 
head facilitated by the powerful neck musculature. (Extensive side-to-side 
striking movements of the head, as employed by extant crocodylians, appear 
unlikely in view of the narrow occiput as well as the weak development of 
the basal tubera.) Whereas fish formed part of the diet in at least _B. 
walkeri_, there is nothing to suggest that spinosaurids were exclusively or 
even predominantly piscivorous. Previous anatomical comparisons between the 
feeding apparatus of crocodylians and spinosaurid theropods were based only 
on superficial resemblances.  The postcranial skeleton of _Baryonyx_ lacks 
any obvious specializations suggestive of an aquatic or semiaquatic mode of 
life (Charig and Milner, 1997). Charig and Milner (1986, 1997) interpreted 
the greatly enlarged and strongly curved ungual of manual digit I as a 
?gaffing? device for catching fish, but this intriguing hypothesis remains 
untestable in the absence of a close analogue among extant tetrapods."

Also, don't forget that there is evidence that pterosaurs were also part of 
the spinosaurid diet.  The remains of one pterosaur were found with a 
spinosaurid tooth embedded in its neck vertebra.


References

Buffetaut, E., Martill, D., and Esculli, F. (2004).  Pterosaurs as part of a 
spinosaur diet. Nature 430: 33.

Sues, H.-S., Frey,E., Martill, D.M., and Scott, D.M. (2002).  _Irritator 
challengeri_, a spinosaurid (Dinosauria: Theropoda) from the Lower 
Cretaceous of Brazil.  Journal of Vertebrate Paleontology 22: 535?547