Re: Swiming spinosaurus

["Adam Britton" <abritton@wmi.com.au>]

From: <GUNTER.VANACKER@ELECTRABEL.COM>
Sent: Monday, July 09, 2001 8:04 PM


> All these features indicate that
> spinosaurids actively hunted fish, however, I do not believe that they
> pursued fish while swimming, but that they caught fish like most bears now
> do; looking for fish while standing on the shoreline (or the waterline to
> their knees at most). When the spinosaur spotted a fish, the theropod
would
> use its powerful hands, with the large thumbclaws, to grab it (like most
> basal tetanurans such as allosaurids and megalosaurids, spinosaurids had
an
> unusually large manual ungual I). The theropod would often immerse its
head
> (but not the rest of its body) under water to look for fish, or actually
> catch them if he had the chance.

In crocodilians that include high percentages of fish in their diets (eg.
Indian gharial, _Gavialis gangeticus_), the principal advantage of having
longirostrine jaws is to reduce resistance when the jaws are swept through
the water. If you catch prey with your hands, then diet doesn't necessarily
influence skull shape (look at bears) but if you catch prey with your jaws
directly, then suddenly shape becomes an issue. I wouldn't be surprised if a
spinosaur was far more "agile" with its head movements than with its
forelimb movements, making the jaws the best means of prey acquisition.
Another big plus for crocs is that the principal sensory apparatus are on
the head, include pressure-sensitive pits lining the jaws to detect subtle
pressure changes underwater (ie. nearby fish swimming past the head).

Longirostrine jaws do not necessarily indicate an obligate fish specialist,
though. Several living crocodilians have extremely elongated jaws (eg.
_Gavialis_, _Tomistoma_, _Crocodylus cataphractus_, _C. johnstoni_, C.
intermedius_) yet only one of these (_Gavialis_) is considered a fish
specialist. The others are generalists that include a reasonable proportion
of fish in their diets, but can exist on a variety of creatures that venture
too close to their head (taken in water or on land).

Adam Britton