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Re: Suchomimus vs Baryonyx: Dare to Compare

Betty Cunningham <bettyc@flyinggoat.com> writes:
> what precludes the whole body of Suchomimus from ungulating while
> swimming?

You mean "undulating," of course.

> I think that might be a tad hasty, unless the article reinforced
> something I haven't seen yet. (not having seen teh article, I can't say 
> ;]
> Even terrestrial lizards (such as the one in the study at Vassar I
> posted) have no problems ungulating while swimming to locomote-as long
> as they don't have fused vertebrae or rigid tendons along the spine to
> keep it from doing so.

Let's bear in mind that the spinosaurids are considered to be basal members
of the clade _Tetanurae_, which means "stiff tails" (Gauthier 1986).  Such
theropods tend to have rather stiff torsos and their tails are generally
more mobile proximally (at the base of the tail) than distally (most
pronouncedly in the highly derived dromaeosaurids), whereas crocs and
lizards have a body plan of ribs and vertebrae which is more uniform along
the whole length of the animal, and facilitates the undulating movement you
describe.  Among extant reptiles who swim, and we can certainly include
snakes in this description, the undulations of the swimming reptile are no
different than the serpentine undulations employed in terrestrial
locomotion.  I do not imagine such twisting motion would come naturally to
a theropod dinosaur.   Is there evidence for enlarged transverse processes
on either side of the caudal vertebrae in theropods, as we see in
crocodiles?  Do the neural spines taper gradually toward the tip of the
tail (as in crocs) or rapidly dwindle?  Would the animal be submerged to
such an extent that the whole length of the body would come into play, or
would the air sacs in the body serve it to keep the head and back well out
of the water?  Regarding tail mobility, I wonder if the tall, blade-like
neural spines characteristic of _Suchomimus_ -- which, you will note, are
tallest atop the hips and the proximal section of the tail (unlike the
condition seen in _Spinosaurus_) -- would be a hindrance to the sinuous
undulation you propose.

I would like to see more data to resolve the issues regarding the
functional morphology of this animal, (and I obviously must read Thomas
Holtz' paper!), but I would agree with Tracy Ford that, based on a
preliminary review of the _Suchomimus_ reconstructions, and a consideration
of basic theropod anatomy, it would appear to be improper to ascribe a
full-body undulation swimming style to _Spinosuchus_.  On the other hand,
this doesn't mean that _Spinosuchus_ couldn't swim, for theropods typically
had large feet and mobile tails.  Based on the design of the body, would it
not be more appropriate to compare a swimming _Suchomimus_ to a swimming
bird?  The typically large theropod foot would also be helpful to stabilize
a spinosaur up to its belly in water, supported by a muddy substrate,
snapping at fish and other creatures with its long narrow snout.  Perhaps,
once the mouth captured an animal, the meathook manual claws came into play
to assist in landing the larger prey items.

There may be other papers which address the subject of dinosaur swimming
ability, but until I run across them, I recommend Bakker's _The Dinosaur
Heresies_, and Paul's _Predatory Dinosaurs of the World_, both of which
have helped me to imagine the motory abilities of dinosaurs.

-- Ralph Miller III     gbabcock@best.com