Re: SUCHOMIMUS STUFF

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 12:18 PM 11/16/98 +0000, Brian Franczak wrote:

>My local paper, the Hartford Courant, ran a skeletal drawing with the
>_Suchomimus_ article the other morning. I assume this illustration is
>from the SCIENCE article (I haven't seen the magazine yet). It appears
>that there are four fingers on the manus in this reconstruction. Is this
>correct?

>From the material described, there is no evidence for more than three fingers.

>> Will talk some more about spinosaurs later: artists, please note,     >
though: according to Sereno et al.'s reidentification of skull        >
material in the _Baryonyx_ holotype, the skull of that animal does  > not
look like what has been restored before.  (One of the footnotes > in the
paper details the reidentification of these elements).
>
>Just how much of the _Suchomimus_ skull is actually known? The skeletal
>drawing showed only (what I took to be) the quadratojugal, the
>premax/maxilla up to about mid-antorbital fenestra, and a similar length
>of the dentary. Is there more known than has been figured here?
>
What cranial material is known for _Suchomimus_:
Articulated premaxillae and maxillae
right quadrate
partial dentaries

What was reidentified in the _Baryonyx_ skull:
(Charig & Milner's IDs -> Sereno et al.'s IDs)
left postorbital -> posterior portion of right surangular
left jugal -> right prearticular
right atlantal neural arch -> central body of left pterygoid
left angular -> right angular

Sereno et al. also reorient some features of the posterior part of the
cranium of _Baryonyx_.

As I have said, the current re-examination of _Irritator_ will hopefully
clear some of this up.

Now, on to spinosaurids as croc-mimics:
I) Ecomorphology
There are a LOT of croc like features in the skulls of spinosaurids:
elongate snout, conical teeth, dentary rosette, secondary palate, very deep
basipterygoid articulation, the "double wave" in the snout.  Most (all?) of
these features have been associated with the adoption of a piscivorous diet
in crocodylomorph evolution.  As Charig & Milner have pointed out, the
retracted nares might be considered sort of a reverse of the "snorkles" of
parasuchians or crocodilians: a spinosaur could keep its snout dipped in the
water (or inside a prey item) while still breathing (just as a croc or
parasuchian can/could float in the water and keep breathing).

However, as Charig & Milner, and Sereno, and I have all pointed out, modern
crocs (at least many croc taxa) are not *obligate piscivores*: they can and
do eat tetrapods as well.  The fact that acid-etched _Iguanodon_ bones as
well as acid-etched fish scales are found in the belly of _Baryonyx_ suggest
it ate both.

II) Ecology
It is true that grizzlies only eat salmon during the salmon run season.  It
is also trye that this is the only season in which large fish exist at
sufficient quantaties in the streams of the American Northwest.

The paleoenvironments from which known spinosaurids come all support diverse
communities of fish, some of which were very large.  There is currently no
evidence that these were seasonal communities, and would seem likely to be
permanent residents.  Thus, we have very large packages of fish meat not
otherwise easily exploitable by theropods.  (A potential advantage of
spinosaurids over giant Cretaceous crocs is their mobility: a spinosaurid
would probably have an easier time getting from stream to stream and lake to
lake).

Noting that in the modern world no large endotherms get the majority of
their food as fresh-water fish is interesting, true.

However, be careful of this argument.  By similar reasoning, as there are no
multitonne terrestrial endothermic carnivores in the modern world, one might
argue that a multitonne predator would have to be an ectotherm.  In
particular, the theropods of the mid-K of North Africa were necessarily
ectothermic, because we have at least three multitonne theropods
(_Carcharodontosaurus_, _Spinosaurus_, and _Deltadromeus_ in the Cenomanian;
a carcharodontosaur, _Suchomimus_, and a _Deltadromeus_-like in the
Aptian/Albian) without (as far as we know) huge herds of the contemporary
herbivores.  (Maybe that's what Sereno et al. will discover for a 2000 issue
of Science: the first _Ouranosaurus_ bonebeds!!).

That's it for the moment: have to get my paper finished!!!

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661