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Re: competition

A typical technical glitch held this up.  Chris, I hope I fixed the
right thing; re-check your first paragraph...  -- MPR

------- Start of forwarded message -------
Date: Wed, 1 Aug 2001 10:18:31 -0700
To: dinosaur@usc.edu
From: chris brochu <cbrochu@fieldmuseum.org>
Subject: Re: competition

>    There is no doubt that Tomistomines gradually radiated, and supplanted
>the dyrosaurs.  Both taxa probably frequented estuaries and would have
>competed.  Occasionally, floods could have swept dyrosaurs out to sea where
>they were buried, just like some near-coastal nodosaurs.  Had the dyrosaurs
>been truly pelagic, they would have vanished with the mosasaurs when marine
>productivity collapsed.

Actually, there is some doubt about that, as tomistomines were not
estuarine when they first appear (e.g. "Crocodilus" spenceri, Dollosuchus,
etc.).  Moreover, they first appear in the lower Eocene.  Gavialoids first
show up in the late Cretaceous (e.g. Thoracosaurus, Thecachampsoides) and
definitely WERE estuarine at first.  In fact, their earliest distribution
is circumatlantic and they almost always show up in nearshore facies.

Something that might confuse the issue is that much of the older literature
refers things like Thoracosaurus to Tomistominae.  This is largely because
Thoracosaurus and the like retain some plesiomorphic conditions seen in
extant Tomistoma but not in Gavialis - the premaxillae and nasals are in
mutual contact, for example.  But phylogenetically, Thoracosaurus is closer
to Gavialis.

>    Like many other forms, Tomistomines were initially successful only to be
>supplanted in their turn.  The generalized Nile crocodile was more adaptable
>than the specialized Tomistomines.  Drier conditions or periodic droughts
>more seriously affected crocs which relied exclusively on fish.  So, perhaps,
>did avian competitors for that niche.  In contrast, the Nile crocodile could
>eat a wide variety of prey, and scavenge carcasses.  Specialized crocs
>persist in some environments, but their decline is not surprising.

Not sure why you say this, because tomistomines show their maximum
diversity much later in the Tertiary and don't appear to have been replaced
anywhere by Crocodylus - they died out in the New World prior to the
arrival of Crocodylus at around 5 million years ago, and in Europe (where
Crocodylus may never have existed).  They are no longer present in Africa,
but again their disappearance seems to have predated the origin of
Crocodylus.  Indeed, the really common longsnout in the subsaharan African
Tertiary is Euthecodon, and that is closer to Osteolaemus than to Tomistoma.

Again, though - Euthecodon is frequently referred to Tomistominae.


- ------------------------
Christopher A. Brochu
Assistant Professor
Department of Geoscience
University of Iowa
Iowa City, IA 52242

319-353-1808 phone
319-335-1821 fax
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