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Re: Cretaceous therapods of the south

At 06:27 AM 5/20/98 -0700, Joshua Dyal wrote:
>It seems to me that "traditionally" abelisaurs have been seen as the
>dominant Gondwananan and post Gondwanan predators.  Is this still the
>case?  The recent discovery of a whole slew of allosaur-like
>carnivores (Giganotosaurus, Carcharadontosaurus, Rapator?) from
>various parts of the post Gondwanan continents seems to support the
>idea that maybe they were just as plentiful.  Or maybe there are
>really more abelisaur finds than I am aware of.  It was my impression
>that they have been rare (the finds, that is.)  What is the current
>thinking here?

Abelisaurs seem to have been relatively diverse in the Cretaceous of at
least South America and Indo-Madagascar, and seem to have been present in
Africa and Europe.  South  American forms are the most diverse currently
known.  Abelisaurs in the more general sense include both large and small
bodied forms.

However, there are clearly other theropod groups in southern Cretaceous
sites: spinosaurids, carcharodontosaurs (considered by some to be close to
abelisaurs, but by others to be advanced allosauroid carnosaurs), and
various coelurosaur groups (dromaeosaurs, etc.).

There seems to have been a fairly global distribution of some major theropod
lines in the Jurassic and Early Cretaceous, and that the fate of these
groups varies from region to region in the Late Cretaceous.  For example,
spinosaurids, other basal tetanurines, and carnosaurs are not known younger
than the Cenomanian (early Late Cretaceous).

In the mid-late Late Cretaceous, tyrannosaurid coelurosaurs are the only
large-bodied carnivorous theropods known from continental (non-Indian) Asia
and western North America; abelisaurid ceratosaurs are the only large bodied
carnivorous theropods known from the southern continents (and possibly
Europe) at this time; and the coelurosaur Dryptosaurus is the best known
large-bodied carnivorous theropod known from eastern North America (although
remains of tyrannosaurids are now documented from this region).

>On a related note, has the fairly recent discovery of a titanosaur
>skull done anything to change their position in the cladograms?  The
>skull seems very reminiscent of a diplodicid skull, at least from the
>National Geographic photograph.  Has there been any suggestion that
>titanosaurs are a close sister group to diplodicids instead of
>brachiosaurs, or are the similarities mostly superficial?

There are two currently debated positions for titanosaurs.  The traditional
position, and one still upheld by Paul Upchurch (unless his new analysis
presented this spring supports otherwise) is that titanosaurs and
diplodocoids are sister taxa.  The alternative, that titanosaurs and
brachiosaurs are sister taxa, is supported by some of the Argentine sauropod
workers (Calvo, Coria, Salgado) as well as Wilson and Sereno.

Also, please be careful on how "diplodocid-like" these titanosaur skulls
are: the thin nasals forming the "bump" on the snouts of brachiosaurs are
relatively fragile.  When eroded away, the remaining part of the brachiosaur
skull looks similar in general shape and proportions to a diplodocid: long
and low.  Take a look at a brachiosaur skull illustration or photograph, and
artificially "erode" the nasal bump by using a piece of paper or your hand,
and see what I mean.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661