More on Pangaea

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>4) About the breakup, this has been blamed/credited for dividing
>the dinosaur families according to the area of Pangaea.  Is this
>an over-generalization?  Now that dinosaurs are being found in
>areas thought to be out of reach due to the breakup, suddenly
>we have a "land-bridge".  Forgive me for being sacrilegious, but
>this *sounds* like someone's pet theory was being disproved and
>everyone rushed to save it.

Not sacrilege, but fantasy.  If there's one thing we don't do when
another's theory is failing, it's try and prop it up... ;-)

>It seems that only the surface of
>dinosaur fossils has been touched.  What will happen if more
>out-of-bounds dinosaurs are found?  Will Pangaea start to look
>like a quilt with "land-bridges" and other patches criss-
>crossing the dinosaur map?  ( This is the list's area of
>expertise.  Feel free to be offended and refute my assertions. )

The basic evidence for dinosaurian distribution is as follows:

During the Late Triassic through the Late Jurassic, dinosaurian faunas of
the world are generally very similar.  In the Early Jurassic, for example,
you can find Syntarsus in North America and in South Africa, and
Dilophosaurus in North America and China.  Even by the Late Jurassic, you
can find Brachiosaurus and Camarasaurus and others in North America,
Europe, and Africa.

During the Early Cretaceous, however, the break up seems to have been
sufficient to stop the flow of organisms between the continents.  Groups
like ceratopsians, hadrosaurids, and tyrannosaurids evolve in the north,
while the abelisaurids appear and the titanosaurids diversify in the
southern continents.

By the Late Cretaceous this difference is very marked.  However, at the
very end of the Cretaceous (the Maastrichtian stage), the geologic evidence
shows that the seaways were draining, allowing passage over previously
shallow marine (but NOT oceanic) regions.  At the same time, the dinosaurs
begin to wander again, and hadrosaurs show up in South America and
titanosaurids in North America (for example).

Several of us are working independantly (using different techniques) on the
subject of contiental drift and dinosaur distribution.  Most of our results
are similar, so it looks like contiental drift did have a big effect on
dinosaur evolution.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.