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RE: Dr. Bakker, Dr. Horner, The 'Dino-Pandemic' & The Canard About Declining Dinosaur Diversity in the

I agree with Vlad's main point, that the Hell Creek is only a tiny portion of the Maastrichtian globe, and that no other Late Maastrichtian dinosaur locality has been sampled nearly as well. But a few statements deserve some comment.

3.  I believe that evidence from Western Asia of the
late Cretaceous also shows diversification and the
appearance of new groups of critters (Therizinosaurs,
for example);

A large number of groups appeared and diversified in the Late Cretaceous, even in Western North America (tyrannosaurids, hadrosaurids, ceratopsids, etc.). What the issue involves is whether this continued through the Late Maastrichtian at the very end of the Cretaceous. From my very limited understanding, the argument for less diversity centers on the fewer taxa of tyrannosaurids (Tyrannosaurus), hadrosaurids (Edmontosaurus) and ceratopsids (Triceratops, Torosaurus) in the Late Maastrichtian Hell Creek / Lance / Scollard Formations; compared to the higher number of tyrannosaurids (Gorgosaurus, Daspletosaurus), hadrosaurids (Brachylophosaurus, Kritosaurus, Prosaurolophus, Corythosaurus, Hypacrosaurus, Lambeosaurus, Parasaurolophus), and ceratopsids (Anchiceratops, Chasmosaurus, Centrosaurus, Styracosaurus) in the Late Campanian Judith River Group. Now I don't think you'll find Judith River level diversity anywhere else in the Mesozoic except in the Morrison Formation, so I view it more as a fluke of great habitat, preservation and/or excavation than anything else.
Therizinosaurs are known from the Barremian onward (assuming Early Jurassic Eshanosaurus isn't one), and the earliest known remains are from England and Utah, in addition to Russia and China. So who knows where they originated. If you just meant Therizinosaurus itself, there are still a couple problems. First, it's known from the Late Campanian to the Early Maastrichtian, not the Late Maastrichtian. Second, as just the largest in the long line of therizinosaurids, it's appearance is no more special than Tyrannosaurus' or Triceratops' in North America. Unfortunately, we don't have any good Late Maastrichtian dinosaur-bearing localities in Western Asia, and the exact age of most of their formations is contentious anyway. So we really can't say if they were anything like Western North America.

So, to sum up:  given the KNOWN fossil evidence from
around the globe, it would appear that Dinosaur
diversity in the late Cretaceous was alive and well,
with new, even stranger groups of critters appearing
(at least in the Northern Hemisphere), while the old
Sauropod and Allosaurid prototypes were so successful
that they continued undisturbed in the Southern
Hemisphere, and even allowed a Sauropod return to North
America via a land bridge towards the end...

Looking at allosauroids in the Southern Hemisphere doesn't support your hypothesis. The only ones still around in the Late Cretaceous were carcharodontosaurids, and those are only known from a couple of specimens past the Santonian in South America (Martinelli and Forasiepi, 2004; Alcober et al., 1998?). None are known from the (admittedly very poor) post-Cenomanian African record, or from India.
The latest evidence suggests Alamosaurus is more closely related to Asian titanosaurs (e.g. Opisthocoelicaudia, Borealosaurus) than South American ones. In any case, none of the productive formations in South America or India have been shown to be Late Maastrichtian (they're not very precisely correlated). So again, we don't know if the situation was like Western North America.

Mickey Mortimer