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Re: Hell Creek (long)


On Monday, June 3, 2002, at 05:22 AM, John Bois wrote:



On Sun, 2 Jun 2002, Allan Edels wrote:

I want to remind everyone that nearly ALL THE ANIMALS THAT WEIGHED MORE
THAN 30 KG DISAPPEARED! I'm not sure of the numbers of small
(non-avian) dinosaurs that existed in the late Maastrictian, perhaps
someone can supply this info. My current understanding is that dinosaur
genera had reduced drastically prior to K-T - mostly the larger sized
animals (_T. rex_, _Triceratops_, etc.). It seems like all the small
dinosaurs were birds! 

If true, this is a startling fact!  Why, since species don't just
_surrender_ niche space, would there be fewer small non-avian
dinosaurs?  This is a smoking gun of sorts.  Were birds
outcompeting/preying on small non-avian dinosaurs?
  Did birds do the same
thing to pterosaurs?

We apparently get a change in species-level diversity change between Hell Creek and some other formations. I'm not sure that this means anything, in that we would expect dinosaur diversity to be lower or higher variously at various points through their history, and there's no evidence that this is part of a larger pattern rather than just something that is typical of Hell Creek's ecology or depositional patterns. The question "is X different from Y" (yes) is a lot less useful than "is X significantly different than Y" (we don't know). Personally I think looking at individual species of dinosaurs is just not the way to look at dinosaur diversity here, it's like looking at those pointillist paintings under a magnifying glass, all you're going to see is dots, we need to zoom out and take things in with less resolution but broader scope.
And if you look at the family level everything's still there right up to the end of the Cretaceous. Among theropods, dromaeosaurids, troodontids, ornithomimids, caenagnathids, tyrannosaurids and Ricardoestesia are there into the Late Maastrichtian, and on the herbivore front ceratopsians, protoceratopsians, hadrosaurids, ankylosaurids and pachycephalosaurs are all known from the end of the Cretaceous. Dinosaurs continued to employ a wide variety of ecological strategies and body plans and sizes right up until the K/T boundary; if in fact dinosaurs were somehow in trouble we should expect to see lots of families dropping out of the fossil record before the end Cretaceous, we don't see this.
In addition, it's only after the K/T that mammals really begin to radiate and produce bats, whales, large land herbivores, large land predators, etc. If dinosaurs were really in trouble before the K/T we should expect to see signs of this radiation into large body sizes and radically different ecologies well underway, but there are no large mammals. The timing of the explosive radiation of mammals- post K/T- is every bit as suggestive as the continued morphological and ecological diversity of dinosaurs pre-K/T.