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Re: "No Bolides!"



On Thu, 4 Apr 1996, Virginia Tidwell wrote:
>   Within the last few years some good evidence has been presented 
> showing a marked decline of general dinosaur diversity on the generic 
> level during the late Cretaceous.  Although the taxa that are found 
> near the K/T seem to be well represented by large numbers of 
> individuals,  faunal lists from the Western Interior of N.A. for that 
> time period suggest that a large number of genera had become extinct 
> during the late Cretaceous with out any replacement groups filling in 
> the empty niches.  It may be misleading to say that dinos were doing 
> well until the impact.  Rather, it would appear that select taxa were 
> thriving at the time.  
>   It is also dangerous to make projections about global dinosaur 
> populations based on the limited data currently available from 
> Cretaceous terrestrial localities.  

        What do people think about the Pleistocene extinctions in North 
America? They seem comparable to the Late Cretaceous. We've lost wooly 
mammoths, sabertooths, dire wolves, teratorns, lions, giant lions, 
horses, ground sloths, phorusrhacids, cheetahs, rhinos, those dog-bear 
things, and plenty of other animals. As for niches not refilled, we've lost 
large grazers like mammoths, rearing browsers like sloths, big-game 
predators like sabertooths, etc. etc. The Late Cretaceous saw groups like 
Centrosaurines on the way out, but we did lose elephants, rhinos, etc., 
so it was not merely at the generic level.
        So are we "doing well" in North America? Or are we "on the way 
out", as many would say of Late Cretaceous dinosaurs? Dinosaurs must have 
seen comparable lows in diversity many times, we seem to be in one now. 
On the great plains, for example- I'm not really familiar with the 
ecology, but if we count cougars and wolves as predators, and pronghorn, 
bison and maybe deer as herbivores, we've got the same situation: 
individual species doing well, number of genera low ( I mean, 
pre-colonial days. There were something like 30 million bison and 
millions of pronghorn as well.) That's not even necessarily all that 
abnormal anyway- it's the rule in colder regions to have fewer species 
and more of them. 
        So even if the diversity decline is a real phenomenon, that 
doesn't necessarily mean that it has much significance in the overall 
extinction pattern, any more than that the end of the ice age means we're 
in for major mass extinctions (although if you count that as triggering 
the rise of agriculture and hence modern civilization- well, it could 
cause a major mass extinction yet).