RE: Hell Creek and stegosaurs

["Williams, Tim" <TiJaWi@agron.iastate.edu>]

Nick Longrich wrote:

> 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, 

I completely agree.  As I said in a previous post, dinosaur diversity
most likely had its 'ups and downs' within each period.  The apparent
decline in dino species diversity in the final few million years of the
Cretaceous of North America cannot be taken as representative (or
indicative) of a global decline.

>And if you look at the family level everything's still there right 
> up to the end of the Cretaceous. 

Also true.  Sure, there was less *species* of ceratopsians, hadrosaurids
and tyrannosaurids than earlier strata; but the groups themselves were
still in existence - and doing quite well.  The fact that, at the very
end, N. American terrestrial environments were dominated by a smaller
number of dino species than previously may reflect local environmental
factors.  

Gavin "Frogfoot" wrote:

> This question has been bugging me for a long time actually.  The first
> few dinosaur books I ever got were still of the days when Stegosaurs
had
> these plates for armor protection.  I guess that its well established
> that they used them for regulating body temperature now, or 
> differentiating between species?

I had thought that all of the above theories were currently regarded as
possible.  Still, the big triangular osteoderms of _Stegosaurus_ seem
poorly adapted for defense (or offense), and were perhaps better
utilized as thermoregulatory devices or in threat displays.  

> The only Stegosaur I know that was around til the end of the
Cretaceous
> is Dravidosaurus.  

_Dravidosaurus_ is probably not a stegosaur - or even a dinosaur.  I
have heard that the scrappy material from southern India named
_Dravidosaurus_ actually comes from a sea reptile (plesiosaur?)

> Only a handful of others made it into that same era. So what happened 
> to them, climatic change, different predators/food?

One theory is that stegosaurs, with their relatively unprotected flanks,
were vulnerable to the new 'breed' of maniraptoran killers - especially
deinonychosaurs.



Tim




----------------------------------------------------------- 



Timothy J. Williams, Ph.D. 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 9359