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RE: Why did small dinos become extinct?




Philip Chalmers wrote:

> David Marjanovic said in an earlier post, "Incidentally, no flightless
> theropod the size of Compsognathus or smaller is currently known from
> the Maastrichtian, AFAIK," and that's the impression I've got from what
> I've read.

There's _Richardoestesia_, and a few others.  The small Maastrichtian theropod 
_Rahonavis_ is usually considered to have been flighted.  However, two recent 
discoveries make me less confident of this: (1) phylogenetically _Rahonavis_ 
nests within the Dromaeosauridae, not the Avialae; (2) quill knobs, which were 
once thought to have been indicative of flight (or at least flight ancestry) 
are now reported for _Velociraptor_, suggesting (but not proving) that this 
character may not have originally evolved for flight. 

There's also the juveniles of large theropod species, which you mention...

> My _guess_ is
> that they were squeezed out by some combination of birds, mammals and
> the young of larger predatory dinos (doome dbecuase they grew too large
> for this niche in a few years).

Also, it appears that most bird groups went extinct at the end of the 
Cretaceous, including the very diverse and widespread Enantiornithes clade.  It 
was only the Neornithes that "fell over the line" into the Cenozoic, so any 
extinction scenario has to factor this in.  Birds (especially ornithuromorphs) 
radiated into many aquatic/marine niches, so this may have played a role in 
their success during the Cretaceous and/or their survival beyond the 
Cretaceous.  Then again, as I said, it was the Neornithes alone that survived, 
and not the various other ornithuromorph taxa, many of which show 
aquatic/marine adaptations.
 
Cheers

Tim
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