Re: Cretaceous taeniodont

["Tim Williams" <twilliams_alpha@hotmail.com>]

John Bois wrote:

> How  many species need to be above, say, one kg. before we can say 
> predation and/or competition was no longer keeping mammals small?

and Phil Bigelow wrote:

> If the amply-endowed _Didelphodon vorax_ can thrive amongst the hungry Hell 
> Creek dromaeosaurs (which are still undescribed beasts, BTW), then being 
> large and furry
> probably wasn't a significant handicap in the Mesozoic.

I agree that, for a mammal, being big (or at least bigGER) may have been an 
advantage.  But I'm still not hooked on the idea that mammals got bigger 
because there was a relaxation in predatory constraints.  This implies that 
predatory theropods somehow 'let down their game" in the Cretaceous, 
allowing mammals to get larger and more visible.  Nor do I accept that the 
large size of a mammal species necessarily indicates that that particular 
species was "thriving".

Here's one possible scenario... (I'm not claiming that this scenario is 
correct, or that we could even test it if it was correct - but I think it's 
worth thinking about.)  From around the Late Jurassic we see the emergence 
(or at least, an increase) of new types of smaller-sized theropod predators 
(< 2.5m long, at least half of which was tail).  Many of these theropods 
show wicked raptorial specializations: the enlarged thumb-claws in 
compsognathids; the slashing foot-claws in deinonychosaurs, some birds and 
(?)noasaurids.  Perhaps these theropods specialized in targeting small 
tetrapod prey, and getter bigger was one defense open to mammals.  A 
_Microraptor_ or _Sinosauropteryx_ might go for a shrew-sized mammal in a 
heartbeat, but would baulk at pouncing on a mammal the size of a badger or 
wolverine.  Of course, the theropods could get bigger too, and so on...  But 
these sort of two-way interactions undermine the simplistic notion that 
increased body size in mammals is an unambiguous indicator of success in 
these furballs.


Tim

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