Re: Cretaceous taeniodont

[John Bois <jbois@umd5.umd.edu>]

Phil Bigelow says:
> I suspect that small islands played a large role in the evolution and
> radiation of Mesozoic mammals.  If some islands lacked dinosaurs (and I'd
> guess that many of them did), then the conditions would have been similar
> (although by no means identical) to the conditions in the earliest
> Paleocene.

Could be.  But bear in mind that most of the adaptations possessed by the
continental mammals (speed/predator avoidance/prey capture, etc) were forged
_within_ the niches they came to dominate.  I mean, I have this image of a
jolly Dodo-like mammal rafting onto the raptor infested continents...

> Most
> typical floating debris bundles probably could support the weight of, at
> most,  a Troodon- or Dromaeosaurus- size dinosaur, but only one
> individual at a time.  In contrast, that same small debris bundle could
> transport a breeding pair of mammals from the island to the mainland.

True.  But dino _babies_ could raft...and don't forget birds having access
to islands.

> Somewhere in my garage I have a Norway Rat skull that is about 8/10s the
> length of a typical adult house cat skull.  A single breeding pair can
> theoretically produce at least 100,000 descendants in a year.  What if
> those two Norway Rats rafted over to a predator-free island?........

My guess would be that they might experience a size increase but that if
their descendants rafted back they would experience the same pressures that
limited them in the first place.

> And you asked, does size matter?  Penthouse Magazine says YES!! (and who
> are we to argue with the experts, eh my friends.

Phoohey...if organ-size were important to females, we would be displaying
them as we do our other visible means of support.

> 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.

So, how _did_ these three (maybe four) oversized furballs make it through?
What is the ecological picture like just then?  What did dromaeosaurs eat,
hunt?

> So why were most
> other Mesozoic mammals small?  My guess is that, since dinosaurs didn't
> get that small, the mammals "conquered" that niche way back in the middle
> Triassic, and they never let go of their conquered territory.  Perhaps
> the dinosaurs *didn't* keep the Mesozoic mammals small.  Perhaps it was
> the other way around:  perhaps the Mesozoic mammals kept the dinosaurs
> large.  _D. vorax_ proves that Mesozoic mammals could Live Large if they
> "chose" to do so.

This line of argument assumes a stasis throughout the Mesozoic.  But--and I
see the "" around chose--there were forces operating on species that changed
over time.  Just because we call it the "Mesozoic" doesn't mean it was a
discrete period in reality.  But there is a tendency for us to see these
units as chapters in a book: "this is the end of that episode, now we'll
allow some different forces to enter the plot line in the next chapter."
We've had several threads that discussed vast speciation changes w/in the M:
pterosaurs, birds, major dino lineage booms and busts--a lot was happening.
My own bias tells me that size were responses to these changes, and not a
willy nilly wander through ecospace.

> On the other hand, how many shrew-sized flightless birds (fossil or
> living) can we name?