Re: Cretaceous taeniodont

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

Eric M. said:
>...I know of
no species that has managed to evolve to an _intermediate_ size, where it's
too big for one set of predators but too small for the next.  The only
ultimate safety in size lies in being bigger than _everything_, which the
dinos managed with some success.

And yet there are lots of species of "intermediate size".  But who says the
only advantage to size is protection against being eaten.  For example, if
by increasing size this allows you to expand your diet (e.g., bigger insects
and/or small mammals and lizards), you can sustain more predation because
your reproductive success is better.  Anyway, I am arguing that the removal
of a size cap in the Late Cretaceous (if true) was not about protection from
predators.  Rather, it may have been due to a relative absence of predation.
Well, the scenarios are myriad.  But the bigger point, I think, is that
there is a feeling that niches are unchanging--that those species that
occupy them have squatter's rights; that the only thing that can change the
ecological status quo is some catastrophy.  Clearly, speciation w/out
abiotic catastrophy was having catastrophic effects on some species.
Pterosaurs, at least, show that.  And now, perhaps, mammals.

>Mammals, OTOH, wouldn't be able to evolve
fast enough to reach that point before being eaten.  If you evolve in
lock-step with your local predators, you can get bigger, but the mammals
existed in a world where all the predator niches were already filled.

Based upon a true story: a new, more fecund mammals species (1) invades a
continent.  This provides more food for a predatory bird who was previously
excluded from the area.  The bird preys incidentally on the mammalian
predator (2) of another mammal (3).  This other mammal is nocturnal and
therefore immune to predation by the bird.  Having no predators, the third
mammal is in a Garden of Eden.  It experiences a general relaxation in size
constraints.  Large extremes that were previously culled, are now successful
in interspecific battles for females.  And so on.