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Post K/T carnivore lag.



Robert Carroll, in Patterns and Processes of Vert. Evol. cites
the following interesting, unpublished data of Dan Riskin (does anyone
know if this has been published yet?): over the K/T boundary, the
generally carnivorous/insectivorous Cimolesta flip-flopped in percent
composition of placental fauna with the generally herbivorous
Arctocyonidae.  In the Lancian, Cimolesta comprised 50% of the total, and
Arctocyonidae, 29%. By the Peurcan, Arctocyonidae were up to 51%,
Cimolesta down to 10%.

I think this is interesting on several counts.  Post K/T pred/herbivore
ratios are more in line with general expectations, i.e., predators higher
up the energy pyramid and less numerous.  So what was going on before the
K/T?  And, why didn't carnivores (if so they were) enjoy the same kind of
ecological release as mammalian herbivores?  Is it just that mammalian
predators were not capable, were too small to prey on the released
herbivores?  In this were true, they were still stuck in their old,
pre-K/T niches, that is, until they could evolve size and morphologies to
avail themselves of the new prey.

Finally, perhaps there is here a hint of a solution to the South American
problem, viz., why on that continent placentals dominated the herbivore
niches, marsupials the carnivorous.  I say "problem" because when modern
placental carnivores are pitted against marsupials, the placentals usually
win.  So why didn't they win in the early Tertiary SA carnivorous niche?


Speculating: SA was filled with marsupials until late Cretaceous
connections (this much is known isn't it?).  In the size-range/niche
requirement of the carnivore/omnivore/insectivores (i.e., possum size and
down), marsupials are as good or better competitors--this is supported by
continued success of _D. virginia?_.  So, when dinos exited, new herbivore
niches were created for mammals.  Higher evolution rates in placentals
would favor their movement into new niches.  But, and this is a tentative
conclusion I'm making from Riskin's data, there _was_ no new carnivore
niche--or not yet, anyway, until carnivores could make the radical
adjustments for taking prey in the open field.  Denied any advantage of
rapid evolution, placentals must compete bone for bone, cricket for
cricket, against entrenched marsupials.  They were just not yet up to the
task.  
I am doing little more than thinking aloud here, but would appreciate
comments.