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pre vs. post K/T mammal diversity: A fair question!
Thanks to Archibald's find we can now hypothesize one of the
following two scenarios or something in between: either the
(probably) placental ancestor produced proto-ungulates, proto-
rodents, and proto-everything else 20 million years _before_ the
K/T, OR the placental ancestor produced only proto-ungulates 20
million years ago and the rest of the orders branched off later.
Whether or not these clade-generating mammalian adaptations were
significant in terms of contemporary mammals depends entirely on
one's opinion. Pharris, Rowe, and Orenstein (who moonlight for
the law firm of the same name) believe that pre-K/T adaptations
were insignificant compared to those which followed the
ecological release of the K/T. I believe the pre-K/T adaptations
were more significant and that Archibald's _important_ discovery
(it is, after all, the putative ancestor of whales, cows, camels,
pigs, sheep, deer, and more successful groups!) is itself a sign
of the selective advantage of these new phenotypes. In other
words, advantageous adaptations were functional branch points
which led to further diversity. True, their ultimate fruition
may have had to wait until the dinos were no longer around, but
this was, in my view, merely a dramatic denumen of a story whose
ending was predictable.
Pharris, Rowe, and Orenstein emphasize the importance post K/T
adaptations, particularly of size, adding that mammals could not grow big
until after the dinos exit.
Pharris: "What Archibald found was a tiny ...shrew-like
insectivore-herbivore that lived in the shadow of the
Rowe: "Archibald isn't claiming to have found that cows lived
with dinosaurs." These proto-ungulates were, he says, "little
Orenstein: "Obviously there must have been niche diversification
in Cretaceous placentals but this would have been in a much
narrower compass than that available to them after the K/T (even
if the only major new increase was in body size, this opens up a
huge range of diversity in phenotype and ecology)."
If size _per se_ were so important, why couldn't _any_
animal of big dimensions step into the niche left vacant by the
dinos? Why don't we see big herds of grazing snakes crawling
across the plains? Why no big plain-living lizards, crocodiles,
turtles? Why so very few ground-laying big birds (Ratites had
their day, too. But still mammals dominated)? So let me pose the
question to Pharris, Rowe, and Orenstein: Given the species
existing immediately after the dino extinction, why was it the
mammals which finally prevailed in the niche formally occupied by
This is my answer: Mammals were preadapted to move into more
and bigger niches. 1. They had secure reproduction. This gave
them an edge in the reproduction sweepstakes whether by avoiding
predation or merely by keeping the embryonic environment in
homeostasis (I know Ron has some good arguments against these
assumptions in a recent post--I am still working up a response to
this). This critical adaptation most likely happened _before_
the K/T; 2. They had undergone significant cognitive, sensory,
motor, and probably social advances prior to the K/T; 3.
Dentition and jaw structures gave them greater mechanical
advantage and niche-flexibility; 4. They were most likely
endothermic allowing high metabolic rates; 5. They had insulation
and could exploit cold niches while keeping up metabolic rates;
6. They had mammary glands. This meant they could cash in on the
advantages of altriciality or, on the plains, feed baby on the
We think that all of these adaptations happened _before_ the
K/T. These adaptations (or something) paid a diversity dividend
in the late Cretaceous. They were the defining characteristics
of the mammal and remain so.
So I wait for a response, not a point for point criticism
of_my_ arguments, but a deeper explanation of your positions, an
answer to the fair question: Why mammals?