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Re: pre vs. post K/T mammal diversity: A fair question!
At 10:28 07/06/96 -0400, John Bois wrote:
>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.
I think this exactly what we all are saying. That is what I meant by
the difference between genetic or cladistic diversity and phylogenetic
or ecological diversity. But the point I was making was relating to
the situation before, not after, the "dramatic denouement [I assume
this was what John meant to say]" of post K/T mammalian evolution.
After all part of what we are talking about here was the effect (if
any) of mammals on dinosaur extinction, and that can only be judged by
the ecological role of mammals (to the extent that this can be
determined at all) before the K/T. As I pointed out in some ways
mammals were MORE diverse before the K/T than after, because other
groups in addition to Monotremes, marsupials, multituberculates and
placentals were present (though most or all (?) of these didn't make
it to the K/T). But that diversity was still diversity among small
ratty, shrewy or squirrely things. The fact that protoungulates
produced rhinoceroses, whales and elephants in the fullness of time
and multituberculates didn't (for example) says nothing whatever about
the role of these two groups vis-a-vis dinosaurs in the Cretaceous.
And, of course we know nothing (I suppose) about the reproductive
strategies of multituberculates.
> 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)?
Well, actually, there WERE big open-country lizards (and still are, on
Komodo), terrestrial crocodiles and huge turtles around for mich of
the Cenozoic (the crocs lasted until fairly recently and the big
turtles are still around on islands - however, there were even bigger
ones in continental India and Australia, both of which had large
mammal faunas. As for birds, don't forget Diatrymids, Phorusracids
and Dromornithids, none of which were ratites.
>Dentition and jaw structures gave them greater mechanical
>advantage and niche-flexibility
I think this is the biggest advantage mammals had - much bigger than
reproductive strategy. However I would note that it is not altogether
fair to compare mammals, which had already achieved (as we all agree)
considerable genetic diversity, to narrower groups such as crocs.
Also, I think one of the big mammalian advantages as survivors in a
post K/T world with many open niches was the fact that they were
pretty generalized - rather the oposite of what John is saying. As
such they had not gone down roads of specialization that it would be
extremely difficult to retrace, as snakes have through their loss of
limbs, highly-evolved jaw structure, reduced eyesight etc. Remember
that despite the diversity of snakes today there is not one
herbivorous species - something I suggest is easier to attribute to
specialization early on than to the lack of an open niche.
>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.
Birds had the first two of these too, and pigeons have today something
vaguely like mammary glands. BTW - there are actually more species of
birds, and more species of reptiles (sensu lato), alive today than
there are species of mammals, though mammals are more phenotypically
diverse than birds or lacertilians.
> This meant they could cash in on the advantages of altriciality or,
>on the plains, feed baby on the run.
I am not sure what "advantages of altriciality" are here. In fact
altriciality imposes certain costs as the young are helpless for a period of
time, whether they pass that time in a nest, a den or a marsupium.
> 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.
Actually the defining characteristic of a mammal is the jaw
articulation - don't forget that monotremes are mammals, too (in most
people's opinion) and lay eggs. As I said, we don't know (to the best
of my knowledge) what multituberculates did, or any of the other odd
mammal groups of the Mesozoic. I think that it is parsimonious to
assume that the common ancestor of marsupials and placentals was
live-bearing, so that Cretaceous placentals were at least that (and,
yes, probably did have a system like that in living placentals).
But the fact that an adaptation exists does NOT mean that we can
assess its contribution to fitness - particularly overall fitness - in
prehistoric ecosystems. For example, it used to be a truism that the
placental system was "fitter" than the marsupial one, but I do not
think modern mammalogists draw this conclusion. Certainly it is
almost impossible to determine the effect a reproductive strategy we
can only assume was present (and which varies enormously in many ways
among living placental mammals) might have on competition with
> 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?
To some degree the answer may be, in Stephen Jay Gould's sense, Why
not mammals? Or, Mammals, because it wasn't something else. The luck
of the draw, in other words. However, I am inclined to think that the
chief cards in the mammalian pocket were related to niche flexibility
- and these may have included dental structure, small size, endothermy
or any combination of these or other factors.
Remember also that the question may be divided into "Why mammals and
not non-mammals", which may have one answer, and "why placentals and
marsupials and not monotremes or multituberculates?" - which may in
fact relate to reproductive strategy, but I don't know for sure. It
is possible that multis and monotremes may have been more specialized
by the K/T than placentals and marsupials were, and had less
Also, I make no apologies for point-by-point criticism of arguments
when I think that they are either fallaciously reasoned or based on
inadequate consideration of the facts. Any referee in a journal would
do the same.
Ronald I. Orenstein Phone: (905) 820-7886 (home)
International Wildlife Coalition Fax/Modem: (905) 569-0116 (home)
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