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Mammal diversity takes a 20 milion year step backwards!



     Derek Smith claims there is no advantage to a particular
strategy, whether egg-laying or live-bearing, in the open-field. 
Instead, he agrees with Gould and just about everyone else that neither
mammal reproduction, nor anything else about mammals, had anything to do 
with their success.  Their radiation was made possible only by the 
disappearance of the dinosaurs--the mammals were a strictly neutral force 
bearing no more potential for success than any other creatures.  But now
comes Archibald's find of "zhelestids", an apparent ungulate
forebear, in Asia.  As the headline of the article accompanying
the _Science_ paper says: "MAMMAL DIVERSITY TAKES A 20 MILLION-
YEAR LEAP BACKWARDS."

     I am tempted, especially with the additional "molecular
clock" evidence which puts the branching of all placental lines
back to 100 million years, to speculate not only about proto-
ungulates, but proto-everything else as well.  I am tempted to
note that this predates the last known contact between Gondwana
and Laurasia (Sereno) and so it may yet be possible that
placentals moved to Southern lands and have just not been found
(in this regard we must admit the awesome ability of mammals to
hide their fossils, if not their offspring!)--though, relative to
dinosaurs, marsupials are equal to placentals in their
reproductive security and are equally good candidates as are
placentals, birds, crocs, lizards, and snakes for causing
dinosaurs problems.  Finally, I am tempted to speculate that all
the pieces were in place for a multi-species onslaught on the one
thing all the dinosaurs had in common and COULD NOT CHANGE: they
laid eggs and could not hide them!  But I won't speculate on
these things because I want to do some serious speculation later
in this post.  However, the one thing we can say for certain is
that mammals were not null species, inert until "ecologically
released" by the accidental fall of their betters--the dinosaurs. 
Important functional branch points (dentition and reproductive
"advances") were being attained a good 20 million years before
the K/T "just so" story maintained!

I welcome this first crack in the K/T edifice.

But back to egg-laying vs. live-bearing.  Perhaps, as I claim,
the mammal's secure reproduction played a part in this increase
in evolutionary creativity.  Species have different strategies
the effectiveness of which depends entirely on their niche.  Egg-
laying in birds is preferable to live-bearing because of the need
for lightness and, anyway, the eggs can be hidden in out-of-the-
way places.  Egg-laying in crocodiles works because, sovereigns
in their swampy habitat, they can guard their nest with ferocious
efficiency, on their own terms.  Egg laying in most snakes is
okay because the nest is either guarded by venomous parents, or
the eggs are down some slithery hole which only the parent can
reach.  Lizards are small and can hide their eggs well.  Turtles
make a single stealthy trip to lay their eggs, then they forget
them.  But dinosaurs, big and parentally invested, had no means
of avoiding detection.  The only strategy they could resort to
was defence of the nest.  This is an expensive strategy and
violates the first law of self-preservation: it is better to run
than fight.  Mammals holding the baby within could obey this law
and have their offspring as well!  

     Derek is very generous in letting me compare two species
that didn't ever exist together, i.e, a dinosaur and a precocial
mammal, both the same size, herbivorous, and herding.  But he is
stingy in his praise of mammalian reproductivity!  He says the
pregnant female is a liability for mammals.  Pregnant and heavy,
so the idea goes, she is a better target for predators.  Not only
that, the predator gets a "two for one" deal.  But there is
intense selection on females in the open-field to remain speedy
despite their offspring load.  Pregnancy can endow females with
_enhanced_ powers.  In addition, a pregnant female only has
significant weight-gain towards the end of gestation.  This
means, relative to the eggs which have been sitting out in the
open for several stressful weeks, the mammalian babies have
enjoyed _complete_ security.  Anyway, I have never heard of any
significant predation on pregnant females.  Derek, where did you
get this idea from, or is it just gainsaying?  Predators target
the young, the sick, and the old, not the pregnant.  Given the
well known fecundity of plain animals of today, I regard this
claim as self-evident, and, without evidence against it,
supported.  Pregnant females in the open-field are _not_ at
significantly greater risk than other adults!  This means that
the mammalian offspring, since it can be removed from the scene
of attack, is more secure than the shell-bound and stationary
dinosaur babies.

     I agree with Derek when he notes that baby open-field
mammals, precocial though they may be, are at risk.  But then
elephants surround the calves for protection, zebras confuse
predators with stripes, wilderbeasts threaten predators, and so
on.  True, such tactics were available to dinosaurs.  But they
were not as effective for a stationary offspring (the egg). 
Ultimately, the dinosaurs had to resort to blood and guts,
expensive defence.  Again, I would argue the _prima facie_
evidence of fecundity in modern herding mammals.  And I deliver
the _coup de grace_ to Derek's _pregnancy liability_ argument
thusly: Greg Paul, in _Dinosaur Eggs and Babies_ reports that
dinosaurs were r-selected.  That is, few reached adulthood.  In
contrast, many if not most mammal babies do reach adulthood.  As
a result, open-field mammals only need bear one or so offspring a
year to maintain their populations at carrying capacity!  Indeed,
the very term used to describe this sort of reproduction is _K-
selection_ and it is itself defined as low mortality among
offspring.  Dinosaur age structures were like those of an oyster,
bottom heavy.  The strategy was to lay lots of eggs in the hope
that some would live.  This, however, is less than just an
alternative strategy.  It was, in a world full of increasingly
adept egg and offspring predators, increasingly expensive.

     But Derek uses the fiction of the _pregnancy liability_ to
bolster another fiction, i.e., eggs have a shorter window of
insecurity than foetuses.  This counter-intuitive claim rests on
the assumption that time from fertilization to viability in
mammals and dinos is roughly equivalent and that a pregnant
female is at much greater risk for the duration of her long
gestation period.  But common sense and science refute these
claims.  Compare the two modes in equivalent animals:
Dinos.
Fert->eggs------>juvenile------>viability.

Mammals.
Fert->gestation--------------------------->enhanced viability.

As argued above, for most of the mammals gestation there is _no_
increased risk.  And there is little risk _at any time_ except
during delivery (by the way, I know some plain animals deliver at
night so that by the morning mother and baby can be off and
running).  Derek's claim also depends on the notion that egg
defence was inexpensive and effective.  Again the evidence
suggests otherwise: 1. dinosaurs were r-selected (see above); 2.
dinosaurs were precocial.  We know why plain mammals are
precocial, so they can _repeat_ their uterine experience of high
mobility.  Dinosaurs were precocial to _avoid_ their pre-hatching
experience of being completely immobile--they had to get away
from the stationary mode as quickly as possible!  Derek also
implies that the heightened risk during pregnancy "necessitates"
the herding of mammals.  While it might be true that pregnant
females get protection from this tactic, it is misleading to see
herding only a temporary counter to the pregnancy liability. 
Plain mammals _always_ herd!

     Moving now to speculation: a species of herbivorous
dinosaurs must have been distributed, in breeding season at
least, not in large ranging herds but in nest-bound clumps.  The
size of a herd, notwithstanding the intense predation at all
stages of development bar adults (which makes them an r-selected
species), would be determined by the productivity of the range
within (probably) a six-hour forage (not a six-hour walk!) from
the nest (in daylight hours only, six-hours out and six in to get
back before dark).  If they were at carrying capacity and some
more herbivores moved in, it would pay them to defend, not only
the nest, but their territory as well!  This is an additional
cost.  Across a plain you would see a uniform series of guarded
territories, each supporting a few adults and a lot of juveniles.
But even the juveniles, who likely had quite different niches
from their parents, would eat down their range, especially since
(as trackway evidence shows, I think) they ranged with the adults
staying close-by for protection.  And now factor in the
predators.  Big predators, stealthy predators, small predators,
flying predators, burrowing predators (perhaps), predators on
juveniles, predators on eggs, predators on hatchlings, predators
on adults.  Again, the dinosaurs had no flexibility of response. 
They must stand and fight.  And the predators could stake out the
nest area.  Unlike the relatively simple life of a large and
vigilant herd of mammals, dinosaurs were required to return to
the same place, to stay within a certain range, to protect a
large number of juveniles as best they could from a ferocious
gang of predators who knew where they were at every minute of the
day!!!!  I, for one, am glad to be a placental.

     Derek says that dinosaurs suffering intense predation at one
site can simply abandon the nest and move to a new one.  "After
all," he says, "there's always next year."  This sounds like one
of those "Famous Last Words" jokes, perhaps said on the last day
of the K/T.  Small deficits in fitness can lead, in a short time,
to extinction!  In any case, it would not have been that simple. 
As in birds, nest sites are a limited resource and are vigorously
defended.  In addition, perhaps the range itself was defended. 
In any case, moving to a new home would first mean a costly
eviction of the previous tenants!

     But then Derek challenges me to argue the benefits of
mammalian reproduction in a _carnivorous_ territorial mammal. 
Again, this is a situation which never existed, i.e., carnivorous
mammals vs. carnivorous dinos in the open-field.  I'm not sure
whether a lion would rule over a lion-sized dinosaurian carnivore
or not (actually, I'd pick the dino in 3).  But still we can
argue the relative fitness of their respective strategies.  A
carnivorous dinosaur has some of the same liabilities of its
reproductive mode as an herbivorous dino has.  It must forage yet
provision.  It still has stationary eggs and relatively
defenseless hatchlings.  Mammal carnivores could dig burrows and
move their offspring--they were stealthy reproducers--dinosaurs
could do neither.  Altricial mammal babies could cling to mummy's
fur, or mummy could move baby by the scruff of its neck.  A
dinosaur trying this tactic would have scrambled babies.  Mammals
had a smaller window of reproductive insecurity.  But not needing
as much foraging time the carnivore dino is probably in a much
better position to defend the nest than an herbivore.  Perhaps
its main problem is that an ever more efficient group of
predators (including itself) are eating down its resources.  Its
population used to oscillate with the availability of its prey. 
But now, new and old evolutionary products, even though they are
omnivorous and not fully dependent on the carnivore's prey, are
eating it nevertheless.  Indeed, as _all_ the extinction
scenarios have concluded, as go the herbivores, so go the
carnivores.  But unlike the other extinction scenarios, mine also
kills off the small (chicken-sized) dinosaurs.  After all, in a
bolide-winter _some_ small dinos should be able to survive on
rotting dino carcasses, small mammals, birds, lizards, and
snakes.  But none did.  In my non-stealthy nest idea, small dinos
are lumped in with the rest.