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Response to Nick Pharris: Patterns



1. Nick Pharris maintains that mosasaurs, plesiosaurs,
pterosaurs, and crocodiles were abundant at the end of the
Cretaceous.
     From my own readings and discussions on this net my
impression has been that the first three of these taxa were in
serious decline and were, therefore, not very abundant.  Rather
than run off a list of citations which have already recently
appeared here, perhaps I could challenge Nick to name a site
close to the K/T where numbers of these creatures are found.
     Crocodiles over 25kg. perhaps were abundant it is true.  But
big crocodiles must have been dependent big dinosaurs.  This
negates having to explain this in some arcane way: "they were
selectively targeted by a bolide because they were heavier than
25kg."  Again, _all_ dinosaurs became extinct--those under and
those over 25kg.  There is no pattern here, only the fact of
dinosaur extinction.

2. Nick justly pins me on two word usages.  I know better for
one.  But for ease of usage, and because the issue of
classification is still disputed, I'll stick to dinosaurs and
birds rather than the clumsy non-avian and avian dinosaurs.

3. Then he wonders: If dinosaur eggs were a fixed liability,
"how, then, did concealed nests ever develop in birds?"
     Birds had wings.  They could fly to places that egg-
predators couldn't--and lay their eggs there.  Perhaps this was a
primary adaptive advantage to flying.  Certainly, today, such
things as leaf cover are prime considerations for birds in nest-
site competition (Martin, BioScience, Sept 1993 v43 n8 p523(10)). 
But dinosaurs did not have that option.  Not only could they not
fly, they were all above the threshold of discovery.  Part of
concealing your nest is being unobserved when you are laying the
eggs.  I imagine this was difficult for an ultrasaurus.

4. "There were egg-eaters all throughout the Mesozoic...so that
does not explain why they suddenly wiped out the dinosaurs, all
over the world, at the K/T."

     Eggs were a _fixed_ liability.  Yes, eggs did evolve in such
areas as protection and aeration.  But ultimately this was a
Hobson's choice, i.e., whether to grow a thick shell to stop the
egg-predator getting in, or to grow a thin shell to let the
hatchling out!  There were limits on what could be done.  On the
other hand, agents better suited to eating this rich resource
_could_ evolve.  I maintain that the static egg was fine as long
as nothing could exploit it.  But its structural limitations
doomed it--in the open, at least.

5. "(Dinosaurs probably did conceal their nests). I think it's
probably just that the only nests we've been able to find so far
are the ones made out in the open."

     Some dinosaurs covered their nests with vegetation, some
with sand.  Some layed their eggs in shallow pits.  These
strategies, as well as others involving social behaviors,
undoubtedly slowed down the predation of eggs.  But, with the
arrival of mammals and skillful fliers, their effectiveness was
limited.  I can imagine several likely scenarios.  
a) Small burrowing mammals with keen senses built networks of
tunnels and gnawed at the eggs (along these lines, I should note
that small mammals have been found in association with dinosaur
nests, recently (1986, I think) by Horner, and back in the 1920s
by Osborne in Mongolia (in protoceratops egg beds).  I don't know
of any tunnels, though.
b) Mammals in burrows near dinosaur nests scurry over to them at
night and drain the dinosaurian inheritance.
c) Mammals wait for dinosaurs to go out foraging and steal into
their nests.
d) Birds scratched away any covering and pecked into the shells. 
I have seen crows (I know they are a relatively modern species)
unceremoniously flipping over garbage to expose food underneath. 
Bird species could use wait-and-see, low risk crow-like
strategies, where they patrol the nesting grounds looking for an
unprotected nest.  They could use the constant nagging and
attacking of a gull-like strategy.
e) Birds perched in trees near egg beds wait for hatching.  They
swoop up the little hatchlings with impunity.

     Note that in these scenarios, any environmental stress
involving food scarcity only makes the egg more of a liability. 
The dinosaurs are faced with another Hobson's choice: forage and
lose the babies, or starve at the nest!  In this scenario, which
is the primary agent, the climate change or the open and
advertized egg?  It's a philosophical question, but one the
mammals did not have to consider.

6. "I was not aware (that mammal and bird diversification was
coming to a functional branch point)."

     Fully volant fliers (enantiornithines) existed in the
Cretaceous and were, according to Alan Feduccia (Science Vol 267
3 Feb 1995), diversifying up to the K/T.  He has also found, just
before the K/T, what he calls a "transitional shore bird" which
he says is essentially modern at the K/T (this is from memory as
I have lost all but the first page of the paper).  But he has the
temerity to claim that this shorebird--the very species _he_
found, was the ancestor to _all_ modern birds.  They survived the
impact winter, he says, by living in the micro-environment of the
shoreline and cracking crabs.  This is disputed by biochemical
evidence which suggests that modern birds have a common ancestor
which is older than this bird (this was in the same issue of
Science).
     My point here is that the birds were certainly diversifying
and were most likely capable of the roles I have suggested for
them.  If you could specify a doubt about a particular avian
ability I think I could answer this more effectively.  Suffice to
say that birds could improve their tactics, open-field eggs could
not.  At some point, a functional branch point perhaps, they
became skillful enough predators  such that the dinosaurs could
not respond to them.

7. "What does (birds being essentially modern in their flying
ability) have to do with your argument?"

     Like a Spitfire to a bomber, the better to harry the
dinosaurs, to peck at an egg and escape untouched.  The better to
snatch up a hatchling.

8. "...there aren't alot of (modern) birds that eat eggs."
     
     Because there just aren't the eggs there used to be. 
Besides, all the eggs are well hidden.  But eggs are still eaten
by birds.  The crows around my house relish eggs.  They eat them
on the spot (shells and all), or they carry them off in their
beak, or they spear the egg with their beak.  I have read of Sea
Crows each with an egg in its beak, flying from flamingo (I
think) rookeries in a line stretching across the horizon.

9. "(Placental birth gives greater) security for the offspring,
yes, but greatly increased danger to the mother."

     The development of mammals represents an evolutionary flight
from the egg liability--from the egg-laying platypus, to the
echidna, to the marsupial, and finally to placental birth.  What
more powerful way could nature demonstrate the adaptive value of
keeping your offspring safe.  Modes of reproduction have always
been targets of selection.  The more babies that make it into the
next generation, the more successful you are.  Successful
reproduction is the gold-standard of natural selection.  That
placental delivery makes up in successful offspring what it loses
in pregnant mothers must be admitted.  Look at mothers on the
Serengeti.  It is not mothers but the old and the young who are
preyed upon most often.  Mammals in close cover can stealthily
hide while they give birth (at night time, for example).I don't
have figures for this, but my sense is that pregnancy is seldom a
liability.
     But that is not even the point.  Successful reproductive
strategies depend on all stages of the life cycle.  And here is
another place the dinosaurs suffered.  Having made it through the
egg-predation gauntlet, now the little hatchlings (no bigger than
the egg) must continue the fight anew.  Compare this with the
drop-and-run babies of the Serengeti (zebra, elephants, etc.).
     Passerine birds of the U.S., not that the species is
relevant but just to give you an idea of the dimension that
different life-stages give to predation, is as follows: Out of
100 eggs, 53 are eaten or abandoned (due usually to predator
fear).  Of those that hatch, only 29 fledge.  Of those, only 12
live to the first year.  A hypothetically equivalent mammal would
have less predation at the egg stage.  Why?  1. Because they're
better at hiding, 2. because, if they are discovered, the mother
takes the eggs!  3. They don't have to make as many predator
alerting trips to the nest.  Then, in the juvenile stage the
mammal again has better luck because her babies are at a further
stage of development.

10. Nick says: "It is a well known ecological principle that
predators do not consume their entire food supply."

     I don't know this principle.  The snow shoe hare and the
lynx are a nice, tidy example of predator prey oscillations.  And
it is true (as you say in a subsequent post) that scarce prey
leads to a drop in predator populations, thus allowing for
constant renewal of all concerned.  Unfortunately nature is not
always this magnaminous.  In fact, _extinction_ is the rule for
most species.  And extinction is generally caused by predation,
isn't it?  In any case, the hare/lynx example is not analogous to
egg-predators/dinosaurs.  A better analogy would be those
laboratories of evolution--islands.  An immigrant comes in and
wipes out the defenseless endemics.  This happens all the time
_especially_ between immigrant mammals and endemic birds.  In the
sense that egg evolution was severely constrained by structural
considerations--the egg is, after all, structurally more akin to
a rock than an animal--development of egg-predation tactics,
sympatric though they were, could develop without reciprocal
defenses in the egg.  Yes, parents could do things, but they were
constrained, too.(See later comments in response to Jeffrey
Martz).

11. (And last) Nick says that the K/T bolide "...might be
expected to have some pretty serious repercussions on the
environment."

     Yes.  But what were they, exactly?  Apart from the oft
stated problems in timing (i.e., significant extinctions and/or
severe drop-offs in diversity were asynchronous with the
"event"), no feasible mechanism has been given either for the
atmospheric suspension of light-blocking materials for the time
required, or, for that matter, any regime of light intensity
which explains the observed extinctions and survivors.  Even
deposit feeders should have died but didn't (Levinton,
Paleobiology 22(1) 1996 pp104-112).  And, although he proposes a
high latitude refugium hypothesis, Levinton closes with this
tantalizing, dogma-breaking idea: "It may also be that the end-
Cretaceous extinction crisis may  not have been one of reduced
food, or even the result of an impact."