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Re: Response to Nick Pharris: Patterns



On Mon, 15 Apr 1996, John Bois wrote:

> 1. Nick Pharris maintains that mosasaurs, plesiosaurs,
> pterosaurs, and crocodiles were abundant at the end of the
> Cretaceous.

It doesn't really matter if they were *abundant*.  It is simply the fact 
that all those distantly related groups (along with other, even 
more distantly related groups) independently snuffed it at the same time.

>      From my own readings and discussions on this net my
> impression has been that the first three of these taxa were in
> serious decline...

I'll let someone with better info at their fingertips tackle that one...

>      Crocodiles over 25kg. perhaps were abundant it is true.  But
> big crocodiles must have been dependent big dinosaurs.  

Or, possibly you must admit, on big fish.

> Again, _all_ dinosaurs became extinct--those under and
> those over 25kg.  There is no pattern here, only the fact of
> dinosaur extinction.

But, as I'm sure will be endlessly pointed out to you, non-avian 
dinosaurs (yes, I will continue to make that very important 
qualification, dammit!) were *not* the only groups that died at the K/T.

> 3. Then he wonders: If dinosaur eggs were a fixed liability,
> "how, then, did concealed nests ever develop in birds?"

You seemed to be implying that dinosaurs were genetically programmed to 
seek out nest sites out in the open.

>      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)).

Do you think small dinosaurs didn't look for leaf cover when they were 
picking a nest site?
 
> But dinosaurs did not have that option.  Not only could they not
> fly, they were all above the threshold of discovery.  

How in the world have ratites survived so long?  They probably survived 
the K/T event.  they tend to be quite large, and they all lay their eggs 
out in the open, IN THE PRESENCE OF VARIOUS AVIAN, MAMMALIAN, AND 
LACERTILIAN PREDATORS.  Not all neognathous bird groups nest in trees or 
on rocky outcrops, either.  Gruiforms and galliforms, for example, as I 
recall, typically nest on the ground, and these are both successful 
groups.  What also of the big tertiary phorusrhacoids and diatrymids?  
These had to deal with mammals and birds more sophisticated than those in 
the Cretaceous, and they were certainly "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.

But probably not for an _Othnielia_ or a _Compsognathus_.  This may 
actually be an impetus for colonial and herding behavior in the larger 
dinosaurs.  Predators may have been able to get to eggs in nests around 
the periphery, but it would be much harder to get past the watchful eyes 
of the parent dinosaurs all the way to the middle of the colony.

> 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."
> 
> I maintain that the static egg was fine as long
> as nothing could exploit it.  

I can't believe there weren't mammals, lacertilians, and, later, snakes 
all throughout the Mesozoic that were just as capable of exploiting eggs 
as a modern mammal.

> 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.  

Those are all characteristics of the nests we've *found*.  But we've only 
found nests for a handfull of n-a dinosaur species, and these were 
reasonbly large species that indeed could not hide their nests very 
well.  But I find it hard to believe that smaller species that were able 
to conceal their nests would not have done so.

> I can imagine several likely scenarios.  
> a) Small burrowing mammals with keen senses built networks of
> tunnels and gnawed at the eggs...
> 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. 
> e) Birds perched in trees near egg beds wait for hatching.  They
> swoop up the little hatchlings with impunity.

Yes, all of these things probably did happen from time to time.  The 
question is whether these factors could have *completely annihilated* 
many diverse groups (among the n-a dinosaurs alone) which had been so 
successful for over 150 M yrs.  I contend that there were creatures (if not 
mammals and birds, then non-mammalian therapsids, pterosaurs, lizards, 
rhynchocephalians, and snakes) that did all of the above THROUGHOUT THE 
MESOZOIC and at least as well as mammals and birds.

> The dinosaurs are faced with another Hobson's choice: forage and
> lose the babies, or starve at the nest!  

Many birds face this choice all the time, to this day.  They do fine.

> 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.  

Those had little to do with neornithines.

>      My point here is that the birds were certainly diversifying
> and were most likely capable of the roles I have suggested for
> them.  

But other animals had been capable of similar roles for a long time already.

> Suffice [it] to say that birds could improve their tactics,
> open-field eggs could not.

Nobody is suggesting that the eggs hide themselves better.  The parents 
could improve their nest-hiding strategies, as you suggest birds later 
did.  This would not be a hard feature to evolve--if, indeed it was 
necessary at all, which has not been established.

> 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.  

I'm just trying to counter some placental chauvinism here.  Let's take a 
closer look at the development of mammal birth (this is a strategic 
series, not an evolutionary one):

Platypus:  disappears into a burrow to lay its eggs and care for the 
hatchlings ==> many placentals:  disappear into burrows to bear their 
young and care for the babies.

Echidna:  travels around with eggs in a pouch on its underside ==> 
marsupial:  travels around with its baby in a pouch on its underside ==> 
placental:  travels around with its baby in its belly.

Forgive me if I see no great leap forward here.  Sure, a monotreme egg is 
somewhat less protected than a marsupial or placental baby, and sure, 
there are *a few minutes* when a marsupial baby is less protected than a 
placental one, but I don't see that great a difference.  Besides, a 
monotreme could theoretically drop its eggs and run and go lay some more 
eggs, but a placental mother is permanently weighed down by her offspring.

> Then, in the juvenile stage the mammal again has better luck because
> her babies are at a further stage of development.

This is by no means always the case.  Compare a precocial bird hatchling 
like a duckling to an altricial mammal newborn like a puppy (not even to 
mention a human!)

> And extinction is generally caused by predation, isn't it?

I certainly wouldn't say "generally."

> 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.  

Excuse me.  Why is this a better analogy?  Dinosaurs had been evolving 
next to egg eaters on all continents for over 150 million years.  The 
reason immigrants to islands can kill off the native inhabitants is that 
they have evolved in isolation.  Are you suggesting that some new breed 
of ovovore suddenly arrived in the dinosaurs' environment (planet earth, 
that is) from somewhere else?  *That* would be analogous to the island 
scenario.

> 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?  

How am I supposed to know?  But are you saying that life just sailed 
blithely on through?  Wouldn't there be some impact on any dinosaurs 
around at the time?

> 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."

I don't believe that an impact alone could have killed off the n-a 
dinosaurs, either.  But I think the effects of a bolide impact would be 
far worse for the dinos than the effects of egg predation!

Nick Pharris
Pacific Lutheran University
Tacoma, WA 98447
(206)535-8206
PharriNJ@PLU.edu