Re: Armadillos at the K/T! (long)

["David Marjanovic" <david.marjanovic@gmx.at>]

----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
Sent: Wednesday, October 03, 2001 3:36 AM
Subject: Re: Armadillos at the K/T!


> On Mon, 1 Oct 2001, David Marjanovic wrote:
>
> > > They may well have been stressed by birds.
> >
> > Why birds?
>
> Birds are today fearsome predators of eggs and hatchlings of ratites.  An
> earth bound predator may fear a parent; a bird can nail a hatchling and
> fly off without penalty.  This is such a threat to rhea that they strike
> at airplanes flying thousands of feet in the air.

Really makes me wonder why ratites have survived to the present instead of
dying out in the LK (right when they probably evolved)!

> Two comments and a question: niche occupation is not a God-given
> right.

Sure. But is there a firmly known case of sympatric evolution of something
into an occupied niche?

> Pterosaurs occupied the flying diurnal niche--they were summarily
> booted out of it by better competitors (birds).

>From what I know it seems to me that in the end-K all pterosaurs were super
albatrosses and there were no soaring birds at all, so they had very
different niches; and if the K-T impact hadn't happened, the pterosaurs
would be still there, and osteodontornids and albatrosses would never have
evolved. Of course the first part of this sentence relies heavily on the
patchy fossil record, and the second is entirely speculation; however, the
impact is still the only event that I can think of that could be responsible
why it hasn't become true and azhdarchids suddenly died out in what looks,
from the known fossil record, like their full bloom.

> I think there are reasons
> that dinosaurs are less competitive in the very
> small animal niche ([...] but then there are rails).

Indeed no very small nonavian dinosaurs are known -- but *Microraptor*,
found in a site with exceptional preservation, may IMHO be a sign of things
to come. Thanks for mentioning rails -- in mammal-free (save the two bat
species) New Zealand there are big grasshoppers (weta) instead of small
herbivorous mammals and small rails (weka) instead of small
carni-/insectivorous ones. (I'd appreciate confirmation or correction from
New Zealanders -- I know NZ from TV.)

> I am under the impression that the mean size of dinosaur fossils increases
> toward their extinction--

The mean size of nonavian dinosaur fossils is something difficult to
calculate, of course. When I look at the extremes then there are really big
sauropods in the LJ of NA and Africa as well as the mid-K of SA and Africa,
while theropods the size of *Compsognathus* (1.4 m long adults) seem pretty
widespread in time and space (but few are known -- bad, bad fossil record).
*Microraptor* is EK, 30 Ma later than *"Amphicoelias" fragillimus*.
Personally I can't see any trend here.

> do you have a reference as to the distribution of
> fossil sizes (adult) as related to the progression of the Cretaceous?

No. AFAIK nobody has ever done such a study because the fossil record just
doesn't permit it to say anything useful.

> > There were small non-dinosaurian potential egg eaters around all the
time.
>
> But they were qualitatively different--at least this is the
> hypothesis.  If you grant that predatory guilds today have a limiting
> effect on non-concealed nests, this must have become true _at some
> time_.  If this is true, then your above argument doesn't hold.

If this became true before the K-T boundary (say LTr, MTr...), it does hold,
doesn't it?

> I mean,
> there were always small non-dinosaurean egg eaters around--

not to forget the probable dinosaurian ones!

> but, at some point they became more effective.
>
> > *Gobiconodon* was [...]
>
> I hope you don't think it an admission for me to say that I don't know
> that my hypothesis is correct.

I can't ask for a proof, of course. I just ask for a little evidence :-}

> But, I could heavily speculate the
> following: Gobiconodon may have been limited by its reproductive
> mode; later placentals had lower mortality rates and thus could survive
> better to prey on nests and hatchlings.

I can't argue against that. It would help to know _anything_ on its
reproduction mode... it could have laid eggs, it could have been nearly
placental*, nobody knows :-)

*Depends on what the marsupial method is derived from -- from something like
the placental condition or something speculative approaching the marsupial
condition without a pouch.

> However, the hypothesis is based
> on a simple idea: the ancestors of organisms that first appeared near dino
> extinction in sizes which today cause trouble, today cause
> trouble.

We don't know whether *Cimolestes* is ancestral to anything alive (it has
been claimed to be related/ancestral to Creodonta + Carnivora, but that was
long ago, or just to hang around somewhere near to the base but maybe even
out of the crown clade Placentalia). We know that *Stagodon* and
*Didelphodon* aren't ancestral to anything alive (Boreometatheria is
extinct). Same for *Gobiconodon* (whatever "triconodonts" it belongs to were
gone before the K-T as far as is known) and the AFAIK fairly big
*Deltatheridium* (sister group to apparently all other metatherians).

> > > We must remember that the extinction of dinosaurs is only recorded in
one
> > > place.  I am arguing that mammals and birds contributed there and that
> > > they have had an abiding effect everywhere else in the Cenozoic.
> >
> > Well, everywhere else in the world where terrestrial sediments are known
> > from these times there are nonavian dinosaurs in the LK and never in the
> > Paleocene.
>
> A much greater expanse than the 2 month K/T window...

Real terrestrial K-T sections are rare, but very early Paleocene sites
aren't.

How would you explain the marine extinctions?

> Indian iridium layer is trouble--doesn't correlate to layers in other
> parts of the world (ref. if needed).

Sounds very strange. Ref would be appreciated :-)

> > IMHO [...] competitive exclusion. No egg-layer -- and nothing else --
can
> > evolve into the niche held by elephants because there already are
elephants
> > everywhere where that niche exists [...]
>
> So, you're saying that dinosaurs could re-evolve were it not for elephants
> getting to that niche first.  Dinosaurs could be a lot smaller than
> elephants and still be a lot bigger than they are now.  I mean there are
> lots of niches between elephant and ostrich!

True. I was using the elephants as an example and should have said so.

> Or are you saying that every
> single mammal that exists has preempted every niche they are now in
> forever and back into the Cenozoic past?

No, but I say that every terrestrial niche now occupied by whatever mammals
has happened to have been occupied by mammals, birds, crocs and/or varanids
all over the Cenozoic, save the first few million years, when many of them
were totally empty, it seems.

> What I mean to say
> is that the niche dimension of reproduction/parental care/nest
> defense/concealment bears a resemblance in many if not most end Cretaceous
> species.  Specifically, that most relied on obligatory nest defense (ala
> crocs).

Croc nest mounds are hidden, and when you've found one you still have to dig
deep for the eggs.

> This enabled their existence; w/out the ability to guard nests,
> most of these big creatures would have not been able to reproduce at
> all.

and in another post,
----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
Sent: Wednesday, October 03, 2001 5:40 AM
Subject: Re: Armadillos at the K/T!

> On Mon, 1 Oct 2001, David Marjanovic wrote:
>
> > Have you got evidence that we _should_ decouple other extinctions?
(Sorry if
> > I'm annoying you by repeating the same questions all the time. I'm just
> > trying to believe that you have more evidence than the three big mammals
of
> > LK western NA on which to build a hypothesis.)
>
> No. The hypothesis comes from the simple observation that a
> strategy that was once widespread is today non-existent.  With the
> exception of crocs, all oviparous species avoid confrontation with nest
> predators by concealing, laying remotely, or some other tactic.  Nest
> defense is a last resort.

Now I realize what your basic assumption is: that nonavian dinosaurs didn't
make any attempt to hide their nests but always preferred battle! Right? :-)
If so, it has been falsified with the discoveries of Egg Mountain and Egg
Island -- islands in an alkaline lake where *Maiasaura* nested -- and of
apparent colonial nesting in oviraptorids. I should also mention the
pterosaurs from Chile that nested in the desert (as some seabirds in the
region do today -- there are IIRC boobies that nest in the Atacama).
Extremely generalizing hypotheses of that sort are generally... oops :-)