Re: Armadillos at the K/T!

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

----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
To: "Michael de Sosa" <ofsosa@uclink4.berkeley.edu>
Cc: "Dinosaur Mailing List" <dinosaur@usc.edu>
Sent: Monday, October 01, 2001 12:45 PM
Subject: RE: Armadillos at the K/T!


> On Mon, 1 Oct 2001, Michael de Sosa wrote:
>
> > I'm sure this has been brought up before, but how come dinosaurian
> > egg-and-young-eaters didn't wipe out the larger species a hundred
million
> > years before this? They were above badger-size for a lot longer than the
> > mammals were, if that means anything.
>
> They may well have been stressed by birds.

Why birds?

> In any case, It seems that by
> the end of the Cretaceous non-avian dinosaurs were not competitive
> in the small animal niche.

To mee it seems that non-avian dinosaurs were competitively excluded from
the rat-and-below-sized niche_s_. Okay, *Microraptor* approaches that size,
but it's the only adult non-avian dinosaur of its size that we know. I mean,
mammals (and tritylodontids) were around all the time. All niches for e. g.
rodent-like gnawing herbivores have been occupied by mammaliamorphs since
AFAIK the beginning of the Late Triassic, and therefore no dinosaur has ever
entered them.

>From what little I know about ecology, ecological niches are much, much
smaller than one for all big egg-layers (whatever big is), and they are
based on what animals do as a job, so to speak, mostly on how they get food,
rather than how they reproduce. Of course one can split most niches very
finely, as proven by related sympatric species that never "do" exactly the
same (except in times when there's plenty of food).

> They had put all their eggs in the big animal
> niche.  It is likely that they had most to fear from other big dinosaurs
> (after all, they were stuck to one site for 3 months or so).  They were
> diametrically evolved to be unable to protect themselves against small,
> nocturnal, fossorial beasts--there were few to none of these among
> dinosaurs).

There were small non-dinosaurian potential egg eaters around all the time.

> > Sorry if I seem skeptical before knowing all the facts, but it just
seems
> > unlikely to me that diverse groups of mammals on every continent waited
> > until the very end of the Cretaceous to simultaneously grow large enough
to
> > be a threat to dinosaur eggs, and that they found and devoured every
single
> > nest of every single dinosaur species on every single continent.

*Gobiconodon* was 40 cm long without the tail and is known from Mongolia and
NA in the EK. It was without any doubt some sort of predator (think of some
mixture of cat and opossum). Plesiomorphic dentition, except that the
incisors are big and pointed instead of the canines (don't ask me why).
Skull length around 10 cm -- compare *Sinornithosaurus* with 13 cm!!!
Definitely able to do a lot of damage to eggs.
        Is the pdf for *Repenomamus* or however its spelling is already
available? I'd like to know whether it was bigger (it's said to be the
biggest Mesozoic mammal).

> > Not to
> > mention that this remarkable feat of dietary prowess occurred around the
> > same time as a huge meteor hitting the Earth and severe climate changes
due
> > to regressions in sea level.

And at the same time as a Strangelove ocean, I'd like to add...
(How much do we really know about severe climate changes at that time? I've
read the regression in NA may have been limited to two quarters of the globe
and was caused by some rotational stuff, as I've posted long ago.)

> 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. In South America there are several Early Paleocene places stuffed
with mammals but not one dinosaur (don't know how old the ?ratite
*Diogenornis* exactly is). In India there is a site where the K-T boundary
layer (thin brown layer with iridium anomaly and stuff) is in the middle of
an intertrappean bed; dinosaur fossils continue right to the end and no
further. And so on.

> > There were probably tons of factors affecting the K/T mass extinction.
Nest
> > predation may be one of them, even an important one, but I have a hard
time
> > believing it was the primary one.
>
> Possibly true.  However, the apparent continuing selection against big egg
> layers--that is, we see the mechanism in action--should make it a prime
> (if not the prime) suspect.

IMHO what we see today is not selection against big egg layers but -- as
always -- 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 (kindly forgetting Pleistocene
extinctions -- nostra culpa, nostra maxima culpa).

More later, gotta run, university has begun :-)