Re: Armadillos at the K/T!

[John Bois <jbois@umd5.umd.edu>]

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.

> > 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.

Two comments and a question: niche occupation is not a God-given
right.  Pterosaurs occupied the flying diurnal niche--they were summarily
booted out of it by better competitors (birds).  I think there are reasons
that dinosaurs are less competitive in the very small animal niche (I
have suggested in the past that bipedalism may have something to do
with it; that scuttling through the dense matrix of a small universe might
be better done on four legs or a flat belly--but then there are rails).  
I am under the impression that the mean size of dinosaur fossils increases
toward their extinction--do you have a reference as to the distribution of
fossil sizes (adult) as related to the progression of the Cretaceous?

>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).

I may be stretching the definiton too far--not sure.  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).  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.

> 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.  I mean,
there were always small non-dinosaurean egg eaters around--but, at some
point they became more effective.

> *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.

I hope you don't think it an admission for me to say that I don't know
that my hypothesis is correct.  There is heavy speculation involved in
parts (but by no means, all) of the idea.  The exact mechanism, if it
happened, must be total guess work.  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.  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.  Simple idea: impossible syntax, I'm sorry!  This could not be
said of G. I think.

> > 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...

> 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.

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

> > 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).

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!  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?  That's a stretch if you ask
me.  I favor the view that there are reasons for things--adaptive reasons,
structural reasons, body plan reasons, phylogenetic reasons, reproductive
mode reasons; that preemptive filip cannot explain all ecological
residence for all localities for all times.