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Re: The extinction of small dinosaurs (long again)



Original Message by John Bois
Thursday, 19 December 2002 03:09

> So what is it that selected against enantiornithines and small dinos at
> the K/T, or against small pterosaurs during the Cretaceous?

A few more fossils could come in very handy. How many late Maastrichtian 
Enantiornithes are known? *Avisaurus* in the Hell Creek... er... What is a 
small dino?

If a small pterosaur is any one that is smaller than the biggest contemporary 
bird, then, AFAIK, the last small (adult!) pterosaurs were the anurognathids 
of the EK of China. Could have died out in the underresearched Aptian-Albian 
mass extinction (cool Oceanic Anoxic Event at that time), the 
Cenomanian-Turonian one (even cooler OAE) or who knows when. In any case the 
Jurassic-Cretaceous boundary (surrounded by a few considerable impacts and 
ammonite mass extinctions...) may have marked much extinction among 
pterosaurs.

If a small pterosaur is any one that is smaller than the biggest contemporary 
bird _in a similar ecological niche_, then there may never have been any... 
assuming the anurognathids were swallow- or swift-like insectivores* and none 
of the so far known birds of the entire Cretaceous appears to have been.

* Conclusion in Wellnhofer's 1991 book. Based on the extremely long, narrow 
wings that have turned out to be an exaggeration...

> > > If we're talking about pterosaurs vs. birds, I don't think we can rule
> > > out predation for the same reason (dispersal).
> >
> > Still, they don't have continental-sized or global ranges,
> > so a global predation-mediated extinction event seems
> > unlikely, unless somehow all of the bird-pterosaur
> > interactions worked out the same way.
>
> My understanding is that pterosaurs did not disappear in an event--rather
> a gradual loss of diversity--first small then large species becoming
> extinct.  Is this not true?

Impossible to tell at the moment. Far too few fossils. It does look like the 
gigantic clades went last, but if this reflects a drawn-out affair over the 
whole K, or 2 mass extinctions, or whatever, is not for me to guess, and 
probably for nobody at the moment. Or have I ignored too many of the LK 
"*Ornithocheirus*" fragments?

> > However, competition is actually an individual
> > trait, though we speak of it at the species level at times.
> > For a species to radiate into an occupied niche, you would
> > have to select _for_ individuals that are similar to the
> > species that is the current 'niche-holder'.  This should be
> > rare, because it would put increased pressure on those
> > individuals.
>
> But there is _always_ competition for resources.  Perhaps the idea of
> fundamental vs. realized niche is helpful here.  I would argue that most
> species are under pressure from other species for many of their
> resources.  They are able to exist because they outcompete others for
> essential resources.

Indeed. And therefore the trend is to reduce the fundamental niche so that it 
isn't much bigger than the realized niche, in order to reduce competition 
because competition is so expensive in time and energy. Or at least that's 
what the lecture "Introduction into Ecology" says.

> Now an individual is slightly more competitive for a
> specific crucial resource that you were already in competiton for.  That
> individual has more offspring because of it and--bang, zoom--pretty soon
> you have a new species whose entire population is more
> competitive.

If the former resident has somehow not evolved in the meantime. I mean, it 
all happens in its presence.

> Depending upon the resource, this might put the former
> resident out of business.

Then you have competitive displacement between _two species_. But you're 
talking about much bigger clades. What could compete with an albatross-like 
pterosaur? (To regard a furcular fragment from the Nemegt Fm as an albatross 
is... very bold.)

> > > I mean, competitive exclusion refers to almost
> > > identical niche utilization--

But so does competition in general.

> > > I just don't think this was the case with
> > > bird v pterosaurs.

Agreed. :-)

> > Not always.  If the two groups are competing heavily enough
> > to have extinction consequences, then that should be enough
> > for competitve exclusion to be a factor.
>
> Agreed.  But this is a _process_.  Species may compete for non-critical
> resources, and gradually encroach on each other's territory.  Arms races
> may ensue

I only know of arms races between prey and predator. Do you know an arms race 
between competitors?

> > However, I really don't see the
> > evidence that they were basically predated to death at a
> > family level.
>
> Right, there is no evidence.

So is there a way to test your speculations?