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Re: Response to Nick Pharris: Patterns



> 1. Nick Pharris maintains that mosasaurs, plesiosaurs, pterosaurs,
> and crocodiles were abundant at the end of the Cretaceous.  From my
> own readings and discussions on this net my impression has been that
> the first three of these taxa were in serious decline [ ... ]
> perhaps I could challenge Nick to name a site close to the K/T where
> numbers of these creatures are found.

While plesiosaurs and pterosaurs do seem to have been declining at the end,
mosasaurs were not.  The type of _Mosasaurus_, for example, is from VERY
close to the K-T boundary, and other taxa were also present at the time.

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

While this is likely, it is not certain.  _Purrusaurus_, an alligatorid as
large or larger than any of the Cretaceous crocs, is from the Miocene of
Amazonia, where the largest prey items were 3 m long fish, giant capybaras,
and giant anhingas (and maybe the occasional ground sloth).

> This negates having to explain this in some arcane way: "they were
> selectively targeted by a bolide because they were heavier than
> 25kg."  Again, _all_ dinosaurs became extinct--those under and those
> over 25kg.  There is no pattern here, only the fact of dinosaur
> extinction.

NO!  Birds survived.  Thus, one lineage off (very small) dinosaurs made it
through the extinction.

> 2. Nick justly pins me on two word usages.  I know better for one.
> But for ease of usage, and because the issue of classification is
> still disputed, I'll stick to dinosaurs and birds rather than the
> clumsy non-avian and avian dinosaurs.

Nevertheless, the fact of avian surivival reflects that this small part of
the dino genome survived the extinction.  Making a taxon paraphyletic in
order to wipe it out is not fair in extinction studies.

> 3. Then he wonders: If dinosaur eggs were a fixed liability, "how,
> then, did concealed nests ever develop in birds?"
>
>   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.  [ ...] Part of concealing your nest
> is being unobserved when you are laying the eggs.  I imagine this
> was difficult for an ultrasaurus.

Which was long extinct at the K-T boundary.  Why would a _Troodon_ or a
small hypsilophodont nest be so hard to conceal?  Surely one of these little
guys could hide under the brush.

Also, as stated before, there is no evidence that Mesozoic birds were
tree-nesters.

> The dinosaurs are faced with another Hobson's choice: forage and
> lose the babies, or starve at the nest!  In this scenario, which is
> the primary agent, the climate change or the open and advertized
> egg?  It's a philosophical question, but one the mammals did not
> have to consider.

Actually, some Mesozoic mammals DID have to consider this, since some (the
ancestors of monotremes, and possibly all taxa up to the metathere-euthere
divergence) were egg layers.

> 7. "What does (birds being essentially modern in their flying
> ability) have to do with your argument?"
>   Like a Spitfire to a bomber, the better to harry the dinosaurs, to
> peck at an egg and escape untouched.  The better to snatch up a
> hatchling.

Now THAT's funny!!  I suggest you check the size of the known Cretaceous
birds.  Even though baby nonavian dinosaurs were small, the were much larger
than the typical K birds.  Okay, _Hesperornis_ and _Patagopteryx_ were
pretty big, but these were more like submarines and ATVs, respectively, than
Spitfires.

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

Not directly, actually.  There is abundant evidence that extinctions are
correlated with times of climate (and thus habitat) fluctuation.  In the
case of modern extinctions, however, the ultrapredation of humans does seem
to be a major factor.

> Paleobiology 22(1) 1996 pp104-112).  And, although he proposes a
> high latitude refugium hypothesis, 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."

Well, this breaks with the dogma since 1980, but it is typical of the
hypotheses of most of the century.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661