Natural Science Forum / Biology / Paleontology / April 2004

This study made several unfounded assumptions. The most significant is
that the sex of dinosaurs was determined by temperature. Since no
variety of bird determines sex  in this manner it would be very
surprising if all non avian dinosaurs determined sex like crocodilians.

Since the evidence indicates a global extinction event that affected all
life on the planet it is impossible that this factor was significant in
the extinction event. It is more likely that the impact caused a
"nuclear winter" scenario that caused a collapse of ecosystems which
lasted for several years.

Ken

In most issues, one single failure won't produce a catastrophe. Also
nature can handle single problems.
Most catastrophes are caused by the coincidence of several problems. The
sex hypothesis may account for part of the extinction, but I *guess* not
alone.
Accordingly, I doubt the hypothesis that the Chicxulub impact was the
*only* reason for the extinction. I *guess* that the huge eruptions of
the Deccan flood basalts, which occurred at the same time, had serious
influence on life on earth.
I belive that one reason for the over-estimation of the meteorite impact
is the vicinity of the impact site to the USA compared to India. It's
quite human that one tends to prefer studying nearby phenomena.
Indian scientist rather tend to prefer the Deccan basalt influence.

As you mention the length of time required for the KT boundary clay to
deposit, be aware that most dinosaurs bones are far thicker than the
boundary clay, so it's hard to determine the exact stratigraphic
position (if I say "exact", I mean several tenthousand years).
In many cases (especially fluviatile environments), bones can be
reworked and deposited in early tertiary strata.

Cheers, Christof

Of course, nobody "exactly" know the time length of the deposition of the
KT boundary layer. There are various guestimates & estimates. See:

Mukhopadhyay S, Farley K A, Montanari A, A short duration of the
Cretaceous-Tertiary boundary event: evidence from extraterrestrial
helium-3, Science 2001; 291: 1952-55

Their estimate is about 10,000 years. Short enough to be considered almost
instanteneous in geological time scale but long enough not to allow any
dinosaur fossil to sit on top of it. But also see :

Fassett J E, Dating the extinction of paleocene lazarus dinosaurs based on
magnetochronology, San Juan basin, New Mexico,
http://gsa.confex.com/gsa/2001AM/finalprogram/abstract_25767.htm

Fassett claimed to have found dinosaur fossil a million years younger
than the KT boundary - and it is not a reworked specimen.

It is not really the case that there are no dinosaur fossils after the
KT boundary.  Quite a few saurian bones have been found in Lower
Paleocene deposits.  The problem is that many (most? all?) of them
probably washed out of older deposits and were redepositied in the
Paleocene beds.  What percentage are redeposited?  No one knows.  In
the past few years, a partially articulated skelaton and one very
large bone found in a context where water transport was deemed
unlikely have turned up in the Lower Paleocene Ojo Alamo formation of
New Mexico.

Also, I'm unsure of the status of the debate about whether some groups
that disappeared near the KT boundary were in decline prior to the
Chixulub impact.  I believe, for example that the diversity of
ammonites in the US Western Seaway drops precipitiously in the last
few million years of the Cretaceous.  At one time, dinosaurs were
thought to have been declining also.  I believe that hypothecated
decline has become controversial.

What I've always wondered about was the effect on light--especially
UV--caused by the impact/Deccan eruptions.

It seems to me the dinosauria sported very few unambiguously nocturnal
adaptations, aside from a few Antarctic species perhaps. I wish I knew
more about the need for UV in various lineages, but I at least know
that humans can't make much vitamin D without it (or rather, certain
wavelengths of it). Are other vertebrates--especially birds--dependent
on it at all?

It seems to me the land vertebrate's key to surviving KT was (very
roughly speaking) to be either rather cryptic (mammals, lizards,
snakes), or somewhat aquatic (crocodilians, turtles). Such creatures
would have already compensated for lower levels of UV, and others
would have compensated for higher levels. Could the impact have thrown
a tinted window across the sky long enough to cause a severe crisis in
lineages that had skins and eyes and metabolisms geared for high UV
levels, or, more subtly, at least a certain balance in the spectrum?

The pattern of marine extinction would seem to be less
supportable--but, then, I have no idea what UV would have meant to
plankton (or plesiosaurs)...