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K-T extiction (fwd)
---------- Forwarded message ----------
Date: Thu, 17 Nov 1994 09:41:45 -0600 (CST)
>From: Peter Sheehan <email@example.com>
To: Dinosaur list <firstname.lastname@example.org>
Subject: K-T extiction
Since the gradual extinction crowd has been active on the list I thought
I should put out a bit from the other camp. I find that at this point in
time the two sides are essentially talking at each other, rather than
trying to convince each other. This is much like I remember as a student
when plate tectonics was becoming accepted. Conversion from one camp to
the other occurs when an individual decides the weight of evidence is
overwhelming--and the conversion is almost instantaneous. Some never
From my perspective I am pleased that most movement seems to be
from the gradualist camp to the impact camp. Does anyone know of any
movement the other way?
To the extinction. I present my view--the background has been
published--I will not attempt to document it here.
1. Was there an extinction event--some gradualists deny it.
There was a huge extinction. Sepkoski data base shows it as one
of the big 5. On land, just consider what the land was like in the
earliest Paleocene. There were no large bodied herbivores. There were
no large bodied carnivores (except some stream dwellers that might get
onto land around water--but they would have no food source on land.)
Mammals were tiny, mostly insectivores and omnivores. This community
was unlike anything before or after. It reflects the enormity of the event.
2. Dinosaur communities died out suddenly and there is no evidence they
were in decline.
The record is too poor and spotty to look at global diversity
over the last 20 my but we can look at communities over shorter time
spans. My group found that over the last 2 my of the K, dinosaur communities
were essentially stable. There was no evidence of any sort of decline in
the communities as would be expected if a gradual extinction were in
3. What does the sea level decline mean? Not much--3~ seas went in and out
numerous times in Mesozoic--with no extinction events (I assume the
Triassic event was not caused by sea level flux.) Look at Greg
Retallack's paper in the lastest GSA Bull. There was no drastic change
in soils during Hell Creek. Flora in Hell Creek changed modestly, but
there was a huge event at the time of the impact (followed by disaster
floras world wide). The point is the sea level change did not seem to
have much effect on the land climate or biota. And in the marine realm it
was not much of an event either.
4. Was there a gradual or step-wise extinction. The Signor-Lipps effect
explains virtually all known instances of supposed gradual and step-wise
extinction. The poorer the record the longer the supposed "gradual
extinction" occurs. People still talk about a 10 my long decline of
dinosaurs. But dino record is basically hopeless for this kind of change.
5. Birds--idle speculation with almost no record.
6. Loss of sunlight (from dust and Sulfur gases) and other associated
effects of impact caused collapse of food chains based directly on primary
production. There was a crash of marine phytoplankton and land plant
production at the impact layer. Animals in these food chains died, eg.
ammonites and dinosaurs. Animals in detritus based food chains
preferentially survived, eg many
detritus feeding bivalves, insectivorous and ominivorous mammals.
Animals in stream communities preferentially survived because stream
communities have very little primary production and receive much energy
from detritus on land.
Mammals ate larvae, worms, and other animals you find in a rotten log
which in tern fed on dead rather than living plants. On top of the crash in
primary productivity there was general destruction from acid rain, forest
fires, etc. etc.
There is virtually no evidence for gradual extinction.
Inoceramids and rudists declined or died out before. This is not
something foreboding the extinction. Groups of animals, especially
highly specialized groups die out through out geologic time these groups
died, but were not part of the extinction event.
There was a huge extinction event both on land and in the oceans
which, to the level of resolution allowed by the fossil record, was
1) geologically instantaneous, and 2) at the impact layer.
But this will not convince a gradualist in the near future. Nor will a
gradualist convince me. We are involved in a kind of scientific
exchange that differs from most science. To move from one camp to the
other requires a huge reevaluation of a scientists basic understanding of
nature. This is not an issue of normal science, where new data is just
an addition to our understanding. We are in the sort of controversy
that happened during the plate-tectonic revolution. For those of you
viewing this process from the outside, my guess is that in 20 years
the abrupt, impact extinction will be accepted--but only time will tell.
Those of us involved in this debate are simply talking at each other at
this point in time.