The main bone (:-)) of contention here is wether the dinosaur extinction
was very rapid (catastrophic) and very numerous (mass extinction) or very
slow (gradual) and very few (normal extinction) or something in between. In
other words it is a question of *Rate* and *Magnitude*.

On the subject of Magnitude, a recent study[1] looked at 111 vertebrate
species of undisputed latest Cretaceous age in Montanna and tried to find
them in overlying localities of undisputed Early Paleocene (Tertiary) age.
Using species-level taxonomy involves complicating factors:-

1) species disappear locally but survive elsewhere - local extirpations,
not extinction.

2) rare species (eg a top preditor in an ecosystem) have something like a 5
times higher disappearence rate - artifact caused by rarity.

3) pseudoextinctions - where species evolve into new species (eg mammals,
if this process is not considered when looking at the mammals, it appears
that the mammals underwent a 95% extinction at the K/T boundary - which is
highly unlikely).

Allowing for these factors, this study found that the vertebrate extinction
rate across the K/T boundary was between 35%-47%, not approx. 75% as has
been stated previously. Significant yes, but not quite up the the great
mass extinctions of the past.

As to the question of rate, a recent study (and the one to which you may
have been refering to Mickey) was by Sheehan et al[2]. This study looked at
diversity in 8 families of dinosaurs in three sequential sampling levels in
the uppermost Cretaceous (Maastrichtian) Hell Creek Formation in eastern
Montana. Acccording to the study, all 8 families made it to the K/T
boundary. The study concluded that there was no discernable change in the
relative abundance of individuals assignable to a family and therefore,
"data from the Hell Creek are compatible with abrupt extinction
scenarios"([2] p 838).

However, there are two main problems with this study. One was that the
study was done via surface collecting *only* and in areas which were
*thought* not to have been collected for 10 years - one of the criteria for
choosing a site was the absence of plaster jacket material from other digs.

The second and more important problem was that the study was looking at
*ecological* diversity, not taxanomic diversity and as a result the
taxanomic level was too crude to deliniate taxanomic diversity and
subsequent changes in it. The study was at the Family level and as a
result, up to 43% of genera could become extinct without causing one family
to disappear. Increases in numbers of surviving genera may make up for the
loss of specific genera but nevertheless, *taxamonic* diversity would be
decreased.

The next question is "Does taxanomic diversity decrease in the
Maastrichtian?" and the answer is - Yes.

Emphasising a more global approach and at a generic (not familial) level,
another recent study[3] has highlighted both a decrease in taxanomic
diversity in the Maastrichtian and a bias imparted by North American
studies on this interval. This study found that of 73 Maastrichtian
dinosaur genera, only 20 are known from the late Maastrichtian and of these
20, 14 are known only from North America. Four hundred specimens of
articulated Masstrichtian dinosaurs were identified, of these only 100 are
from the late Maas. and of these 95% are from North America. Thus within
the Masstrichtian "trends toward reduction in diversity are evident". The
record of this interval is biased not only toward North Americal
localities, but also toward overrepresentation by North American genera and
specimens. North America in the Maastrichtian probably represents the last
bastion of dinosaur rule as dinosaur numbers dwindled elsewhere in the
world.

Thus a meteorite impact theory must explain how an impact could
retrogresively affect dinosaur numbers up to 1 million years before it hit
- if it is to be considered as the 'prime suspect'. IMHO it may well have
toasted a few dinosaurs, but they were already on the slippery slope to the
big X.

Another problem presents itself. Since the impact ejecta is taken as
demarking the K/T boundary, how come (to my knowledge) no dinosaur bones
have ever been found in this layer. Since it must have been layed down
fairly rapidly, and there must have been a 'great dying' at the time (if it
was the main cause) where are the bones surrounded by the ejecta layer - or
if it was fairly instantaneous, where are the bones lying above it. Where
it is found (I think) there are bones below it, but no bones in it or
obviously, above it.


[1] Archibald, J. D. & Bryant, L. (1990) Differential Cretaceous/Tertiary
extinctions of nonmarine vertebrates; evidence from northeastern Montanna.
_Geological Society of America Special Paper_ 247: 549-562.

[2] Sheean, P. M. et. al. (1991) Sudden extinction of the dinosaurs: latest
Cretaceous, upper Great Plains, U.S.A. _Science_ 254: 835-839.

[3] Dodson, P. M. (1991) Maastrichtian dinosaurs. _Geological Society of
America, Abstract with Programs_ 23(4): A184-185.

See also:-

Archibald, J. D. (1992) Dinosaur extinction: how much and how fast?
_Journal of Vertebrate Paleontology_ 12 (2): 263-264.


> Mickey Rowe     (rowe@pender.ee.upenn.edu)
Shouldn't that be Mickey Rowe DP ? :-)

Chris

================================oOo====================================
Christopher Nedin                                                 -- --
Dept. Geology, University of Adelaide, South Australia          / \   /
cnedin@tellus.geology.adelaide.edu.au                          /   \ /
"How can Nedin be trusted"                                      -- --
C Wieland  Director, Institute of Creation Research  QLD. Australia
================================o0o====================================