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Re: Extinct vs Disappeared

There have been many studies done over the decades which show a high 
statistical correlation between unconformities and high numbers of extinctions. 
 Phrased another way, if nature puts a big enough time gap in any 
randomly-chosen section of strata, there is a high probability that flora and 
fauna disappear from the overlying rock beds, and, further, that the number of 
dissappearing species increases the longer duration of the time-gap.

In contrast, there have been MANY fewer studies which show a "paleo-migration" 
of a species out of an area and into a new area had occurred.

Back in the 1980s, J.D. Archibald suggested that the Bug Creek Beds that 
"straddle" the K-T Boundary in Montana (actually, they don't straddle it at 
all, but I digress) represent a gradual replacement of "Cretaceous-aspect" 
mammals with "Paleocene-aspect" mammals.

Archibald's hypothesis is extremely hard to prove for a variety of reasons, but 
mainly because the Bug Creek Beds are so localized that they only represent a 
small geographic area (1 mi^2 is all there is of them).

You have to use Occam's Razor in situations like this.  If you look at the Bug 
Creek Beds, their bottoms are channeled deeply into Cretaceous rocks. The 
stratigraphic relationships between the beds and the Paleocene Tullock 
formation is less clear, but the proximity of the beds to the K-T time horizon 
should be taken into account.

The most likely scenario is that the Bug Creek Beds represent mixing of animal 
skeletons from different time periods (K and Paleocene).

We don't know if 20-foot-long methane-eating worms live on a small planet 
orbiting Alpha Centauri.  Given a total lack of data, our best guess would be 
the null hypothesis.....that the worms don't exist.

When a fossil dissappears from the fossil record, pending new data, the most 
prudent way to view the data as showing extinction.

The dataset can be added to, however.  That's why there are grad. students.


---------- Original Message ----------
From: Andrew Simpson <deathspresso@yahoo.com>
To: tholtz@umd.edu, tijawi@yahoo.com, dinosaur@usc.edu
Subject: Extinct vs Disappeared
Date: Fri, 5 Mar 2010 09:56:53 -0800 (PST)

With Stegosaurs it is often said that they went extinct midway through the 
Cretaceous. This always bothers me slightly as I prefer the phrase 'disappeared 
from the fossil record' over the word extinct do to the fact that disappeared 
from the fossil record is a factual statement were as went extinct is 
speculative. I for one tend to assume that Stegosaurs would have likely still 
existed in small remote niches and on islands where advanced predation and food 
niche taking new herbivores with better defenses couldn't get to them and thus 
wouldn't cause their end. 

Am I being nit-picky?

Andrew Simpson
Dinosaur Enthusiast 


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