Extinctions

[Ralph Chapman <MNHAD002@SIVM.SI.EDU>]

John Atwood is concerned about the lack of a mass death horizon and
suggests it indicates that the dinos were gone before the impact, if
it did occur at the boundary. Actually, many have worried about this
problem and it is pretty much been discounted because of developing
work by Kay Behrensmeyer & Alan Cutler and, in developing form - the
two of them with me. The logical progression goes like this:

1) Most "fossil deposits" of bone as recognized represent time-averaged
deposits of bone over a 1-10 thousand year time range, including many
that include mass deaths as a major component that lasted only a year
or two or less. The whole deposit as excavated, however, lasts the longer
time frame and mixes the faunas that done died there over that time.

2) Given that population mortality in a major area probably represents
5% or more of the population per year.

3) A mass extinction of the whole fauna would then represent only
20 or 30 years of total average deposition = 100%/20 or so.

4) Consequently, a 100% extinction would just look like an extra 20 or
30 years in a deposit that time-averages over 1000 years or more.

5) So, it don't look like much in the record.

6) What you really need is a big spontaneous deposit associated with
a sedimentary structure that nails the time period down - perhaps a
mega-tsunami deposit inland. With the K-T extinction event, you might
get these - or you might just get non-condensational deposits.

7) So the K-T boundary may represent something like a mass death or may
just be drowned out by the standard time-averaging in the fossil record.

Behrensmeyer and Cutler have given this at the last Snow Bird conference
and the three of us are going to take the simulations we did as part of
the Paleontological Society's short course on Taphonomy, and show this
also by simulation.

So, the lack of stacks o' bodies does not discount a K-T impact cause,
although we could get some, which would be neat.

It is a complicated thing this business of figuring out the K-T, especially
figuring out if the Marine and Terrestrial records are coupled or whatever.

Ralph Chapman, NMNH