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Re: Extinctions



On Wed, 19 Apr 1995, Ralph Chapman wrote:

> 
> 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
> 

See also my paper in Dinofest vol: Sheehan, 1994, The extinction of 
dinosaurs: Dinofest, Rosenberg & Wolberg eds., p. 411-423.

All the dinosaurs living at the end of the Cretaceous would have died 
within less than 100 years even had an impact not happened.  
And 100 years of sedimentation is almost nothing in a flood plain.  The 
Hell Creek Fm. is 300 feet thick and represents 2 m.y. of time.  What is 
the expected sedimentation in 100 years?  Clearly the problem is that 
very few dinosaurs ever make it into the record.

Peter