Re: Fossil Scenario

Arthur V. Chadwick (
Wed, 03 Dec 1997 11:32:21 -0800

At 06:04 AM 12/3/97 -0600, Glenn wrote:
>Not necesarrily. It depends upon how deep the burrows go, and the density of
>burrowing animals. I would point out however that there are many layers in
>which bioturbation HAS destroyed the bedding. I would have to look them up
>but how do you account for these beds?

There are *VASTLY* more beds with burrowing that have preserved bedding
than the occasional bed that has all bedding obliterated by burrowing.
This is not as it should be regardless of what model one wants to use to
explain the existence of the burrows themselves. It is a problem that
won't go away. In the modern Mississippi, burrowing organisms obliterate
any sign of bedding features in sedimentary environments where burrowing
organisms exist. Estimated rates of sedimentation on the modern sea floor
vary from 1000mm/year for deltas to 10 mm/year for reefs and shallow
carbonate (Schindel, D.E. Paleobiology, 1982, 8: 340)in areas of rapid
sedimentation. If this sedimentation occurred in areas of high biotic
activity, most of the sedimentary features should be obliterated (Darwin
estimated that earthworms in one acre of soil moved 10 tons of soil through
their bodies each year). There are far more burrowing organisms per acre
in the marine environment than in Darwin's yard.

This raises another question: if modern rates of sediment accumulation on
deltas are one meter per year, in 1000 years that should give one kilometer
of sediment. What would happen in a million years? Yes, I know, the
sediment is reworked into deeper water, etc, but we don't have all that
many sequences in the fossil record that are deep water (something that I
think is itself long overdue for correction, but that is for another day),
so we invoke recycling, which is convenient, except that it eliminates the
evidence for evolutionary sequences, which is also convenient, except that
it just ain't so. Stratigraphic sequences I have studied in detail(mostly
early Paleozoic) show very little evidence for reworking between beds, and
certainly lack any substantial evidence for displacement of massive amounts
of sediment to deeper water. Hypothesizing that sediment rates were far
lower in the early Paleozoic doesn't cut it since generally they are
believed to have been much greater, since there was presumably no
vegetation to prevent erosion. In fact the whole problem of sedimentation
rates through time has been the subject of a lot of study. The whole
affair calls into question our current understanding of sedimentary
processes. (Schindel, op.cit; Anders, M.h., S.W. Krueger and P.M. Sadler,
J. Geol. 1987. 95:1) The conclusions (borrowed from a friend) are worth

"Sedimentation rates [estimated by every conceivable technique] are
alarmingly variable, spanning at least a dozen orders of magnitude. In
every environment the rates decrease progressively as longer time spans are
considered. The regression is partly a function of environment. There are
several consequences of this pattern and they are not all intuitively obvious.

1. Stratigraphic completeness is a function of time scale and can be
meaningfully quoted only for a specified time resolution.

2. Short term sedimentation patterns are a disasterous guide to long term
accumulation and we should distance ourselves from the doctrine of

3. If high resolution is required then stratigraphic sections must
generally be expected to be disasterously incomplete.

4. Even abyssal ooze accumulations may be quite incomplete.

5. Beyond time spans of about 10^4 yr all sections at or above marine wave
base seem to be controlled by tectonic and not sedimentological factors.
Thus, we should not automatically expect a fluvial section to be less
complete than a shallow marine section. That we usually do, is an
unfortunate consequence of uniformitarianism.

6. Very few, if any sections of sedimentary rocks will yield a record
adequate to study species-level biological phenomena, if a fairly complete
sequence of generations is needed. The staratigraphic record may not
qualify as a testing ground for punctuated, allopatric speciation. It may
not ever be able to tell us if an asteroid arrived at the right time to
cause a catastrophic extinction at the end of the Mesozoic."