K/T, Sig-Lipps (or is it Signor-Lip?)

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I was wondering how long it would take us to get around to a K/T debate.
I'll try to keep this one short.  First, let me re-iterate what Jere said
about the negative evidence nature of the Signor-Lipps Effect.  One of the
most important (but rarely stated) conclusions of the 1982 Signor-Lipps
paper was that the biostratigraphic record of first and last occurrences
cannot be used to decide the question of abrupt vs. gradual extinction
patterns because of the inherent bias toward gradual patterns that arises
as a result of a number of factors including differential preservation
potential, differential environmental tolerances, and differential sample
size relative abundance interactions between species.  We see patterns of
species distributions in the fossil record (not the processes responsible
for those patterns) and on a very fine spatial scale in single sections it
is exceedingly difficult to know whether the patterns reflect biotic
processes in a completely unbiased manner.  As a result of this uncertainty
the pattern of last (or first) occurrences leading up to the K/T boundary
(or proceeding away from it) is ambiguous with respect to distinguishing
between level of paleontologic support for various causal explanations.
Applied to the vertebrate record in the American West the 1 m gap may
represent a true absence of dinosaurs worldwide or it may represent a
sampling bias (e.g., they were eliminated from that locality but survived
to the boundary elsewhere).  Until we get independent evidence bearing on
this question we cannot know.  [Note: at the Seattle GSA meeting molecular
phylogenetic evidence was presented which suggests that all modern orders
of birds (btw birds are dinosaurs) had diversified prior to the K/T
boundary.  This is the sort of independent evidence that Jere means.]
Unfortunately, the Signor-Lipps effect is being used by many to argue that
because a gradual extinction pattern is observed in a local K/T boundary
section, this observation (via the Signor-Lipps Effect) provides
corroborating evidence in favor of a catastrophic extinction mechanism.
Localized gradual extinction patterns by themselves are neither necessary
nor sufficient predictions of catastrophic extinction models. In fact,
these patterns are not predictions of any mass extinction model, gradual,
catastrophic, or otherwise.  All that can be said with certainty is that
(for the present) such patterns are irrelevant to resolving the
controversy.

That having been said though, I'm much more optimistic than Jere about the
possibility of testing the Signor-Lipps explanation.  True, some form of
Signor-Lipps pattern may arise regardless of how well a section is sampled.
However, if we separate the Signor-Lipps bias out into its component parts
(biases due to environmental tolerance, diagenesis/preservation potential,
and sample size/relative abundance interactions) it seems to me that we
should be able to determine whether or not the observed pattern can be
statistically accounted for via appeals to these processes.  In my view,
it's not a question of whether a Signor-Lipps Effect may or may not exist
in a particular dataset (it always must be assumed to exist and I suspect
that it almost always does indeed exist).  Rather, the question is whether
or not the observed pattern of last (or first) occurrences can be explained
as a result of the class of Signor-Lipps biases to a specified level of
confidence.  Tests are available to examine the predictions of these biases
individually and compare them to appropriate null models.  Until those
tests are carried out for individual paleo. datasets, I think it's best to
look to some other test of the catastrophic vs. progressive extinction
scenario for the K/T or any other extinction event.

Two other quick points.  First, recoveries from mass extinction events are
interesting but they don't address questions dealing with the nature of the
mass extinction event itself.  To do that you must look to the data
provided by species that survived the mass extinction event.  Any
explanation for a mass extinction event that cannot account for the
composition of the survivor fauna, in effect, explains nothing.  Second, I
assume Jere mispoke (mistyped?) the following:

The K/T extinction event evidence comes not
from paleo so much, but from geochem, mineralogy, and geology.

I don't understand how evidence for an extinction event (which I understand
to be a biotic phenomenon) can come from non-biotic data.  Of course one
can use the evidence provided by these fields to constrain causal scenarios
that seek to account for the biotic data.  But the biotic data must remain
the subject of the explanation and never be relegated to a "supporting"
role.

Norm MacLeod


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Norman MacLeod
Senior Research Fellow
N.MacLeod@nhm.ac.uk (Internet)
N.MacLeod@uk.ac.nhm (Janet)

Address: Dept. of Palaeontology, The Natural History Museum,
                     Cromwell Road, London, SW7 5BD

Office Phone: 071-938-9006
Dept. FAX:  071-938-9277
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