Nature of the fossil record

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Just one observation and one question to (hopefully) extend the discussion.
First, the Signor-Lipps effect was originally described in the context of
mass extinction studies to address the question of what we might expect the
fossil record to look like as we approach a stratigraphic horizon that, on
the basis of other lines of evidence (e.g., geochemical anomalies), we
might believe represents a mass extinction event.  However, in reality, the
Signor-Lipps Effect is much more general than that.  Indeed, it constitutes
a ubiquitous feature of the fossil record that we should expect to see
regardless of where we are in the stratigraphic column.  In my opinion
there is a deep connection between the Signor-Lipps Effect and the more
biostratigraphically-oriented literature dealing with the question of
assigning confidence intervals to first and last appearance datums.  Work
toward resolving this question can be traced back at least to Shaw (1964)
and so the Signor-Lipps Effect might be more appropriately termed the "Shaw
Effect" [Note: we just can't seem to get away from these nomenclature
debates can we?]  In any event, does our new found appreciation for the
"Shaw" (or Signor-Lipps) Effect mean that we should be changing the way we
interpret biostratigraphic data?  I think it does.

One final thing (to bring this full circle), is that the horizon taken as
the Signor-Lipps "event" horizon seems to be defined in a completely
arbitrary manner.  Since Signor-Lipps is everywhere how do we know where to
place the event horizon?  In terms of the biotic data and the Signor-Lipps
Effect is there any real distinction in assuming the event horizon to be
midway through the "barren interval" in Hell Creek rather than at the Ir
anomaly.  I think this gets at the some of the points being made by some of
the postings referring to the interpretation of the Hell Creek (non-avian)
dinosaur record.  You can build just as strong a Signor-Lipps case for the
dinosaur extinction event being located anywhere within the barren
interval.  Negative evidence is inherently ambiguous.  Perhaps the best way
to approach this problem is to turn the Signor-Lipps Effect on it's head
and ask how far above the last dinosaur fossil occurrence you have to go
before you are sure (in a statistical sense) that dinos are indeed missing.
If that exercise is carried out and given the "thousands of fossils" that
have been collected from that (and nearby) localities, I suspect that
you'll be 95% sure you've seen the last of the dinos before you get very
far into that barren interval.  Moreover, if we have hiatuses in that
section (as we most certainly do) that fact complicates the situation and
those complications must be taken into account.

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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