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

[Una Smith <una@xxxxxxxxxxxxxxxxxxxxxxxxx>]

Norman MacLeod <N.MacLeod@nhm.ac.uk> wrote:

> 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 ...

Ecology, particularly spatial ecology of diverse communities such as
tropical forests, is loaded with similar problems of distinguishing
biological signals from patterns of bias that are intrinsic to the
statistical methods being used, and to the stochasticity of the very
processes we try to identify.  We are gradually learning how to work
around these biases, as we become more sophisticated about our tools
(and pay more attention to statisticians).

> 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.

Oops!

> 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.

Right.  But if you can develop a sound null hypothesis about how a gradual
process should appear in the record, and you apply approriate statistical
tests, and the gradual pattern you observe is significantly less gradual
than the pattern you expect, then maybe you have evidence for a non-gradual
process.  I have seen a few paleofloristic studies that attempt this,
qualitatively, but none that go all the way.  Give paleoecologists a few
years to do it right.

As for the acid rain/blank meter story, I don't know much about it first
hand, because plants appear not to have been affected, but I agree with
Peter Harries that so much effort has go into searching the uppermost
Cretaceous strata that, qualitatively, the Signor-Lipps effect does not
seem sufficient to explain the blank.


> [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.]

GSA?  I wondered where the molecular clock folks had gone.  These studies
depend on pulling a starting date (for some branch point in the lineage) 
from the fossil record, so to some extent they aren't independent.  And
there's a lot of debate in the molecular biology community about whether
these studies are sound, if they assume that the clock runs at the same
speed in all lineages, etc., which other evidence (variation in abundance,
diversity, and morphological change) suggests is not the case.  What does
the fossil record of birds look like?  How many orders are recognized in
the Cretaceous?  (Gee, I should know this, being at Yale...)


> ... recoveries from mass extinction events are interesting but they
> don't address questions dealing with the nature of the mass extinction
> event itself. ... Any explanation for a mass extinction event that
> cannot account for the composition of the survivor fauna, in effect,
> explains nothing.

Northern temperate flora in the early Tertiary were not diverse;  they 
were dominated by two families of angiosperms (flowering plants) that
are today specialists in both wind-pollination and wind-dispersal of
seeds, and by ferns.  The angiosperm taxa were most probably derived
from groups that were insect-pollinated and employed animals in the
dispersal of their seeds (see Friis, Chaloner and Crane, "The origins
of angiosperms and their biological consequences", 1987, Cambridge U.).
This implies various things about the animal communities and/or climate
changed from the Cretaceous to the early Tertiary, but I don't see how
it addresses the issue of the extinction event per se.

In sci.geo.geology (Usenet) right now, there's a raging debate about
the K/T boundary, the demise of presumably open-ground egg-laying
dinosaurs and presumably egg-eating small mammals.  Some lay readers
are in favor of putting the dinosaur extinction at the feet of hungry
mammals, while various geologists defend the idea that a catastrophic
event could have seriously interfered with dinosaur reproduction via
lower temperatures or some other effect.  How this might have worked
is not clear to me, though I think the idea is worth serious thought.


> 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 suppose he meant the K/T impact event evidence, or simply evidence that
the event was an abrupt "pulse", does not come from paleontology.  The
paleobotanists seem ready to accept a catastrophic pulse event, but there
are also clues that things were changing fairly dramatically for several
million years prior to the K/T boundary (papers by Kirk Johnson et al.).


	Una Smith		una.smith@yale.edu

Department of Biology, Yale University, New Haven, CT  06520-8104  USA