A Review of Lazarus Taxa and Fossil Abundance at Times of Biotic Crisis,
P.B. Wignall, and M.J. Benton; and Discussion on Lazarus Taxa and Fossil
Abundance at times of Biotic Crisis, R.J. Twitchett, et. al. By David
Syracuse
Wignall and Benton discuss the various theories that might help explain the
appearance of Lazarus taxa. These taxa are those that disappear (are absent from the
fossil record) at the point of an extinction event, but later return after a period of time
known as an outage. Their first explanation is simply that the abundance of the species in
any given area became so low after the extinction event that, due to the various factors
that can affect the formation of fossils, none were formed. When conditions improved
and populations grew, the probability of an organism being preserved increased, and thus
it reentered the fossil record.
The authors go on to suggest that this phenomenon might be a reflection of the
quality of the fossil record, or the movement of these Lazarus taxa to other areas (refugia)
that were safer but not conducive to fossil formation. Their data suggests, however, that
the Lazarus effect is not due to any drop in the quality of the fossil record. Nor, they
find, is it due to the migration of species away from disaster sites to safer ground. With
the exception of one species of sponge, the authors determine that the most likely cause is
a simple drop in abundance. I’m inclined to agree – for many species, the shock of
moving from one area to another in the span of just a few days might be just as bad as the
extinction event itself. It’s doubtful that a species could survive or reproduce enough to
recover a viable population in this situation.
Twitchett and others are skeptical of the methods of the Wignall and Benton
paper, though they accept the explanations. They find fault with the sampling techniques
used, as well as with some of the data presentation (e.g. figure 3c being mislabeled).
Overall their argument is rather weak and while it seems to discredit much of the work
done by the authors, it makes no attempt to put forth any other explanations or data.
I’ve noted that we need to address the genetic effects of a truncated population –
the bottleneck effect would certainly come into play with a vengeance, and would affect
species survivability tremendously. This might have further implications for the refugia
hypothesis.