The continental fossil record of vertebrates is notoriously patchy and there is a risk that studies
such as this reflect little more than poor sampling (Smith 2007). It could be argued, for example, that the
range chart and the plots of
extinction and origination metrics merely document fluctuations
in the quality of preservation of the fossils, variations in the environments represented, or in
the number of localities and specimens recorded for each time division.
Such problems with sampling could reflect human efforts - perhaps geologists and
palaeontologists have worked with different degrees of vigour on different rock units, and so
their collecting efforts might then bias the apparent patterns of diversity through time. On the
other hand, sampling might more probably reflect the nature of the rock record. Terrestrial
sediments in such red-bed successions reflect sporadic deposition in rivers, lakes and dune fields.
A great deal of deposited rock could well be eroded by subsequent sediment movements under water
or air. So, how can palaeontologists attempt to rule out such sampling problems?
In our paper (Benton et al. 2004), we presented three tests for sampling that sought to determine
whether we were looking at a geological or a biological signal.
- First, we plotted the numbers of genera and families against numbers of localities and
specimens. If sampling intensity drove apparent diversity, then stratigraphic intervals that are
well sampled (lots of specimens, lots of localities) might very well show higher diversity than
more poorly sampled stratigraphic intervals (few specimens, few localities). Our plot (see
Figure right, a and b) shows no correlation: if anything, time bins with large numbers of
localities and specimens are associated with low-diversity faunas and vice versa. Further, when
the distributions of generic and familial diversity through time are compared with the distributions
of numbers of sites and numbers of specimens per time bin (see Figure right, c), there is no
apparent tracking. Peaks and troughs in the diversity data do not match peaks and troughs in
richness of the fossil record. And, crucially, the time of diversity decline across the PTB
corresponds to a rising trend in numbers of sites and specimens.
- Secondly, we looked at sample sizes. Five of the 13 stratigraphic units are represented by
small (number of specimens <50) sample sizes, the Osinovskaya, Belebey, Bolshekinelskaya,
Gostevskaya and Bukobay svitas, of which only the Gostevskaya
falls near the PTB. Because the sample sizes are much smaller than those of the remaining eight
time zones, we re-examined the data with those samples either omitted, or combined with
neighbouring time bins. Both adjustments have the effect of increasing mean sample size; both had
no effect on the patterns of diversity, extinction or origination.
- Thirdly, we applied a statistical technique called rarefaction analysis, which is
designed to adjust sample sizes to the lowest common level. The question is asked: what result
would we find if we drew a subsample of a particular size from the overall samples? The idea is
to pick a subsample size that matches the smallest actual sample, and to use the rarefaction
analysis to determine how much of the pattern might be generated by variation in sample sizes
through the 13 stratigraphic units. Our rarefaction analysis showed that
the better-sampled time units - the Kopanskaya, Kzylsaiskaya
and Staritskaya svitas - may overestimate diversity by one, or at most, two families, in
comparison to the other time bins. Normalizing all time bin sizes to the range of 49Ð63 specimens,
cuts diversity of the first three Triassic gorizonts by one or two families and, hence, makes the
PTB extinction seem larger (91% instead of 82% extinction rate) and depresses earliest Triassic
diversity even more than has been indicated from the raw figures.
Our conclusion is then that the patterns we see in the Russian sections are more biological than
geological. Sampling effects are not ruled out completely, of course, but the pattern of data
cannot be passed off simply as a geologically driven signal. It seems reasonable for the present
to read the patterns as evolutionary and then to compare them with other areas.
References
- Benton, M.J., Tverdokhlebov, V.P. and Surkov, M.V. 2004. Ecosystem remodelling among vertebrates at the Permian-Triassic boundary in Russia. Nature 432, 97-100. pdf. Download the original of Figure 1 as a gif or pdf. Download the Excel data file here.
- Smith, A.B. 2007. Marine diversity through the Phanerozoic: problems and prospects. Journal of the Geological Society, 164, 731-745.
|
