Biozone defined by the range of one taxon, B of lineage A ^ B ^ C

Figure 2.2 The main types of biozone, the operational units of a biostratigraphy. (Based on Holland 1986.)

the observation that stratigraphic ranges are always shorter than the true range of a species, i.e. you never find the last fossil of a species. So, incomplete sampling means that the disappearances of taxa may be "smeared" back in time from the actual point of disappearance. The Signor-Lipps effect is particularly relevant to mass extinctions, when this backsmearing can make relatively sudden extinction events appear gradual. This can be corrected to some extent by the use of statistical techniques to establish confidence intervals that are modeled on known sampling quality (see p. 165).

Many different animal and plant groups are used in biostratigraphic correlation (Fig. 2.5). Graptolites and ammonites are the best known and most reliable zone macrofossils with their respective biozones as short as 1 myr and 25 kyr, respectively. The most unusual zone fossils are perhaps those of pigs, which have been used to subdivide time zones in the Quaternary rocks of East Africa where hominid remains occur. Microfossil groups such as conodonts, dinoflagellates, foraminif-erans and plant spores are now widely used (see pp. 209-32, 493-7), particularly in petroleum exploration. Microfossils approach the ideal zone fossils since they are usually common in small samples, such as drill cores and chippings, of many sedimentary litholo-gies and many groups are widespread and rapidly evolving. The only drawback is that some techniques used to extract them from rocks and sediments are specialized, involving acid digestion and thin sections.

Dividing up geological time: chronostratigraphy

Geological time was divided up by the efforts of British, French and German geologists between 1790 and 1840 (Table 2.1). The divisions were made first for practical reasons -one of the first systems to be named was the

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