Page 40 - Introduction to Paleobiology and The Fossil Record

FOSSILS IN TIME AND SPACE 27

Assemblage biozone Concurrent-range
biozone
2 4 7
3 11 14
strata of 15 strata of
biozone in biozone in
question question
1 5 10 12 B
6 A
89 13
C
Ranges of 15 taxa are shown
Biozone defined by complete Ranges of 3 taxa, A–C, are shown
assemblage Biozone defined by overlap of these
Acme biozone Partial-range biozone
B C

A

Biozone defined by exceptional
abundance of one taxon Biozone defined as within the range of
fossil group B, above the last appearance
of fossil group A and below the first
appearance of fossil group C
Total-range biozone Consecutive-range
(or local-range biozone
biozone)
C
strata of
biozone in B
question
A
Biozone defined by total or local
range of one taxon 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 in the Quaternary rocks of East Africa where
always shorter than the true range of a species, hominid remains occur. Microfossil groups

i.e. you never find the last fossil of a species. such as conodonts, dinofl agellates, foraminif-
So, incomplete sampling means that the dis- erans and plant spores are now widely used
appearances of taxa may be “smeared” back (see pp. 209–32, 493–7), particularly in petro-
in time from the actual point of disappear- leum exploration. Microfossils approach the
ance. The Signor–Lipps effect is particularly ideal zone fossils since they are usually
relevant to mass extinctions, when this common in small samples, such as drill cores
backsmearing can make relatively sudden and chippings, of many sedimentary litholo-
extinction events appear gradual. This can be gies and many groups are widespread and
corrected to some extent by the use of statisti- rapidly evolving. The only drawback is that
cal techniques to establish confi dence inter- some techniques used to extract them from
vals that are modeled on known sampling rocks and sediments are specialized, involving
quality (see p. 165). acid digestion and thin sections.
Many different animal and plant groups
are used in biostratigraphic correlation (Fig.
2.5). Graptolites and ammonites are the best Dividing up geological time: chronostratigraphy
known and most reliable zone macrofossils Geological time was divided up by the efforts
with their respective biozones as short as of British, French and German geologists
1 myr and 25 kyr, respectively. The most between 1790 and 1840 (Table 2.1). The divi-

unusual zone fossils are perhaps those of pigs, sions were made first for practical reasons –
which have been used to subdivide time zones one of the first systems to be named was the