Page 41 - Introduction to Paleobiology and The Fossil Record

28 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 2.1 Zone fossils

The recognition and use of zone fossils is fundamental to biostratigraphic correlation. Fossil groups
that were (i) rapidly evolving, (ii) widespread across different facies and biogeographic provinces,
(iii) relatively common, and (iv) easy to identify make the ideal zone fossils. In the Early Paleozoic
macrofauna, graptolites (see p. 412) are the closest to being ideal zone fossils, whereas during the

Mesozoic, the ammonites (see p. 334) are most useful. The use of efficient zone fossils ensures that
relatively short intervals of geological time can be correlated, often with a precision of a few hundred
thousand years, over long distances through different facies belts around the world. In practice there
are no ideal zone fossils. Most long-range correlations involve use of intermediate faunas with mixed
facies.
For example, in Ordovician and Silurian rocks, deep-water facies are correlated by means of the
rapidly-evolving and widespread graptolites; these fossils are rare in shallow-water shelf deposits
where trilobites and brachiopods are much more common. Nevertheless, facies with both graptolite
and shelly faunas may interdigitate in deep-shelf and slope sequences, allowing correlation through
these mixed facies from deep to shallow water. Parallels can be drawn with the neritic ammonites
and benthic bivalves and gastropods of the Mesozoic seas. Microfossils are widely used for correla-
tion in hydrocarbon exploration; the amount of rock available in drill cores or cuttings is usually
limited and a range of fossil microorganisms including foraminiferans and radiolarians together with

dinoflagellates, spores and pollen form the basis for the correlation schemes used by petroleum
companies.
On a simple plot of space against time (Fig. 2.3), an ideal zone fossil, such as an ammonite or

graptolite, will represent a thin horizontal band reflecting a brief time duration but a widespread
spatial distribution. In reality very few fossils approach the properties of an ideal zone fossil. The
distribution of most is controlled to some degree by facies, the rocks that represent a particular life
environment. A more typical facies fossil, such as a typical bivalve or gastropod, is not tightly con-
strained by time but appears to occur in a particular facies belt (Fig. 2.3).

Time excellent facies

fossils, tracking a
particular environment
with time

excellent zone
fossils, widespread
for short time interval

Space (environment and geography)

Figure 2.3 Behavior of ideal zone and facies fossils through a hypothetical global stratigraphic
section.