Page 42 - Introduction to Paleobiology and The Fossil Record

FOSSILS IN TIME AND SPACE 29

Box 2.2 Quantitative biostratigraphy

Quantitative stratigraphy can be traced back to work by Charles Lyell (1797–1875), who plotted
what we would now call decay curves (analogous to the decay curves for radiogeneic isotopes) for
the molluskan and mammalian faunas of the Tertiary basins of northwest Europe. He wanted to
look at “evolution in reverse”, tracking back in time from the present day to see how proportions
of living taxa changed the farther back you went into the rock record. He found that the proportions
of modern to extinct forms declined the farther back in time he went, and he used this to defi ne
divisions in the Tertiary system. For example his Older Pliocene included only 10% of mammals
and 50% of mollusks living today, whereas in the Newer Pliocene the respective fi gures are 90%
and 80%. These ratios were used as a method of correlating Tertiary strata quantitatively. In recent
years, driven by hydrocarbon and mineral exploration, a range of quantitative, computer-based
techniques has become available (Hammer & Harper 2005). Three – graphic correlation, seriation
and ranking and scaling – are outlined here.
A rigorous, numerically-based mode of correlation was developed by Alan Shaw when he was
working in the petroleum industry during the 1950s. Because hydrocarbon reservoir and
source rocks occur within stratigraphic successions, it is essential that the rocks in oil and

gas fields are accurately correlated; geologists can then locate key horizons on the basis of
biostratigraphy (Fig. 2.4a). Graphic correlation by Shaw’s method requires fossil range data

from two or more measured sections. Data of the first and last occurrences of fossil species are
plotted against a measured stratigraphic section; this is repeated for a second section. Usually
only the more common taxa are plotted. A bivariate scattergram is then drawn with section 1 along

the x-axis and section 2 along the y-axis. The first and last occurrences are then plotted as x–y

coordinates – for example the x coordinate represents the first appearance of species a along section

1 and the y coordinate its first appearance in section 2. A regression line is fitted to all the fi rst

(FAD) and last (LAD) appearance coordinates; this line of stratigraphic correlation can be used for
interpolation, permitting the accurate correspondence of all levels in the two sections. A composite
standard section can be constructed and refined by correlating it against additional actual

sections.
Biostratigraphers also use techniques established by archeologists in the late 1800s. Seriation is
an ordering technique designed to analyze gradients. Usually the gradients are temporal but biogeo-
graphic and environmental data have been investigated by seriation. Biostratigraphers tend to enter
the ranges of organisms on range charts as sequential FADs. In simple terms seriation shuffl es the
original data matrix until the stratigraphically higher taxa are on the left hand side of the matrix
and the stratigraphically lower taxa are on the right; any stratigraphic gradients in the data are then
clearly visible (Fig. 2.4b) and can be interpreted.
Ranking and scaling (RASC) is a method of arranging a series of biostratigraphic events in order,
and of estimating the stratigraphic distance between such events. The technique requires only fi rst
and last appearances measured in meters in a stratigraphic section, perhaps an exposure or oil well.
Events are first ranked or ordered based on the majority of relative occurrences and then the distances

between such events are calculated (Fig. 2.4c).
A dataset of Early Ordovician trilobite ranges is available at: http://www.blackwellpublishing.
com/paleobiology. These data may be analyzed and manipulated using ranking and scaling, seriation
and unitary associations; confidence intervals may also be calculated (see also Hammer & Harper

2005).

Continued