26 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
ized by distinctive groups or assemblages of
fossils. In a traverse from Wales to London,
Smith encountered successively younger
groups of rocks, and he documented the
change from the trilobite-dominated assem-
blages of the Lower Paleozoic of Wales
through Upper Paleozoic sequences with
corals and thick Mesozoic successions with
ammonites; fi nally he reached the molluskan
faunas of the Tertiary strata of the London
Basin (Fig. 2.1c). In France, a little later, the
noted anatomist Georges Cuvier (see p. 12)
together with Alexandre Brongniart (1770–
1849), a leading mollusk expert of the time,
ordered and correlated Tertiary strata in the
Paris Basin using series of mainly terrestrial
vertebrate faunas, occurring in sequences sep-
arated by supposed biological catastrophes.
These early studies set the scene for bio-
stratigraphic correlation. In very broad terms,
the marine Paleozoic is dominated by bra-
chiopods, trilobites and graptolites, whereas
the Mesozoic assemblages have ammonites,
belemnites, marine reptiles and dinosaurs as
important components, and the Cenozoic is
dominated by mammals and molluskan
groups, such as the bivalves and the gastro-
pods. This concept was later expanded by
John Phillips (1800–1874), who formally
defi ned the three great eras, Paleozoic (“ancient
life”), Mesozoic (“middle life”) and Cenozoic
(“recent life”), based on their contrasting
fossils, each apparently separated by an
extinction event. Many more precise biotic
changes can, however, be tracked at the species
and subspecies levels through morphological
changes along phylogenetic lineages. Very
accurate correlation is now possible using a
wide variety of fossil organisms (see below).
Biostratigraphy: the means of correlation
Biostratigraphy is the establishment of fossil-
based successions and their use in stratigraphic
correlation. Measurements of the stratigraphic
ranges of fossils, or assemblages of fossils,
form the basis for the defi nition of biozones,
the main operational units of a biostratigra-
phy. But the use of such zone fossils is not
without problems. Critics have argued that
there can be diffi culties with the identifi ca-
tions of some organisms fl agged as zone
fossils; and, moreover, it may be impossible
to determine the entire global range of a fossil
or a fossil assemblage, so long as fossils can
be reworked into younger strata by erosion
and redeposition, but this is relatively rare.
Nonetheless, to date, the use of fossils in bio-
stratigraphy is still the best and usually the
most accurate routine means of correlating
and establishing the relative ages of strata. In
order to correlate strata, fossils are normally
organized into assemblage or range zones.
There are several types of range zone (Fig.
2.2); some are used more often than others.
The concept of the range zone is based on the
work of Albert Oppel (1831–1865). Oppel
characterized successive lithologic units by
unique associations of species; his zones were
based on the consistent and exclusive occur-
rence of mainly ammonite species through
Jurassic sections across Europe, where he rec-
ognized 33 zones in comparison with the 60
or so known today. His zonal scheme could
be meshed with Alcide d’Orbigny’s (1802–
1857) stage classifi cation of the system, based
on local sections with geographic terms,
further developed by Friedrich Quenstedt
(1809–1889). Although William Smith had
recognized the signifi cance of fossils almost
50 years previously, Oppel established a
modern and rigorous methodology that now
underpins much of modern biostratigraphy.
The known range of a zone fossil (Box 2.1)
is the time between its fi rst and last appear-
ances in a specifi c rock section, or fi rst appear-
ance datum (FAD) and last appearance datum
(LAD). Clearly, it is unlikely that the entire
global vertical range of the zone fossil is rep-
resented in any one section; nevertheless it is,
in most cases, a workable approximation.
This range, measured against the lithostratig-
raphy, is termed a biozone. It is the basic
biostratigraphic unit, analogous to the
lithostratigraphic formation. It too can be
defi ned with reference to precise occurrences
in the rock, and is defi ned again on the basis
of a stratotype or basal stratotype section in
a type area. Once biozones have been estab-
lished, quantitative techniques may be used to
understand the relationships between rock
thickness and time, and to make links from
locality to locality (Box 2.2).
This is all very well, of course, but the fossil
record is rarely complete; only a small per-
centage of potential fossils are ever preserved.
Stratigraphic ranges can also be infl uenced by
the Signor–Lipps effect (Signor & Lipps 1982),