Page 39 - Introduction to Paleobiology and The Fossil Record

26 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

ized by distinctive groups or assemblages of or a fossil assemblage, so long as fossils can
fossils. In a traverse from Wales to London, be reworked into younger strata by erosion
Smith encountered successively younger and redeposition, but this is relatively rare.
groups of rocks, and he documented the Nonetheless, to date, the use of fossils in bio-
change from the trilobite-dominated assem- stratigraphy is still the best and usually the
blages of the Lower Paleozoic of Wales most accurate routine means of correlating
through Upper Paleozoic sequences with and establishing the relative ages of strata. In
corals and thick Mesozoic successions with order to correlate strata, fossils are normally

ammonites; finally he reached the molluskan organized into assemblage or range zones.
faunas of the Tertiary strata of the London There are several types of range zone (Fig.
Basin (Fig. 2.1c). In France, a little later, the 2.2); some are used more often than others.
noted anatomist Georges Cuvier (see p. 12) The concept of the range zone is based on the
together with Alexandre Brongniart (1770– work of Albert Oppel (1831–1865). Oppel
1849), a leading mollusk expert of the time, characterized successive lithologic units by
ordered and correlated Tertiary strata in the unique associations of species; his zones were
Paris Basin using series of mainly terrestrial based on the consistent and exclusive occur-
vertebrate faunas, occurring in sequences sep- rence of mainly ammonite species through
arated by supposed biological catastrophes. Jurassic sections across Europe, where he rec-
These early studies set the scene for bio- ognized 33 zones in comparison with the 60
stratigraphic correlation. In very broad terms, or so known today. His zonal scheme could
the marine Paleozoic is dominated by bra- be meshed with Alcide d’Orbigny’s (1802–
chiopods, trilobites and graptolites, whereas 1857) stage classifi cation of the system, based
the Mesozoic assemblages have ammonites, on local sections with geographic terms,
belemnites, marine reptiles and dinosaurs as further developed by Friedrich Quenstedt
important components, and the Cenozoic is (1809–1889). Although William Smith had

dominated by mammals and molluskan recognized the significance of fossils almost
groups, such as the bivalves and the gastro- 50 years previously, Oppel established a
pods. This concept was later expanded by modern and rigorous methodology that now
John Phillips (1800–1874), who formally underpins much of modern biostratigraphy.

defined the three great eras, Paleozoic (“ancient The known range of a zone fossil (Box 2.1)
life”), Mesozoic (“middle life”) and Cenozoic is the time between its first and last appear-

(“recent life”), based on their contrasting ances in a specific rock section, or fi rst appear-
fossils, each apparently separated by an ance datum (FAD) and last appearance datum
extinction event. Many more precise biotic (LAD). Clearly, it is unlikely that the entire
changes can, however, be tracked at the species global vertical range of the zone fossil is rep-
and subspecies levels through morphological resented in any one section; nevertheless it is,
changes along phylogenetic lineages. Very in most cases, a workable approximation.
accurate correlation is now possible using a This range, measured against the lithostratig-
wide variety of fossil organisms (see below). raphy, is termed a biozone. It is the basic
biostratigraphic unit, analogous to the
lithostratigraphic formation. It too can be
Biostratigraphy: the means of correlation

defined with reference to precise occurrences

Biostratigraphy is the establishment of fossil- in the rock, and is defined again on the basis
based successions and their use in stratigraphic of a stratotype or basal stratotype section in
correlation. Measurements of the stratigraphic a type area. Once biozones have been estab-
ranges of fossils, or assemblages of fossils, lished, quantitative techniques may be used to
form the basis for the defi nition of biozones, understand the relationships between rock
the main operational units of a biostratigra- thickness and time, and to make links from
phy. But the use of such zone fossils is not locality to locality (Box 2.2).
without problems. Critics have argued that This is all very well, of course, but the fossil
there can be difficulties with the identifi ca- record is rarely complete; only a small per-

tions of some organisms fl agged as zone centage of potential fossils are ever preserved.
fossils; and, moreover, it may be impossible Stratigraphic ranges can also be infl uenced by
to determine the entire global range of a fossil the Signor–Lipps effect (Signor & Lipps 1982),