Page 38 - Introduction to Paleobiology and The Fossil Record

FOSSILS IN TIME AND SPACE 25

tions (chemostratigraphy), discontinuities A stratigraphy, illustrated on a map and in
(allostratigraphy), seismic data (seismic stra- measured sections, is required to monitor bio-
tigraphy) and depositional trends (cyclo- and logical and geological changes through time

sequence stratigraphies). The first two have and thus underpins the whole basis of Earth
most application in paleontological studies, history. It is a simple but effective procedure.
although sequence and cyclostratigraphic Successions of rock are often divided by gaps
frameworks are now providing greater insights or unconformities. These surfaces separate an
into the climatic and environmental settings older part of the succession that may have
of fossil assemblages. Here, however, we been folded and uplifted before the younger
concentrate on lithostratigraphy (rock frame- part was deposited. Commonly there is a
work), biostratigraphy (ranges of fossils) and marked difference between the attitudes of
chronostratigraphy (time dimension). the older and younger parts of the succession;
but sometimes both parts appear conformable
and only after investigation of their fossil
ON THE GROUND: LITHOSTRATIGRAPHY
content, is it clear that the surface represents
All aspects of stratigraphy start from the rocks a large gap in time.
themselves. Their order and succession, or Early geologists thought the Earth was very
lithostratigraphy, are the building blocks for young, but the Scottish scientist James Hutton
any study of biological and geological change (1726–1797) noted the great cyclic process of
through time. Basic stratigraphic data are fi rst mountain uplift, followed by erosion, sedi-
assembled and mapped through the defi nition ment transport by rivers, deposition in the
of a lithostratigraphic scheme at a local and sea, and then uplift again, and argued that
regional level. Lithostratigraphic units are such processes had been going on all through
recognized on the basis of rock type. The Earth’s history. He wrote in his Theory of the
formation, a rock unit that can be mapped Earth (1795) that his understanding of geo-
and recognized across country, irrespective of logical time gave “no vestige of a beginning,
thickness, is the basic lithostratigraphic cate- – no prospect of an end”. An example of
gory. A formation may comprise one or several Hutton’s evidence is the spectacular uncon-
related lithologies, different from units above formity at Siccar Point, Berwickshire, south-
and below, and usually given a local geo- ern Scotland, where near-horizontal Old Red
graphic term. A member is a more local litho- Sandstone (Devonian) strata overlie steeply-
logic development, usually part of a formation, dipping Silurian greywackes. Beneath the
whereas a succession of contiguous forma- unconformity, Hutton recognized the “ruins
tions, with some common characteristics is of an earlier world”, establishing the immen-
often defi ned as a group; groups themselves sity of geological time. This paved the way for
may comprise a supergroup. All stratigraphic our present concept of the Earth as a dynamic

units must be defined at a reference or type and changing system, a forerunner to the
section in a specified area. Unfortunately, the current Gaia hypothesis, which describes the

entire thickness of many lithostratigraphic Earth as a living organism in equilibrium with
units is rarely exposed; instead of defi ning the its biosphere. Although the Earth is not
whole formation, the bases of units are defi ned actually a living organism, this concept now
routinely in basal stratotype sections at a type forms the basis for Earth system science.
locality and the entire succession is then pieced
together later. These sections, like yardsticks
or the holotypes of fossils (see p. 118), act as
the definitive section for the respective strati- USE OF FOSSILS: DISCOVERY OF

graphic units. These are defined within a rock BIOSTRATIGRAPHY
succession at a specific horizon, where there Our understanding of the role of fossils in

is a lithologic boundary between the two stratigraphy can be traced back to the work
units; the precise boundary is marked on a of William Smith in Britain and Georges
stratigraphic log. Since the base of the suc- Cuvier and Alexandre Brongniart in France.
ceeding unit defines the top of the underlying William Smith (1769–1839), in the course of

unit, only basal stratotypes need ever be his work as a canal engineer in England, real-
defi ned. ized that different rocks units were character-