The Origin and Evolution of Mammals - Moodle

CHAPTER 2
Time and classification
The age and the classification of a particular fossil
are the two fundamental properties necessary to
begun understanding how it fits into the evolutionary
patterns revealed by the fossil record. There are
often misunderstandings of one or other of these by
specialists. Evolutionary biologists on occasion
express far too optimistic a view of how accurately
fossils can actually be dated, both absolutely and
relative to one another. Geologists have been known
to have a rather limited view of how modern systematic
methods are used to infer relationships from
large amounts of information, be it morphological
or molecular. In this chapter, a brief outline of the
principles underlying the construction of the geological
timescale, and of a classification are given,
along with reference timescales and classifications
for use throughout the following chapters.
The geological timescale
The creation of a timescale for dating the events
recorded in the rocks since the origin of the Earth is
one of the greatest achievements of science, unspectacular
and taken for granted as it may often be. It is
also unfinished business insofar as there are varying
degrees of uncertainty and inaccuracy about the
dates of many rock exposures, none more so than
among the mostly continental, rather than marine
sediments containing the fossils with which this
work is concerned. A geological timescale is actually
a compilation of the results of two kinds of study.
One is recognising the temporal sequence of the
rocks, and agreeing arbitrarily defined boundaries
between the named rock units, the result of which is
a chronostratic timescale. The other is calibration of
the sequence and its divisions in absolute time units
of years before present, a chronometric timescale.
Chronostratic timescale
It is simple in principle to list the relative temporal
order of events, such as the occurrence of fossils, in
a single rock unit, although even here the possibility
of missing segments, known as hiatuses, in local
parts of the unit, or of complex folding movements
of the strata disturbing the order must not be forgotten.
The biggest problem is correlating relative dates
between different units in different parts of the
world. The potential markers available for correlation
are the numerous kinds of signals in the rocks of
particular events that had a widespread, ideally
global effect over a geologically brief period of time.
Historically, the occurrence of particular fossil
species was the most important, followed by evidence
of climatic change such as tillites indicating
glaciation, and evaporates associated with aridity.
Changes in sea levels are indicated by shifting coastline
sediments, and periods of intense volcanic
activity by igneous rock extrusions. More modern
techniques reveal characteristic sequences of reversals
of the magnetic field. Ratios of stable isotopes
have become particularly important geochemical
signals of several kinds of physical and biotic events.
If the ideal marker is a clear geochemical signal of a
brief, but globally manifested event that affected all
environments, then the best are the effects of a massive
bolide impact, such as the enhanced iridium
levels marking the Cretaceous–Cenozoic boundary.
Unfortunately, such ideal examples are extremely
infrequent.
After global correlation has been completed, the
boundaries between the named chronostratigraphic
units have to be arbitrarily defined by selecting a single
point in a particular exposure, known as a GSSP
(Global Stratotype Section and Point) or a ‘Golden
Spike’. To gain maximum international agreement,
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