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 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. 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.