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Chapter 3:
Fossils and the geological record
The most important ways in which fossils can be helpful to earth science is through their ability to indicate
both the passage of time and changes in environments throughout the world.
Divisions of the geological record
All of geological time is divided into two eons: the precambrian Eon, followed by the phanerozoic Eon. The Precambrian Eon is the time from the formation of the earth until evolution had progressed sufficiently to produce animals with hard parts that could commonly be preserved as fossils. This time makes up 88% of the entire history of the earth. The Phanerozoic Eon begins about 600 million years ago and continues to the present day.
Because the Precambrian Era contains little evidence of life, it cannot readily be divided up using fossils. However, the Phanerozoic is divided into three eras: the palaeozoic (which means the time of early life forms), the mesozoic (time of middle life forms), and cenozoic (time of later life forms).
In turn, the eras are subdivided into periods,
which are of most practical use. Thus, we speak of
the Palaeozoic Era being divided into Cambrian, Ordovician, and other periods. Each of the periods is further divided into epochs. Epochs are either early, middle, or late, except for the Tertiary, where they are given individual names such as Eocene and Miocene. The rocks belonging to a period are called systems, and the rocks belonging to an epoch are called series. So, the Cambrian Period contains the Cambrian System rocks, and the Early Cambrian (Epoch) contains the Lower Cambrian Series rocks.
(Below) The geological time scale.
ERA PERIOD
Million years before present
1.6
65
135 190
225 280
345
395 430
500
570
Quaternary Tertiary
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
PRECAMBRIAN
50
4600
PALAEOZOIC MESOZOIC CENOZOIC