Evolution__3rd_Edition

526 PART 5 / Macroevolution
Era Period Epoch
Cenozoic
Mesozoic
Paleozoic
Quaternary
Neogene
Tertiary
Paleogene
Cretaceous
Jurassic
Triassic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
Recent
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
Pennsylvanian
Mississippian
Myr BP
(approx)
0.01
1.8
5.3
24
34
55
65
144
206
251
290
323
354
417
443
490
543
Figure 18.1
The geological timescale.
Permian, for instance, characteristically differ from those of the Triassic and at the
Permo-Triassic boundary there is a relatively sudden transition between the fossil
faunas. In the nineteenth century it was not known whether the transition times
corresponded to mass extinctions and sudden replacements, or to long gaps in the
fossil record while a slower replacement was proceeding. It is now known they
were mass extinctions in short periods. To demonstrate this, techniques to establish
absolute times were necessary. For the smaller divisions of geological time, such as
epochs, there is no exact agreement on the absolute dates and more than one geological
timescale exists.
18.2.2 Geological time is measured in both absolute and relative terms
Geologists date events in the past both by relative and absolute techniques. An absolute
time is a date expressed in years (or millions of years); whereas a relative time is a time
relative to some other known event. The times on Figure 18.1 are absolute times and
were established from the radioactive decay of elements. The exact method is explained
in Box 18.1.
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