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explained by organic evolution and extinction, events that could
be used to mark the time boundaries of Earth’s geologic history.
The major blocks of time in Earth’s geologic history are
called eons, and there have been four eons since the origin of
Earth. The Phanerozoic (Greek for “visible life”) eon is the geo-
logic time of the abundant fossil record and living organisms.
The eon before this time of visible life is called the Proterozoic,
which is Greek for “beginning life.” The other two eons are the
Prearchean and the Archean (Figure 21.17).
The Prearchean, Archean, and Proterozoic eons together
are known as the Precambrian, which refers to the time before
the time of life. The Precambrian denotes the largest block of
time in Earth’s history—more than 85 percent of the total time.
The Phanerozoic eon—the time of visible life—is divided
A Trilobites into blocks of time called eras, and each era is identified by the
appearance and disappearance of particular fossils in the sed-
imentary rock record. There are three main eras, and the pie
chart in Figure 21.18 shows how long each era lasted compared
to the Precambrian. The eras are as follows: (1) The Cenozoic
era refers to the time of recent life. Recent life means that the
fossils for this time period are similar to the life found on Earth
today. (2) The Mesozoic era refers to the time of middle life.
Middle life means that some of the fossils for this time period
are similar to the life found on Earth today, but many are dif-
ferent from anything living today. (3) The Paleozoic era refers
to the time of ancient life. Ancient life means that the fossils for
this time period are very different from anything living on Earth
today. The eras are divided into blocks of time called periods
(Figure 21.19), and the periods are further sub divided into
smaller blocks of time called epochs.

GEOLOGIC PERIODS AND TYPICAL FOSSILS
B Ammonites
The Precambrian period of time contains the earliest fos-
FIGURE 21.16 Since both trilobites (A) and ammonites sils. The Precambrian fossils that have been found are chiefly
(B) were common in the oceans throughout the world, they have been those of deposits from bacteria, algae, a few fungi, unusual
used as index fossils. Trilobites are always found in older rocks than
soft- bodied animals, and the burrow holes of worms. It
are ammonites. Trilobites were common in the Paleozoic era, and
ammonites were common in the earlier parts of the Mesozoic era. appears that there were no animals with hard parts; thus, the
fossil record is incomplete since it is the hard parts of animals
or plants that form fossils, usually after rapid burial. Another
England occurred in the same order, top to bottom, wherever they problem in finding fossils of soft-bodied or extremely small
were located. He also found that he could correlate and identify life-forms in these extremely old rocks is that heat and
each layer by the kinds of fossils in the rocks of the layers. In 1815, pressure have altered many of the ancient rocks over time,
he published a geologic map of England, identifying the rock layers destroying any fossil evidence that may have been present.
in a sequence from oldest to youngest. Smith’s work was followed The Paleozoic era was a time when there was great change
by extensive geological studies of the rock layers in other countries. in the kinds of plants and animals present. In general, the
Soon it was realized that similar, distinctive index fossils appeared earliest abundant fossils are found in rocks from the Cambrian
in rocks of the same age when the principle of superposition was period at the beginning of the Paleozoic era (see Figure 21.17).
applied. For example, the layers at the bottom contained fossils of These rocks show an abundance of oceanic life that represents
trilobites (Figure 21.16A), but trilobites were not found in the up- all the major groups of marine animals found today. There is
per levels. (Trilobites are extinct marine arthropods that may be no fossil evidence of life of any kind living on the land during
closely related to living scorpions, spiders, and horseshoe crabs.) the Cambrian. The dominant life-forms of the Cambrian ocean
On the other hand, fossil shells of ammonites (Figure 21.16B) ap- were echinoderms, mollusks, trilobites, and brachiopods. The
peared in the middle levels but not the lower or upper levels of trilobites, now extinct, made up more than one-half of the kinds
the rocks. The topmost layer was found to contain the fossils of of living things during the Cambrian.
animals identified as still living today. The early appearance and During the Ordovician and Silurian periods, most living
later disappearance of fossils in progressively younger rocks are things were still marine organisms, with various kinds of jawless

21-11 CHAPTER 21 Geologic Time 531