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Approximate age Approximate age
Eon Era Period in millions of years Epoch in millions of years
and symbol
before present before present
Quaternary (Q) Recent (Holocene)
Cenozoic Pleistocene .01
Tertiary (T) 1.6
65 Pliocene
5.3
Cretaceous (K)
Miocene
144
Mesozoic
Jurassic (J) 23.7
208 Oligocene
Triassic
(T R)
Phanerozoic Permian (P) 286 Carboniferous 245 36.6
Pennsylvanian (P)
America)
Mississippian (M) 320 (outside of North Eocene
360 57.8
Devonian (D)
Paleozoic
408 Paleocene
Silurian (S)
438
65
Ordovician (O)
505
Cambrian (C)
551
Proterozoic

2,500

Archean PRECAMBRIAN (Not drawn to scale)

(p C)
4,000
Prearchean

Origin of Earth
4,500
FIGURE 21.17 The geologic time scale. Modified from “Decade of North American Geology,” 1983 Geologic Time Scale—Geological Society of America.

fish becoming prominent. Also, by the end of the Silurian, some
primitive plants were found on land. The Devonian period saw Cenozoic era
the further development of different kinds of fish, including
those that had jaws, and many kinds of land plants and animals. Mesozoic era
Coral reefs were also common in the Devonian. The Carbonif- Paleozoic era
erous period was a time of vast swamps of ferns, horsetails, and
other primitive nonseed plants that would form great coal de- Precambrian
posits. Fossils of the first reptiles and the first winged insects are
found in rocks from this age. The Paleozoic era closed with the
extinction at the end of the Permian period of about 90 percent
of plant and animal life of that time.
The Mesozoic era was a time when the development of life
FIGURE 21.18 Geologic history is divided into four main eras.
on land flourished. The dinosaurs first appeared in the Triassic
The Precambrian denotes the first 4 billion years, or about 85 percent
period, outnumbering all the other reptiles until the close of the of the total 4.5 billion years of geologic time. The Paleozoic lasted
Mesozoic. Fossils of the first mammals and modern forms of about 10 percent of geologic time, the Mesozoic about 4 percent, and
gymnosperms (cone-bearing plants) developed in the Triassic. The the Cenozoic only about 1.5 percent of all geologic time.

532 CHAPTER 21 Geologic Time 21-12