Geoscience Reference
In-Depth Information
Millions of
years ago
Era
Period
Millions of
years ago
Epoch
Quaternary
2.0
Holocene
Pleistocene
13
0.01
2.0
Tertiary
12
Pliocene
65.0
11
9
Millions of
years ago
5.3
Eon
Era
Miocene
Cenozoic
Mesozoic
24.6
Cretaceous
65.0
248
Oligocene
38.0
144
Paleozoic
8
Eocene
545
Jurassic
54.9
No subdivisions
in common use
Paleocene
7
206
65.0
10
Triassic
13
Modern human civilization
develops
6
250
Permian
Global glaciations begin,
further hominid evolution
12
290
Pennsyl-
vanian
Mississ-
ippian
First primitive hominids,
grasses, Himalayas arise
11
2500
320
First large mammals,
Australia separates
10
2
5
Mass extinctions, including
dinosaurs
354
9
Devonian
Flowering plants, Alps and
Rockies arise
8
417
4
3900
First birds and small mammals
7
Silurian
1
438
Dinosaurs begin to dominate
6
Ordovician
First insects, coal deposits form
4550
5
4
First amphibians, seed
plants, trees
505
First fish, shellfish, mosses
3
Cambrian
2
Early atmosphere, first
photosynthesis
540
3
Precambrian
1
Oldest known Earth rocks
Figure 12.19 The geologic timescale.
This timescale is subdivided on the basis of major geological and biological events in
Earth history, such as the appearance of flowers or major extinctions.
corpses and organic waste of the larger animals that were
decimated.
The Cenozoic Era is significant because it represents all
of post-dinosaur time, beginning 65 million years ago and
continuing until the present. During this period most current
landscape features such as mountain chains and river valleys
developed, as well as life as we know it. The Cenozoic Era
is subdivided into two periods, the
Tertiary and Quaternary
.
Given the relative youth of these two periods, plus the fact that
most current landforms developed within this time interval,
scientists know a great deal more about them than about ear-
lier periods. Thus, these periods are further subdivided into
epochs
. The
Paleocene
Epoch, for example, is marked by the
emergence of the first large mammals following the extinction
of the dinosaurs 65 million years ago. Subsequently, the first
primitive hominids developed at the beginning of the
Pliocene
Epoch 5.3 million years ago. The subsequent
Pleistocene
and
Holocene
Epochs are important because they are the intervals
of time during which most human evolution has occurred.
The Pleistocene Epoch is considered noteworthy because it
