Chapter 18 Fossils and Geologic Time

Figure 18-5 The wide geographic
range of trilobites and
the relatively short periods of
time that each species existed
make them excellent
index fossils. Most adult specimens
range in size from 25
to 100 mm (1 to 4 inches).
HOW HAS GEOLOGIC TIME BEEN DIVIDED? 425
Devonian Trilobites
409–360 Million Years Ago
Ordovician Trilobites
500–439 Million Years Ago
only destruction of moon rocks is caused by the impact of meteorites.
The oldest rock samples returned from the moon by
Apollo astronauts have been dated at 4.6 billion years.
Meteorites are another source of primitive rocks because
they are the materials from which the solar system formed.
Meteorites also yield an age of 4.6 billion years. Earth’s formation
marks the beginning of geologic history and the start
of Precambrian time. The oldest rocks from Earth discovered
so far are just under 4 billion years old. However, some mineral
inclusions have been dated at 4.4 billions years.
Early in the development of the geologic time scale, fossil
collectors could not recognize fossils in rocks older than Cambrian
time. Some of them concluded that older rocks (Precambrian)
do not contain fossils. However, scientists now
know that the apparent lack of fossils in these oldest rocks is
deceiving. Fossils are there. Organisms alive at the time did
not have hard parts, like shells and skeletons, that would
form obvious fossils. Although the Precambrian time makes
up about 88 percent of Earth’s history, events of the Precambrian
are not as well known as more recent events. This is be-