Origins of Modern Organisms
Most phyla that exist today probably originated during the Cambrian
period, which lasted from about 540 million to about 505 million
years ago. The Cambrian period was a time of great evolutionary
expansion, as shown in Figure 9.Many unusual marine animals also
appeared at this time, animals for which there are no close living rel-
atives. A very rich collection of Cambrian fossils was uncovered in
1909 in a geological formation in Canada called the Burgess Shale.
The fossils in the Burgess Shale include those of strange animals that
are not like anything alive today.
The Ordovician period, which followed the Cambrian period,
lasted from about 505 million to 438 million years ago. During this
time, many different animals continued to abound in the seas.
Among them were trilobites, marine arthropods that became
extinct about 250 million years ago, shown in Figure 9.262 CHAPTER 12History of Life on Earth
Figure 9 Appearance
of a Cambrian sea.By
studying fossils from the
Cambrian period, such as
the trilobite fossil below,
artists re-create a scene
from the shallow seas of
the Cambrian period.
Age(in Millions of Years Ago)Origin of all major animal phyla
560 •••••• 510
PRECAMBRIAN ERA CAMBRIAN PERIOD