Formation of Fossils
The fossil record, and thus the record of the evolution of life, is not
complete. Many species have lived in environments where fossils do
not form. Most fossils form when organisms and traces of organ-
isms are rapidly buried in fine sediments deposited by water, wind,
or volcanic eruptions. The environments that are most likely to
cause fossil formation are wet lowlands, slow-moving streams,
lakes, shallow seas, and areas near volcanoes that spew out volcanic
ash. The chances that organisms living in upland forests, moun-
tains, grasslands, or deserts will die in just the right place to be
buried in sediments and fossilized are very low. Even if an organism
lives in an environment where fossils can form, the chances are slim
that its dead body will be buried in sediment before it decays. For
example, it may be eaten and scattered by scavengers. Furthermore,
284 CHAPTER 13The Theory of Evolution
Ambulocetus natans apparently walked on land
like modern sea lions and swam by flexing its
backbone and paddling with its hind limbs (as do
modern otters). They were about 3 m (10 ft) long.
They existed about 50 million years ago.
Mesonychids are one hypothesized link between
modern whales and certain hoofed mammals.
They were about 2 m (6 ft) long. They are thought
to have lived about 60 million years ago. Some
scientists favor an alternative hypothesis linking
whales to other ancestral hooved mammals.
These hooved mammals are also ancestral to
hippopotamuses or pigs.
Whales are thought to have evolved from an ancestral line of four-legged mammals, which are represented here by
their fossils and artistic reconstructions showing what scientists think they may have looked like.
Figure 8 Evidence of whale evolution
Reading Effectively
Read the heading “Forma-
tion of Fossils,” and ask one
or more Who, What, Where,
When, Why, or How ques-
tions. For example, How are
fossils formed? As you read,
answer your questions.
