Hard to read parts of page 153 of Changes Over Time

Gaps in the Fossil Record

If every organism that died left an imprint behind, the fossil record
would resemble a very large evolutionary family tree. Figure 6
shows a hypothetical fossil record in which all relationships
between organisms are clearly mapped.

Although scientists have collected thousands of fossils,
gaps remain in the current fossil record, as shown in
Figure7. This is because specific conditions are necessary
for fossils to form. The organism must be buried in very
fine sediment. Also, oxygen-which promotes decay cannot
be present. However, very few places are free of
oxygen.  Because the conditions needed for fossils to form
are rare, fossils are often difficult to find. Nevertheless,
scientists have identified some fossils that complete sections
of the fossil record.

Vestigial Structures

Whales are similar in shape to fish. Yet whales are
mammals-animals that breathe air, give birth to live
young, and produce milk. Although modern whales
do not have hind limbs, there are remnants of hind-
limb bones inside their bodies, as shown in
Figure 8. These remnants of once-useful structures are
known as
vestigial (ves TI] ee uhl) structures. Scientists
think that over millions of years, whales evolved from
land dwellers into sea-dwelling organisms. But
scientists have not had the fossil evidence to support
their ideas-until now. Read the following case study to
learn the story of whale evolution.