by Ross L., Creation vs. Evolution II Team Member
When
animals that live in fresh or salt water die, their remains
settle to the bottom. After a period of time, these remains
can become covered by mud and sediment. After many thousands
or even millions of years, these remains become fossilized.
The space that they occupied in the sedimentary layers that
form through history become filled with dissolved minerals
that harden into fossils.
Since different types of fossils are typically found in different layers of sedimentary rock, their presence at specific depths indicates the period in which the animals lived. The fact that certain species are only found in certain layers is taken as evidence for evolution. It is also used to help estimate the age of the earth. Land animals are not as reliably found in their entirety in sedimentary rock. However, the fossil record is the best window we have into what creatures once lived on the earth. For example, without fossil records, we would not have known about the existence of dinosaurs.
Check out this link for more information on the fossil record, especially transitional fossils supporting evolution: http://www.talkorigins.org/faqs/faq-transitional.html
