4.03 The Fossil Record

Fossils are clues to the past.

• Fossils are remains or traces of past life.

• They tell us about the past area's

environment, climate, and ecosystem.

• Any evidence of past life is considered a

fossil.

Fossil Formation

• An organism's chances of becoming preserved increase if two

conditions are met:

o The organism was rapidly buried.

o The organism possessed hard parts.

• When an organism dies, the soft parts are decomposed by bacteria

or eaten by predators. But if those parts are buried quickly by

sediment, they might be protected from that destruction and

become fossils.

• Hard parts—teeth, bones, and shells—are more likely to be

preserved.

Types of Fossils

• An organism may become petrified- turns to stone. The

original organism's small spaces and pores are filled with

minerals.

• Fossils are formed by replacement. The cell walls and solid

materials in an organism are replaced with mineral matter.

• Organisms can form a cast or mold. Molds show the

outside of the fossil, not the internal structures. If the mold

is filled with sediment later, this creates a cast.

• Trace fossils are indirect evidence of past life, like tracks or

burrows.

Fossils as evidence
of past life

• Thin-shelled clams are found in deeper,

calmer waters. So, a thick-shelled fossil

would indicate that in the geologic past an

area was once under shallow water along a

shoreline.

• Fossilized evidence of pollen would tell

you about the plants that lived in that area

in the past, which would indicate whether

the area was warm, swampy, grassy, or

wooded.

Fossils and Continental Drift

• Continents move around the surface of earth

slowly and have done so since they formed.

• Mesosaurus fossils are found in both Africa

and South America, which is an example of the

fossil record providing evidence of

major tectonic events like continental drift.

Fossils and Major Climactic
Change

• Global climate change—a significant modern issue

due to human activity on the earth—also occurred in

the geologic past.

• When climate changes, some plants and animals

disappear from a location and new ones appear.

• Microscopic ocean organisms called foraminiferaprov

ide some of the best records of climate change. These

tiny organisms are very sensitive to temperature

changes, and they leave behind millions and millions

of fossil shells.

Fossils and Mass Extinction

• Periods of mass extinction have occurred during

earth's geologic history.

• At the end of the Mesozoic era, 65 million years ago,

a mass extinction of nearly half the species on earth

took place.

• Very few fossils are found from that time, which

suggests that a major event caused the mass

extinction.

Fossils provide evidence of past life,
past climates, and past ecosystems.