GRADE 4 REVIEW SHEET CHAPTER 4 LESSONS 5-6
A fossil is the remains or mark of an animal or plant that lived long ago. Scientists study fossils to
learn about species that are extinct (no longer exist)
Fossils form in different ways. Most fossils form when a plant or animal dies and becomes buried
under layers of sediment (bits of rock, sand, shells and other materials). Over time the sediment
hardens into rock and preserves the shape of the buried organism. [Know the diagram on p 170-171.]
Usually the hard parts of animals (bones, shells, teeth) become fossils. The soft parts decay or are
eaten by other animals.
Casts and molds are body fossils. A mold fossil is a space in rock in the shape of the living thing.
When minerals from the surrounding rock fill in the mold, it is called a cast fossil.
Other types of body fossils: Whole bodies of animals preserved in a fossil.
Ex. Insects can become trapped in sticky tree sap called amber which hardens over time and preserves
all of the remains, both hard and soft parts.
Parts of an animal’s body such as bones and teeth can be preserved in oily pools called tar pits when
the animal falls in. The fossils are actual bone and not hardened minerals.
Trace fossils give clues about a living thing’s activities. Footprints (tracks), preserved nests or burrows
and eggshells are all trace fossils.
A scientist who studies fossils to learn about plants and animals of the past is a paleontologist.
Fossils such as skulls, bones, teeth, and footprints can show what extinct organisms looked like and
how they lived (what they ate, how they moved). Paleontologists compare extinct plants and animals
to those that exist today. Fossils can show how plants, animals, and environments have changed over
time.
Scientists determine the age of fossils in two ways:
1. By looking at what layer it was found in-fossils found in deeper or lower layers of rock are older
than those in layers above.
2. Since they know the rate at which certain materials change, they can measure certain materials in a
fossil to determine how long ago it died.
Scientists have used information about the ages of fossils to make a timeline of the history of the
Earth called the geologic time scale. (See p. 178) [YOU DON’T HAVE TO MEMORIZE THE TIME LINE.
YOU SHOULD BE ABLE TO INTERPRET IT AND ANSWER QUESTIONS IF YOU ARE GIVEN THE TIME
SCALE. ] The earliest time span is at the bottom while the most recent is at the top. The time scale
show when different animals existed. The Mesozoic Era is called the Age of the Dinosaurs. This is
when dinosaurs existed and at the end of this era became extinct.
Fossil fuels such as coal and oil come from the remains of plants and animals. It took millions of
years for the remains to become coal or oil. Fossil fuels are used to produce electricity.