The Fossil Record

Ch. 6, section 3

HOW DO FOSSILS FORM

• MOST FOSSILS FORM WHEN ORGANISMS THAT DIE BECOME BURIED IN SEDIMENTS.

• USUALLY BONES OR SHELLS OF ANIMALS FORM FOSSILS

Petrified Fossils

• Fossils that are buried in sediment and turned to rock.

Molds and Casts

• Shells or other hard parts buried gradually dissolve and leave a blank space called a mold. (footprint)

• When minerals fill the space that was left a casts is formed.

Preserved remains

• Animals that have been found in ice and have not changed since the day they died.

Relative Dating

• Scientists look at the layers of sediment and rock to determine how old a fossil is.

• If a fossil is found closer to the surface it is usually younger.

• If it is found in an older layer (deeper) it is usually older.

Radioactive Dating

• Scientists can compare the amount of a radioactive element in a sample to the amount of the element into which it breaks down.

• Which can be used to determine the actual age of the fossil.

Precambrian Time

• 4.6 billion - 544 million years ago.

• Formation of Earth

• Bacteria forms (1st living thing)

• Animals first appeared 600 million years ago.

Paleozoic Era

• 544 million - 245 million years ago

• Sponges, jawless fish, jawed fish, sharks, and amphibians lived during this time.

Mesozoic Era

• 245 - 66 million years ago.

• Dinosaurs, mammals, birds appeared.

• Mass extinction happened during this time.

Cenozoic Era

• 66 millions years ago - present

• Large mammals (mammoths, saber-tooth tigers, etc)

• Humans appeared

Unanswered Qestions

• How did animals become extinct?

• Climate change- asteroid or volcanoes. We are exactly sure.