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.