Fossil Notes
Fossil Notes
Fossil: any trace or remain of a dead organism
 The formation of any fossil is a rare event.
 Most fossils form when organisms that die become buried in sediments
(particles of soil and rock). Layers of sediment cover dead organisms and after
a very long period of time, they harden into sedimentary rock.
 Paleontologist: a person who studies fossils
Fossil: any trace or remain of a dead organism
 The formation of any fossil is a rare event.
 Most fossils form when organisms that die become buried in sediments
(particles of soil and rock). Layers of sediment cover dead organisms and after
a very long period of time, they harden into sedimentary rock.
 Paleontologist: a person who studies fossils
Types of Fossils:
1. Sedimentary (petrified) – forms from the hard body parts of the organism
where minerals replace tissues and then eventually change into rock
2. Mold – a hollow space in sediment in the shape of an organism or part of it
3. Cast – mold filled with hard mineral; a copy of the organism
4. Preserved remains
a. Amber - petrified tree sap; usually small; whole specimen is preserved
b. Frozen in ice
c. Tar
Types of Fossils:
5. Sedimentary (petrified) – forms from the hard body parts of the organism
where minerals replace tissues and then eventually change into rock
6. Mold – a hollow space in sediment in the shape of an organism or part of it
7. Cast – mold filled with hard mineral; a copy of the organism
8. Preserved remains
a. Amber - petrified tree sap; usually small; whole specimen is preserved
b. Frozen in ice
c. Tar
What do fossils tell us?
 They give us clues about organisms that lived long ago (extinct)
 They provide evidence of how Earth’s surface has changed over time.
 They help scientists understand what past environments may have been like.
What do fossils tell us?
 They give us clues about organisms that lived long ago (extinct)
 They provide evidence of how Earth’s surface has changed over time.
 They help scientists understand what past environments may have been like.
Fossil record: fossil evidence of life on Earth; used to study and attempt to explain how
new species formed on Earth (speciation)
Fossil record: fossil evidence of life on Earth; used to study and attempt to explain how
new species formed on Earth (speciation)
Geologic Time Scale: calendar of life’s history based on radioactive dating; divided into
time spans of eras and periods. (pg. 195) Organism complexity has increased.
 Precambrian Era – first living things like bacteria, algae, and fungi
 Paleozoic Era – invertebrate sea animals, land plants, fish, early amphibians,
tropical forests, seed plants, reptiles, mammals
 Mesozoic Era – dinosaurs, mammals, flowering plants
 Cenozoic Era – first monkeys, apes, grasses, humans
Geologic Time Scale: calendar of life’s history based on radioactive dating; divided into
time spans of eras and periods. (pg. 195) Organism complexity has increased.
 Precambrian Era – first living things like bacteria, algae, and fungi
 Paleozoic Era – invertebrate sea animals, land plants, fish, early amphibians,
tropical forests, seed plants, reptiles, mammals
 Mesozoic Era – dinosaurs, mammals, flowering plants
 Cenozoic Era – first monkeys, apes, grasses, humans
How do scientists determine a fossils age?
 Relative dating – sedimentary rocks form layers. The layers near the top are
younger, while those farther down are older. This can only be used when rock
layers have been preserved in their original sequence. This only tells a scientist
which fossil is older than another.
 Radioactive dating – rocks that fossils are found near contain radioactive
elements, which are unstable elements that decay, or breakdown, into
different elements. The half-life of a radioactive element is the time it takes
for half of the atoms in a sample to decay. Scientists then compare the
amount of radioactive element to the amount that has broken down into the
different element. This allows them to estimate the age of the rock, and thus
the age of the fossil.
How do scientists determine a fossils age?
 Relative dating – sedimentary rocks form layers. The layers near the top are
younger, while those farther down are older. This can only be used when rock
layers have been preserved in their original sequence. This only tells a scientist
which fossil is older than another.
 Radioactive dating – rocks that fossils are found near contain radioactive
elements, which are unstable elements that decay, or breakdown, into
different elements. The half-life of a radioactive element is the time it takes
for half of the atoms in a sample to decay. Scientists then compare the
amount of radioactive element to the amount that has broken down into the
different element. This allows them to estimate the age of the rock, and thus
the age of the fossil.