Unit 8 A: The Geologic Time Scale and Fossils Add the new notes to your table of contents, and then turn to the next clean page and set up your title and date

Fossil Formation After an animal dies, any soft tissues animals do not eat break down. Only the dead animal’s hard parts, such as bones, shells, and teeth, remain. Under rare conditions, these parts become fossils.

Add Fossil Record to your index The Fossil Record The fossil record is made up of all the fossils ever discovered on Earth. The fossil record provides evidence that species have changed over time. Based on fossil evidence, scientists can recreate the physical appearance of species that are no longer alive on Earth. Add Fossil Record to your index

The fossil record is evidence that horses descended from organisms for which only fossils exist today.

A trace fossil is the preserved evidence of the activity of an organism. In rare cases, the original tissues of an organism can be preserved.

Determining a Fossil’s Age Instead of dating fossils directly, scientists date the rocks the fossils are embedded inside. In relative-age dating, scientists determine the relative order in which rock layers were deposited. Relative-age dating helps scientists determine the relative order in which species have appeared on Earth over time.

Radioactive Decay Scientists take advantage of radioactive decay, a natural clocklike process in rocks, to learn a rock’s absolute age, or its age in years. To measure the age of sedimentary rock layers, scientists calculate the ages of igneous layers above and below them. Radioactive decay occurs when the nuclei of unstable atoms break down, changing the original atoms into atoms of another element. The rate of radioactive decay is measured in terms of half-life. Half-life is the amount of time it takes for half the atoms of a substance to decay into another element. Different substances have different half-life’s Examples are Uranium 238 and Carbon 14.

If the age of the igneous layers is known, it is possible to estimate the age of the sedimentary layers—and the fossils they contain—between them.

The Geologic Time Scale The geologic time scale is a chart that divides Earth’s history into different time units. Earth’s age is believed to be 4.6 billion years It is broken down into different units and sub-units based upon the rocks and fossils within those rock layers Largest division = EON (ex: Archean, Phanerozoic) Next largest = ERA (ex: Mesozoic, Cenozoic) Next largest = PERIOD (ex: Devonian, Jurassic) Add Geologic Time Scale to your index

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Eras Cenezoic Time of recent life Fossils resemble modern day living organisms Mesazoic Time of middle life. Some fossils resemble modern day living organisms Some are different from any living organism on the Earth today.

Paleozoic Time of ancient life. Fossils are different from anything found on the Earth today. Precambrian Time before the time of ancient life. Very few fossils are found in this time period

Extinctions Extinction occurs when the last individual organism of a species dies. A mass extinction occurs when many species become extinct within a few million years or less. Extinctions can occur when environments change.

The fossil record contains evidence that five mass extinction events have occurred during the Phanerozoic eon.