The fossil Record Addy murr Fossils are the preserved remains or evidence of once-living organisms.
All the fossils ever discovered on Earth make up the.
Fossils help scientists figure out what species that no longer ___ looked like when the organisms were alive.
Fossil Formation
Most fossils are formed of the parts of an organism.
Sometimes when the remains of organisms get buried in mud, wet sand, or other sediments under a body of ___, the molecules that formed the remains get replaced by ___.
This type of fossil formation is called ___.
Most mineralized fossils are formed of shell or ___, but wood can also become a mineralized fossil.
In ___, a fossil forms when a dead organism is compressed over time and pressure drives off the organism’s liquids and gases.
Sometimes organisms or parts of organisms make a(n) ___ in sand or mud.
The kind of fossil that forms an impression in rock is called a(n) ___.
If the impression gets filled with sediments that harden to rock, a(n) is the result.
Molds and casts show only ___ features of organisms.
The preserved evidence of the activity of an organism, such as its tracks, is called a(n) ___ fossil.
Determining a Fossil’s Age
Scientists cannot date most ___ directly. Instead they usually find the age of the ___ around the fossils.
In ___ dating, scientists determine the relative order in which rock layers were deposited.
In a(n) ___ rock formation, the older layers of rock are below the younger layers of rock.
Relative-age dating has helped scientists figure out the order that ___ have appeared on Earth.
Absolute-age dating is more ___ than relative-age dating and involves ___ isotopes that decay to become stable isotopes over time.

Fossils over Time
1. The ___ is a chart that divides Earth’s history into different time units.
2. Earth’s history is divided into four .
3. Earth’s most recent eon - the ___ eon - is subdivided into three ___.
4. Neither eons nor eras are ___ in length.
5. When scientists began developing the geologic time scale in the 1800’s, they did not have ___ dating methods, so they marked time boundaries with ___.

E. Extinctions
1. When the last individual organism of a species dies, a(n) ___ has occurred.
A(n) ___ extinction occurs when many species die off within a few million years or less.
The fossil record shows evidence of ___ mass extinctions during the Phanerozoic eon.
Extinctions can occur if the ___ changes quickly; for example, as a result of a meteorite impact.
Extinctions can also occur if the environment changes; for example, as a result of the formation of mountain ranges.
2. The fossil record contains clear evidence of the extinction of species over time as well as evidence of the appearance of many new ___ .