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Fossils
Fossil – Remains of prehistoric life used to
match rocks of similar age in different
regions
Conditions Favoring Preservation:
Hard parts and rapid burial!!!!!
How Do Fossils Form?
Usually organism dies and is covered with
mud or sediment
(The key is that it has to be in an anaerobic
environment – anaerobic means there is no ?
_______________)
The organism then slowly decays and leaves
either a hole in the shape of its body or has
minerals fill in the hole.
Fossil Types:
1. Petrified – cavities and pores filled with
precipitated mineral matter ex. Wood
Fossil Types Cont:
2. Replacement –Cell material is replaced
with mineral material
- If the minerals are carried in groundwater,
the fossil is called permineralized
Trilobite – living tissue replaced with
calcite (Found in Washtenaw – Devonian
age 360 – 408 mya)
This is a permineralized cross-section of part of
a tree trunk. Because water carried the
minerals that replaced the tissue, sometimes
little bits of the original tissue gets preserved
and we can get DNA from it (like in Jurassic
Park!)
3. Mold –Hard part is buried then dissolved
by underground water – only imprint is left
Tree Mold – formed as lava flowed around a
tree. Moisture in tree kept it from burning up
for long enough for lava to cool around it
(Found at Craters of the Moon Nat'l Park)
4. Cast –Hollow space of mold is filled
with mineral matter ex. Fossil shell
Internal Cast of a Brachiopod Silurian Age, 438 to 408 mya
5. Carbonization –Remains covered by fine
sediment; pressure from above squeezes out
liquids & gases and leaves carbon residue
ex. Some ferns (will look black)
Carbonized Fern
6. Impression –Replica of fossil’s surface
(a carbonized fossil that loses carbon
residue)
An impression and the same type of leaf found now
7. Amber –Petrified wood resin that may
have trapped plants or animals
Spider caught in Amber
8. Indirect evidence- including tracks,
burrows, coprolites (fossilized dung)
Coprolite (dino dung)
Dino Tracks
Principle of Fossil Succession:
Fossils follow each other in a definite order
& any time period can be identified by its
fossils (Esp. it index fossils – common
prevalent fossil of time period)
i.e. Simple shelled organisms followed by
trilobites
Fossil Succession – Fossils organisms change in
dominance in a definite pattern
Name that fossil type!
Name that fossil type!
Name that fossil type!
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