Fossils and Geologic Time Lab
Last Modified: 08/20/09
Fossils (any evidence of past life) are the key to
understanding life of the past. More than that, they provide a
wealth of information to the geologist...
Why are fossils important in geology?
- They aid in the age dating of rocks
- Allow geologists to correlate rock units
- Provide clues for the interpretation of depositional
environments
- Allow us to study past life and determine how it has
changed (evolution)
- Paleontology - the study of the geologic history
of life on earth
- Fossil - any evidence of past life
- Body Fossil - the remains of a part of the
organism
- Trace Fossil - evidence of past life that does not
include parts of the organism
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Fossil Topic List *
Since (body) fossils are the remains of once-living organisms,
fossils are classified in the same manner as living organisms.
The classification scheme is hierarchical (p. 240) and is listed
below with the broadest (most encompassing) division listed
first.
- Kingdom
- Phylum
- Class
- Order
- Family
- Genus
- Species
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Fossil Topic List *
Radial - a radially symmetrical animal can be divided
into halves which are mirror images by an infinite number of
radially positioned planes.
Bilateral - a bilaterally symmetrical animal can be
divided into halves which are mirror images by one plane passing
though a certain location. For brachiopods, the plane passes
through the center of each valve so that one half of each valve
is the mirror image of the other half. In bivalves, the plane
passes between the two valves so that the valves are mirror
images of each other.
Pentagonal - 5-way symmetry (like a starfish).
Coiled - certain fossils can be identified on the basis
of their pattern of coiling.
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Fossil Topic List *
Best environments for preservation:
- Low Energy
- Fine-grained sediment
- Rapid burial
Why is all that helpful for preservation?
- Good question. Well, if it was a high energy enironment
(landslide, beach...), the creature would be quickly
broken into tiny fragments and scattered--so much for an
intact skeleton! Fine-grained sediment acts as nature's
"ziploc" bag--it keeps destructive bacteria,
acidic waters, etc. from attacking (and destroying) the
organic material, bone, etc. of the deceased. Rapid
burial places the body far from scavenging creatures (and
high energy destructive forces)--giving it the best
chance to remain intact and in pristine condition.
Types of fossilization:
1) Fossilization WITHOUT alteration (e.g., frozen
Mammoth, insect in Amber)
2) Fossilization of WITH alteration (Body Fossils, p.
253)
- Replacement = atom by atom replacement (petrified
wood)
- Permineralization = minerals deposited in pores
(fossilized bones)
- Compression = carbon residue left behind from soft
part alteration
- Mold = fossil surrounded by hardened sediment is
dissolved--leaving a cavity in the shape of the original
fossil
- Cast = forms when the mold is filled with sediment
(which then hardens)
3) Trace Fossils - include only signs of the organisms
activity; examples include...
- footprints
- coprolites (fossilized dung; glauconite)
- burrows
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Fossil Topic List *
Preservation Bias - hard parts are easier to preserve
than soft parts.
Collector's Bias - a paleontologist's objective may be
to collect only certain specimens--and therefore what is
collected is NOT necessarily representative of the entire,
original ecosystem.
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Fossil Topic List *
- Superposition - in undisturbed sedimentary rock
layers, younger rocks overlie older rocks.
- Original Horizontality - sediments are initially
deposited in horizontal layers.
- Cross-cutting Relationships - faults and
intrusions must be younger than the rocks they cut.
- Faunal Succession - assumes that organisms change
over time and that a specific body plan is never
repeated.
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Fossil Topic List *
How can we age date rocks using fossils?
- Through the use of Index Fossils!
Index Fossils are fossils of organisms that were...
- abundant
- geographically wide-ranging
- readily preserved
- lived a short period of time
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Fossil Topic List *
- The boundary between the Cenozoic and Mesozoic is dated
at 66 Million Years ago.
- The boundary between the Mesozoic and Paleozoic is dated
at 251 Million Years ago.
- The boundary between the Paleozoic and Pre-Cambrian is
dated at 550 Million Years ago.
Geologic Time Scale
EON
|
ERA
|
PERIOD
|
Phanerozoic
|
Cenozoic
|
Quaternary
|
Tertiary
|
Mesozoic
|
Cretaceous
|
Jurassic
|
Triassic
|
Paleozoic
|
Permian
|
Pennsylvanian
|
Mississippian
|
Devonian
|
Silurian
|
Ordovician
|
Cambrian
|
Pre-Cambrian
|
|
|
| |
|
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Fossil Topic List *
- "the present is the key to the past"
- states that geologic processes have remained essentially
the same throughout geologic time
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Fossil Topic List *
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