Download Fossils

GEOLOGIC TIME
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Geologic time scale spans 4.5 billion years.
We can focus on the last ~500 million years due to
the study of fossils.
Fossils are the remains or traces of prehistoric life.
Fossils have been used to divide geologic time into
eons, eras, periods, and epochs.
FOSSILS
What is a fossil?
Fossils are the remains or traces of
organisms found in sedimentary rocks.
What conditions are necessary for fossils to form?
1. Rapid Burial
If the organism is quickly buried by fine-grained
sediment it is protected from being eaten by scavengers or
decomposed by bacteria (i.e. little to no oxygen).
2. Presence of Hard Body Parts
Fossils of organisms that contained hard parts are
abundant in the fossil record, but only rare traces of soft tissue
organisms are seen as fossils. Soft-bodied organisms could get
buried by volcanic ash.
3. Low Oxygen Environment
In a low oxygen environment, there is less bacteria and
as a result, decomposition is slower.
FOSSILS
AND THEIR IMPORTANCE IN
GEOLOGY:
What is the importance of fossils to geologist? What
does a fossil indicate?
1. Fossils indicate the age of sedimentary rocks.
Approximate age of the rock can be determined if we
know when a life form existed on Earth.
E.g. Olenellus Trilobites indicate Cambrian Period of
Paleozoic Era (540-490 Ma).
2. Fossils indicate the environments in which rocks
formed. For example, fossils of coral indicate a warm
tropical environment.
3. Fossils are used to correlate (match up) rocks.
FOSSIL CORRELATION!
FOSSILS
AND THEIR IMPORTANCE IN
GEOLOGY:
What is the importance of fossils to geologist? what
does a fossil indicate?
4. Fossils provide the basis by which the subdivisions of the
Geologic Time Scale are made.
Division of the Geologic Time Scale is marked by some
significant event in the evolution of Earth.
Examples:
- The extinction of trilobites marked the end of the
Paleozoic Era (248 Ma).
- The extinction of dinosaurs marked the end of the
Mesozoic Era (65 Ma).
5. Fossils can also indicate evolutionary pathways.
Fossil evidence show the progression (evolution) of life forms with
time.
Invertebrates → Fish → Land Plants → amphibians → reptiles → Mammals → Birds →
Humans
METHODS OF FOSSILIZATION
Fossils are preserved in the rock record in several ways!
1)
Petrification (or Petrifaction) By Replacement
2)
Carbonization
3)
Mold and Cast
4.
Preservation (Intact)
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5.
Ice, Mummification, and Amber
Traces (Indirect Evidence)
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Tracks, Burrows/Tunnels, Eggs, gastrolites
(stomach stones), and Coprolites (feces).
For The Exam
You are expected to know how they form
Not their characteristics
PETRIFICATION
Occurs when the small internal
cavities and pores of the original
structure are filled with precipitated
mineral matter.
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Formation:
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cell walls and solid material are removed
It is replaced by mineral material carried by
ground water.
The process is called replacement.
Sometimes internal details and structures are retained.
CARBONIZATION
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Formation:
Fine sediment encloses delicate matter such as leaves (e.g.
ferns) in a oxygen-poor environment.
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As time passes, pressure squeezes out the liquid and gaseous
components of the organism leaving behind a thin residue of
carbon.
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MOLD AND CAST
Often preserve a replica of a plant or
animal in sedimentary rocks.
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Formation:
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An organism is buried in sediment and then
dissolved by groundwater leaving a hollow
depression or an impression called a mold.
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The mold shows only the original EXTERNAL
SHAPE and SURFACE MARKINGS of the organism. It
does not reveal the internal structure!
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When minerals or sediment fills the hollow depression or
impression it forms a cast.
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PRESERVATION
Original remains can be preserved in ice or
in amber (hardened tree sap).
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Formation:
Either ice or amber covers the organism quickly after dying –
or alive
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It is protected from decay (oxygen-free environment) and from
pressures that would cause it to be crushed.
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The entire organism has been preserved; even the soft parts,
which usually decay and disappear.
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Examples:
(1) Woolly Mammoths preserved in ice in Alaska and
Siberia.
(2) Insects preserved in tree sap (amber). Cane in Jurassic
Park.
TRACE FOSSILS
Show traces left in the rock by an
organism.
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Formation:
Not the organism itself
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Imprints or wastes left by the organism
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Examples include:
Tracks - animal footprints made in soft sediment. The sediment later
turns into sedimentary rock.
Burrows/Tunnels - Animal trails made in soft sediment. The sediment
later turns into sedimentary rock.
Coprolites - Fossil dung (feces).
Gastrolites – Stomach stones.
INFORMATION FROM TRACE FOSSILS
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A measurement of the depth and surface
area of a footprint when coupled with the
supposed soil characteristics in which it
was made can give a reasonably accurate
measure of the creature’s body mass.
CORE LAB #2
“Estimating Dinosaur Size and Speed from Trackways”