Geologic History

Daily Dose of Dinos: Are the dinosaurs you know, really dinosaurs?

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To begin this chapter, you are going to get to examine different fossils and try to identify them based on certain characteristics that your samples have.  

SUSD5 Student Version of the Fossil Identification Lab.

Purchase the Fossil Identification Lab for .50 cents at Teachers Pay Teachers.

Section 1: Fossil Formation

Fossils are the remains, imprints, or traces of once living organisms that have been preserved in the rock record.  Fossils can tell us when (relative time), where (type of environments) and how they lived.  Most prehistoric animal traces do not survive because of decay, scavengers, plate movements, subduction, volcanic eruptions, weathering, etc...

Fossils let scientists understand ancient environments.  Rocks have been discovered in Antarctica that contain fossils of tropical plants.  Brachiopods lived in shallow seas but are found in the Midwest.  Fossils help us understand the historical geology of rock layers that they have been found in.  Fossils help us understand and collect information about ancient environments, climates, organism behavior, and we even use them to relatively date rocks.

It is extremely difficult to become a fossil.  Watch the video below and see how difficult it is.

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Conditions that are needed to become fossils are: 

  • The organism must be protected from scavengers and microorganisms.
  • The organism needs to be buried quickly by oxygen poor sediment.
  • Organisms need hard parts like teeth, shells, and bones.  If they don't decays is most likely going to destroy it.

Fossils are usually found in sedimentary rocks.  Think back to what a sedimentary rock is and ask yourself, "Why are fossils most likely found in sedimentary rock?"

Section 2: Types of Fossils

Daily Dose of Dino:  Super Predator Discovered in Utah, The King of Gore

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There are 5 types of fossils.

Permineralization and Replacement

  • Petrified remains are hard and rocklike. The organism's remains have been replaced by mineralization.
    • Example:  Water dissolves the calcium in bone and then replaces it with the mineral quarts.
  • Petrified Wood in the Petrified National Forest is a great example of permineralized fossils.


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Permineralization Petrified Wood



Permineralized Petrified Wood




Carbonaceous Films

  • Most organism tissue is made of carbon compounds.  Sometimes this carbon gets pressed out of the organism caused by a buildup of heat and pressure from overlying sediments.  Gases and liquids are forced out of the organism leaving the outline of the carbon residue through a process called carbonization which chemically changes organic material.  Think of carbonaceous films like a printing press or photocopy.  Nothing is left of the original organism except it's filmed copy.

Carbonaceous Film Fossil


Carbonaceous Film Fossil Crayfish


Carbonaceous Fern Leaf Fossil


Molds and Casts

  • Think of molds and casts like Plaster of Paris impressions. Hard parts of an organism fall into soft sediment like mud from silts and clays.  Compaction and cementation turn these sediments into rock.  Even though it is rock, it is still porous and permeable allowing water and air to reach the organism causing it to decay inside of the rock.  This leaves a hollow spot or a mold.  If sediment enters the mold and fills it in a cast fossil is formed.


Mold vs. Cast fossils

Mold vs. Cast



Mold vs. Cast Fossil Example

Mold on Left, Cast on Right



Mold vs. Cast Fossil Example

Which is Mold and which is Cast?


This video will demonstrate how mold and casts are formed.

How are molds and casts made

Original remains

  • Sometimes the remains of a complete organism are found.
    • Examples:  Amber trapping insects, woolly mammoths are frozen. Animals trapped in tar.
  • In order to be classified a fossil, the organism must be at least 10,000 years old.  So even though a man that had died 5300 years ago and was recently found in a glacier in the Alps, he would not be considered a fossil.


Original remains amber fossil



Original Remains Wooly Mammoth



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Watch this video to see some awesome facts about the wooly mammoth.

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Trace Fossils

  • Trace fossils are the traces, tracks and other evidence of ancient life.  We can use trace fossils to help determine sizes and weights of organisms.  We can use these fossils to discover possible diets.
  • Examples of trace fossils:
    • Worm holes, burrows, footprints, eggs, fossilized poop called coprolite, etc...

Trace Fossil Footprint


Trace Fossil Arthropod


Trace Fossils Eggs