We ended up covering so many topics while doing our geologic history and prehistoric animal study that it was just too much to reasonably fit into a single post. I decided to break it up into 4 sub-topics: Fossils, The Paleozoic Era, The Mesozoic Era, and The Cenozoic Era. This one will focus on fossils.
What is a Fossil?
Fossils are the preserved remains or impressions of prehistoric organisms. The remains go through a process called fossilization. There are several different methods through which an organism can become fossilized, one of the most common methods is called permineralization. Permineralization happens when mineral rich water moves through sediment, depositing minerals into the empty spaces in bones, shells, or wood that is buried in the sediment, turning it into rock.
Scientists use fossils to learn about prehistoric life. Fossils can be used to learn about features of past plants and animals, when the organism lived, and can sometimes even be used to learn about their behavior. Fossils help us understand what the Earth was like in the ancient past.
Types of Fossils
Mold fossils are imprints of an organism in a substrate like rock or sediment. It's basically like a negative of the organism. Mold fossils form when an organism decays and leaves behind an imprint of itself, but leaves behind no organic material.
Cast fossils start out with an imprint like mold fossils, but go a step further. After an imprint is formed the negative space is filled in with minerals, producing a copy of the original organism.
Trace fossils are not actually fossilized parts of an organism, instead they are fossilized traces left behind by an organism like footprints, dung, or nests.
True form fossils are actual body parts of an organism that have been replaced by minerals over time, by the process of permineralization.
Videos and Resources About Types of Fossils-
-Types of Fossils from MooMooMath and Science
-Fossils 101 from National Geographic
-Describe the Types of Fossils from Sciencing
Types of Fossils Activity
This is a simple activity you can do to demonstrate the difference between mold, cast, and trace fossils and how they form.
For this activity you will need:
-Plaster of paris
-water
-sand
-three disposable plastic cups
-scissors
-a small plastic dinosaur or other creature (it needs to be no more than inch or two long so that it can easily fit inside the plastic cups).
Cut your plastic cups so that they are only about two inches deep. Fill one cup most of the way with damp sand. Pack it down and push your plastic dinosaur into it to make an imprint of the dinosaur. Remove the dinosaur.
Mix your plaster of paris and water according to the instructions on the package. Then pour a small amount into the dinosaur imprint in your cup with the sand and set it aside to dry.
Fill the other two cups about 2/3 full with the plaster of paris and let it sit for a couple of minutes until the plaster of paris has set enough to hold form if you press something into it.
In one cup of plaster of paris, push your plastic dinosaur into it to make an imprint. Carefully remove the dinosaur and set the cup aside to dry.
Stick the feet of your plastic dinosaur into the other cup with plaster of paris, to make footprints. Remove the dinosaur and set the cup aside to dry.
When your cups are dry, remove the cast of the dinosaur you made with the plaster of paris in the cup with sand. This is an example of how cast fossils are formed. First an imprint of an organism is made in a substrate and then it is filled in, making a copy of the original organism.
The cup with the imprint of the dinosaur represents mold fossils. It's an imprint of an organism in a substrate.
The cup with the footprints represents trace fossils, footprints left behind by a prehistoric animal.
Finding Fossils
One of my daughter's favorite hobbies is fossil hunting. You don't need a lot of special equipment to do it. A shovel, a flathead screwdriver, and a bucket are all we usually use. Some areas are better for finding fossils than others. Do a little research on fossils in your area to find out where you should look and the types of fossils common in your area. Also make sure to research fossil collecting laws for your state. Some states allow you to keep fossils found on public lands, others have specific rules about how to handle a fossil after it is found. If your searching on private property, always get permission from the land owner first. Both of these sites, have lots of suggestions for places to fossil hunt in various states:
-Best Spots for Fossil Hunting in America
-10 States with Fossil Hunting Sites for the Public
The area we live in happens to be a hotbed for cretaceous age fossils. We find them in limestone deposits just about everywhere. We tend to find the most near the river and in dry creek beds. Here are a few of our favorite finds:
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| Fossilized clam shell |
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| Fossilized shell full of calcite crystals |
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| Fossilized oyster shell |
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| Imprint fossil of a clam shell |
Additional Resources About Fossils
Here are a few more resources with information for learning about fossils:
-How Do Scientists Determine the Ages of Human Ancestors, Fossilized Dinosaurs and Other Organisms- This is a great article from Scientific American about methods used for determining the age of fossils and other remains.
-Earth Science For Kids- Fossils from Ducksters has some general information about fossils and how they form.
-Fossils- This page from DK has lots of great information!
-The 10 Oldest Fossils and What They Say About Evolution- This video from SciShow discusses some of the most important fossils discoveries in relation to the evolution of animals.
If you enjoyed this post check out:
Learning About Geologic History- The Paleozoic Era
Learning About the Mesozoic Era- The Age of the Dinosaurs
Learning About Geologic History- The Cenozoic Era
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