October 15, 2014 marks the fifth anniversary of National Fossil Day. Sponsored by the U.S. National Park Service, the celebration aims to increase public understanding of fossils' educational and scientific value.
Fossils are in many ways the key to understanding prehistoric life: they are the preserved remains of plants, animals, and other organisms (including tiny bacteria!) from the remote past. Evidence of past life, including animal tracks and feces, can be preserved in similar ways and are also considered fossils.
When an organism dies or leaves evidence of its life, it can become fossilized through a number of different geologic processes. An organism can leave an imprint of itself on sediment that hardens and becomes preserved; an organism's bones and teeth can become fossilized as minerals in groundwater slowly replace the minerals in the bones and teeth as they decay; or an organism can become trapped in tree sap and be perfectly preserved once the sap is converted to fossil amber. Organisms that live near sediment, like marine animals that live on the ocean floor, are more likely to be fossilized. It is also easier for fossilization to occur in already-mineralized parts of organisms, such as bones, teeth, and exoskeletons, as opposed to the soft tissue parts. This is why so many fossils of dinosaur skeletons exist, while it is far less common to find fossilized dinosaur skin or flesh.
Once an organism leaves a fossil behind, scientists can use it to paint a picture of biodiversity from that time period. If a fossil is very old (more than 50 million years old), radiometric dating can be used fairly accurately to find when the organism died. For younger fossils, scientists must rely on knowledge of the sediment record to find out how old it is. Layers of rock form on top of one another, creating a layer-cake effect: the layers of sediment toward the top of the cake are newer, while the layers below it get older and older. If scientists can determine how old each layer is, they also will know how old the fossils from each layer are.
To celebrate National Fossil Day, be sure to check for affiliated events in your area! Learn more about fossils with these resources from Science NetLinks: Fossils 1: Fossils and Dinosaurs, Fossils 2: Uncovering the Facts, Comparing Species Through the Fossil Record, Fossils and Geologic Time and Dinosaur Eggs Discovered! Unscrambling the Clues.
Image credit: U.S. National Park Service
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