Fossils
Raiders of the Lost Ammonite: How Fossils Are Made
Mar
When most plants, animals, and other living organisms die, natural conditions collaborate to cause it to completely decay. When conditions are right, these organisms are preserved as fossils. A preserved specimen is called a fossil if it has existed for a certain number of years, usually the arbitrary age of 10,000 years. The sum total of fossils, those discovered and undiscovered, and their placement in sedimentary layers and rock formations, is called the fossil record.
Chemical and physical processes create fossils. On rare occasions, whole-body fossils are created when animal or plant matter is frozen, dried, or encased in a material like resin or tar, but these kinds of fossils are extremely rare. Most organisms that are preserved as fossils get that way through other means.
When most plants, animals, and other living organisms die, natural conditions collaborate to cause it to completely decay. When conditions are right, these organisms are preserved as fossils. A preserved specimen is called a fossil if it has existed for a certain number of years, usually the arbitrary age of 10,000 years. The sum total of fossils, those discovered and undiscovered, and their placement in sedimentary layers and rock formations, is called the fossil record.
Chemical and physical processes create fossils. On rare occasions, whole-body fossils are created when animal or plant matter is frozen, dried, or encased in a material like resin or tar, but these kinds of fossils are extremely rare. Most organisms that are preserved as fossils get that way through other means.
Heat and pressure caused from being engulfed in sediment can cause organisms like plants, reptiles, fish, and other marine life to release hydrogen and oxygen, which leaves behind a carbon residue. This process, called carbonization or distillation, results in a detailed carbon impression in the sedimentary rock of the dead organism.
When an organism’s soft tissues decay in sediment, its hard parts (in particular the bones) are left behind. This is the most common method of fossilization, which is called permineralization, or petrification.
In the fossilization process known as replacement, the original hard parts of an organism dissolve after minerals in the groundwater replace the minerals that make up the organism’s bodily remains.
While it isn’t clear what keeps the organic material preserved, iron might serve as a deterrent to bacteria that would otherwise consume them, much in the way that formaldehyde makes proteins more resistant to decay.
Although fossils are a common occurrence around the world, just a small fraction of life makes it into the fossil record. Most living organisms simply decay without a trace as natural processes recycle their soft tissues and hard parts like bone and shell.
The abundance of fossils in Earth’s geological record reflects favorable conditions where preservation is possible, along with the immense number of organisms that have ever lived, and the enormous amount of time during which they have accumulated.
When an organism completely dissolves in sedimentary rock, it can leave an impression of its exterior in the rock; this is called an external mold. If the mold is filled with other minerals, it becomes a cast. An internal mold is formed when minerals or sediments fill the internal cavity of an organism, such as its skull or shell or skull, and the remains dissolve.
Although experts once thought it impossible for any organic material to have been left behind after the fossilization process, that’s no longer the case. Scientists have discovered in recent years that some fossils aren’t made merely of minerals. Analyses shows that some retained organic material that dates back to the Cretaceous age, a period that spanned roughly 65.5 million to 145.5 million years ago, and the Jurassic period, which took place between 145.5 million and 199.6 million years ago. That organic material is believed to have belonged to dinosaurs, because it matches certain proteins from birds, which evolved from the great lizards.
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