All About Fossils

By | September 9, 2016

Almost everything we know about prehistoric animals comes from fossils. A fossil is the preserved remains or trace of an ancient animal or plant. The word “fossil” comes from the Latin word fossilis. That means “dug up,” and that’s how some fossils are discovered, although most are exposed by erosion. Most fossilized animals have lain buried for millions of years.



Complete fossilized skeletons are rare, but when found they provide a huge amount of information for fossil hunters (paleontologists).


Fossils can be sorted, or classified, into different types, depending on how they formed. All take millions of years to form—fossilization is not quick. Total preservation: If an insect or spider was caught in the sticky sap released by a tree such as a pine, it may be preserved complete. Creatures that are millions of years old have been preserved in fossilized tree resin (known as amber) in this way.


A spider caught in amber



Dinosaur mineralized fossils

Dinosaurs, like us, had hard bones, and sometimes just these parts of an animal are preserved—though not as bone, which is replaced over time with minerals to form rock. Rock has to be carefully removed to expose the fossils.


Tree trunks, just like bones, can be turned to rock by mineralization over millions of years. Petrified trees still look like logs. Petrification means “change to stone.”

External mold

Sometimes the original organism dissolves completely, but leaves an impression of itself in the rock. This impression is called a mold. Natural cast This forms just like an external mold, but the hole then fills in as minerals from water slowly crystallize inside it, forming a rock such as flint.

Trace fossil

Occasionally an animal will leave a hint of its presence: a trace. This may be a footprint, a nest, tooth marks, or even droppings. These are called trace fossils.

What makes a fossil?

All kinds of living things have been discovered in fossil form. We have unearthed fossilized animal skeletons, skin impressions, footprints, teeth, animal droppings, insects, and plants. The hard parts of an animal, such as the bones, are the parts that fossilize best.



A fossil can only form if an animal’s body is buried quickly after death, so fossilized animals are animals that have died in a river and have sunk into mud, for example, or those that have died in a sandstorm and been buried in sand. These five diagrams show one way a dinosaur’s bones—in this case a Baryonyx—may be fossilized and found millions of years later.



A paleontologist analyizing analyizing a dinosaur’s fossil

People who study fossils are called paleontologists. Paleontologists may work in the field, digging up new fossils, or in labs or museums. They work like detectives, carefully gathering as many clues as they can to find out what happened in the past and to figure out where each new discovery fits in the tree of life.


■ Fossils are usually found in rock but may also be found in mud or gravel.

■ The parts of an animal most likely to fossilize are the hard parts: the bones or teeth or a creature’s shell.

■ Teeth are among the most commonly found fossils.

■ The oldest fossils are stromatolites (mounds of rock made by sea-dwelling bacteria). These have been dated to 3.5 billion years ago.

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