Over time the soft parts of the animal decay, leaving only the harder parts like teeth and bones.
More sediment settles on the remains, and time and pressure turn the sediment surrounding the bones and teeth to stone.
A cast fossil is like a mold fossil, but in the case of a cast fossil the mold has been filled with minerals.
An animal that dies in a cold place, like the tundra, can be frozen.
So long as the temperature remains below freezing, the animal stays frozen, sort of like a natural freezer.
Paleontologists have found woolly mammoths perfectly preserved in permafrost.
How Fossils are Dated: Superposition, Carbon-14, and Radiometric Dating Imagine you've found a fossil in your backyard and you want to know how old it is. A long time ago scientists used the concept of superposition, which states that the oldest layer of sedimentary rock is found at the bottom of the strata, or layers, and the youngest at the top.
While they didn't have exact dates for how old these strata were, they could determine which fossils were older than others based on which layer they were found in.