Fossil record stories

A fossil usually forms when an organism's hard parts, like bones, teeth, or shells, are buried quickly in sediment before they can fully decay. Over a very long time, minerals dissolved in groundwater gradually replace the original material, turning it into rock-hard fossil. Because fossils form inside layers of rock that build up over time, with older layers generally buried deeper than newer ones, scientists can use rock layers to place fossils in relative order, from older to younger; more precise numeric ages can also be found using radiometric dating methods.

Some fossils are especially valuable because they are transitional: they show a mix of features between an older group and a later group, helping fill in how one form changed into another over time. The fossil record for whales is one of the clearest transitional sequences known: Pakicetus, found in the oldest layers, was a four-legged land mammal that lived near water and shared certain distinctive ear-bone features with modern whales. Later layers hold Ambulocetus, with strong hind limbs suited for swimming as well as walking; then Rodhocetus, with smaller hind limbs and clearly more time spent in water; then Basilosaurus, fully aquatic, with only tiny hind limb bones too small to support walking. Modern whales, found in the youngest layers, have no external hind limbs at all, and front limbs reshaped into flippers.