Six Layers, Half a Billion Years
1The fossil hall at the city's natural history museum has a wall that looks like a slice through the ground. It is a model built from six real rock layers collected at sites across the region, stacked in the order the layers formed. Museum guide Callum Ferreira tells visiting classes that the wall is the hall's "history book," read from the bottom up. Students start at the floor and work their way up toward the ceiling, stopping at each layer to look at the fossils displayed in glass cases beside it.
2Most of these layers are sedimentary rock, which forms when mud, sand or shells settle and are slowly pressed into stone. New sediment settles on top of older sediment, so in undisturbed layers the lower layers are older. Scientists have also measured the age of each layer using natural radioactive elements in volcanic ash found between some layers. The ages are written on the wall in millions of years ago (mya).
3Museum scientists counted how many different species of five groups have been found as fossils in each layer. Trilobites were sea animals with jointed legs and a hard outer shell split into three lengthwise parts. Ammonites were sea animals with a coiled, chambered shell. The dinosaurs counted here are the non-bird dinosaurs. The mammal fossils are mostly teeth and jaws, and the flowering plant fossils are leaves and pollen.
4Callum reminds visitors that the fossil record has gaps. Fossils form only when remains are buried quickly, usually in mud or sand, before they rot or are eaten. Soft-bodied animals and animals that lived in dry uplands are rarely preserved. A zero in the table means no fossils of that group have been found in that layer, not proof that none lived anywhere at that time. Even so, when the same pattern shows up at site after site, scientists treat it as strong evidence.
5The counts below come from the museum's own collection. Other museums have found other species, but the overall order of the groups, layer by layer, is the same in rock around the world. Callum asks each class to look for three kinds of patterns: when a group first shows up, when its number of species grows or shrinks, and when it is no longer found.