Trilobite fossils are one of the most well-known types of fossil animal groups, perhaps the second most well-known after dinosaurs, or third most after Ammonites. Trilobite fossils are quite common, varied over time (as new species evolved), and are found in many different locations around the world. As a result, trilobites are often used as “index fossils”, which is to say that by identifying the particular type of trilobite found within a rock, it is often possible to assign a date to that rock.
When alive, trilobites were a group of arthropods (the same phylum that contains insects, crustaceans such as crabs and lobsters, spiders, millipedes, centipedes, etc.). All trilobites appear to have been marine living. Most lived in the seas, but there is some evidence that some species of trilobite may have lived in freshwater. During life, trilobites seem to filled many different ecological niches. Some trilobites were bottom-dwelling scavengers or predators, others were swimming plankton eaters, and there even appear to have been a group of trilobites which had a symbiotic relationship with sulfur-eating bacteria that provided them with food.
The very first trilobites evolved during the early part of the Cambrian period, perhaps 530 million years ago. They greatly diversified during the succeeding ages, but many species of trilobites were wiped out during the late Devonian mass extinction (364 million years ago). After this mass extinction, only one order of Trilobite (Proetida) survived, and this order, the last surviving Trilobites, died out during the Permian-Triassic mass extinction (248 million years ago).