Trilobites are an extinct group of marine animals that were once abundant in the oceans but became extinct about 250 million years ago. Trilobites are the oldest known arthropods, a diverse group of invertebrates characterised by jointed limbs and a hard external skeleton. Living arthropods include crabs, spiders, scorpions and insects. The feeding habits of trilobites were highly variable and they may have been active predators, scavengers on dead animal remains and filter feeders on the sea floor.
Trilobites are named for the three (tri) lobes that run the length of their body - a raised central lobe with flatter regions on either side. Other features of trilobites include gills located on their many pairs of limbs, compound eyes that are the oldest visual organs known, and a body size that varies between species from 0.5 mm to about 90 cm.
The trilobite specimen shown here belongs to the genus 'Kolihapeltis'. It was collected in Morocco and is about 400 million years old. The genus 'Kolihapeltis' is characterised by long spines that arise from the back of the head and curve backwards over the body. Trilobites of this genus are relatively rare and display a particularly fascinating form.
Fossils are the remains, moulds or traces of dead plants or animals preserved in rock, generally sedimentary rock such as sandstones, siltstones, shales and limestones. Fossils provide invaluable information about the Earth's past, including the evolution of plants and animals, the age and formation of rocks, and the former positions of the continents.
Some fossils, such as trilobites, that are particularly useful for dating rocks are called index or zone fossils. Index fossils are generally common fossils, easily preserved and identifiable, and widely distributed. Organisms chosen as index fossils need to be species that lived for a relatively short period, for instance a few hundred thousand years, so that dating can be narrowed to a limited time span. They assist scientists to date rock sequences as the age of the rock correlates to the age of the fossils encased in them. This information can then be applied to rocks found in other regions that contain the same fossilised species. Trilobites are one of the most important index fossils.