Trilobites ( meaning "three lobes") are a well-known fossil group of extinct marine arthropods that form the classTrilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (526 million years ago), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, almost all trilobite orders, with the sole exception of Proetida, died out. Trilobites finally disappeared in the mass extinction at the end of the Permian about 250 million years ago. The trilobites were among the most successful of all early animals, roaming the oceans for over 270 million years.

When trilobites first appeared in the fossil record they were already highly diverse and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton an extensive fossil record was left, with some 17,000 known species spanning Paleozoic time. The study of these fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology and plate tectonics. Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata),although several alternative taxonomies are found in the literature.

Trilobites had many life styles; some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton. Most life styles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where there is still scientific debate). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.

Similar to the horseshoe crab, Trilobites - meaning 3 lobes-lived from 550 to 200 million years ago in Cambrian, Paleozoic, and Devonian seas. Trilobites are most closely related to the chelicerates, which include the horseshoe crabs and spiders. They are now all extinct, but their fossil remains are found in many places in the U.S. and worldwide. Some, in the Canadian Rockies, have been found as large as 2½feet in length.

For more information about Trilobites try these links:

http://www.trilobites.info/trilobite.htm

http://www.biol.wwu.edu/trent/alles/Trilobites.pdf

http://geology.utah.gov/surveynotes/gladasked/trilobites.htm

http://www.georgehart.com/trilobites/trilobite.html