Trilobites are hard-shelled, segmented creatures that existed over 300 million years ago in the Earth's ancient seas. They went extinct before dinosaurs even came into existence, and are one of the key signature creatures of the Paleozoic Era, the first era to exhibit a proliferation of the complex life-forms that established the foundation of life as it is today. Although dinosaurs are the most well-known fossil life forms, trilobites are also a favorite among those familiar with Paleontology (the study of the development of life on Earth).

Trilobites were among the first of the arthropods, a phylum of hard-shelled creatures with multiple body segments and jointed legs (although the legs, antennae and other finer structures of trilobites only rarely are preserved). They constitute an extinct class of arthropods, the Trilobita, made up of nine orders, over 150 families, about 5000 genera, and over 15,000 described species. New species of trilobites are unearthed and described every year. This makes trilobites the single most diverse group of extinct organisms, and within the generalized body plan of trilobites there was a great deal of diversity of size and form. The smallest known trilobite species is just under a millimeter long, while the largest include species from 30 to 70 cm in length (roughly a foot to two feet long!). With such a diversity of species and sizes, speculations on the ecological role of trilobites includes planktonic, swimming, and crawling forms, and we can presume they filled a varied set of trophic (feeding) niches, although perhaps mostly as detritivores, predators, or scavengers. Most trilobites are about an inch long.

Whatever their size, all trilobite fossils have a similar body plan, being made up of three main body parts: a cephalon (head), a segmented thorax, and a pygidium (tail piece) as shown at left. However, the name "trilobite," which means "three lobed," is not in reference to those three body parts mentioned above, but to the fact that all trilobites bear a long central, or axial lobe, flanked on each side by right and left pleural lobes. These three lobes that run from the cephalon to the pygidium are what give trilobites their name, and are common to all trilobites despite their great diversity of form.

Classification:

AGNOSTIDA - Among the early trilobites, with a basic, clamshell-like appearance.
Suborders Agnostina and Eodiscina.

REDLICHIIDA - Including the most primitive orders of trilobites in the lower Cambrian. Suborders Olenellina and Redlichiina.

CORYNEXOCHIDA - An often spiny group united by a shared hypostomal attachment.
Suborders Corynexochina, Illaenina, and Leiostegiina.

LICHIDA - Some of the most ornately sculptured species fall into this group.
No suborders, but three Superfamilies.

PHACOPIDA - The well-known Phacops, with its beautiful compound eyes belongs here.
Suborders Calymenina, Phacopina, and Cheirurina.

PROETIDA - Includes some of the last trilobite species before the Permian Extinction.
No suborders, but three Superfamilies.

ASAPHIDA - All share a ventral median suture, and most a similar development.
No suborders, but six Superfamilies comprising ~20% of all trilobites.

ASAPHIDA - All share a ventral median suture, and most a similar development.
No suborders, but six Superfamilies comprising ~20% of all trilobites.

PTYCHOPARIIDA - Bearing the "generic trilobite" body plan, but many diverse variations!
Suborders Ptychopariina and Olenina.

HARPETIDA - Bearing the distinctive, broad, often intricately pitted, cephalic fringe.
Very recently (2002) split out of the Ptychopariida and elevated from suborder to full order.

NEKTASPIDA - The so called "soft-shelled trilobites" such as Naraoia have been classified as an order of trilobites by some.