Trilobites, meaning 3 lobed, were the first multiple celled animals with a hardened exoskeleton to exist on earth! They dominated the seas 570 million years ago and steadily declined in numbers until they became extinct 350 million years later.
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). 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, making trilobites the single most diverse group of extinct organisms.
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!). Most trilobites are about an inch long, and part of their appeal is that you can hold and examine an entire fossil animal and turn it about in your hand. Collectors are especially interested in their eyes: explained further below.
Trilobites developed one of the first advanced visual systems in the animal kingdom. The majority of trilobites had a pair of compound eyes, made up of many lensed units. At least one suborder of trilobites, the Agnostina, are thought to be primarily eyeless. None have ever been found with eyes.
The advantage of good eye design
A Guide to the Orders of Trilobites, a site devoted to understanding trilobites created and maintained by Dr. Sam Gon III http://www.aloha.net/%7Esmgon/ordersoftrilobites.htm
Back to fossils