TRILOBITES

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).

 "THE ANCIENT ARTHROPODS"
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, and part of their appeal is that you can hold and examine an entire fossil animal and turn it about in your hand. Try that with your average dinosaur!  

 Extracted from Introduction to Trilobites (http://www.aloha.net/%7Esmgon/trilobite.htm)