About the Western Trilobite Amateurs (W.T.A.)

The official Website of the Western Trilobite Amateurs (W.T.A.) is here!

What are Trilobites

Trilobites (/ˈtraɪlɵbaɪt/, /ˈtrɪlɵbaɪt/; meaning “three lobes”) are a well-known fossil group of extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (521 million years ago), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except 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.

By the time trilobites first appeared in the fossil record they were already highly diversified and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton an extensive fossil record was left behind, 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 are still scientific debates). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.

The trilobite body plan

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 axial lobe, flanked on each side by right and left pleural lobes (pleura = side, rib). 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 size and form. You can examine the trilobite body plan in more detail using the links on the navigation bar below, or link directly to a page describing trilobite major features.

Mission statement

The organization has a variety of interests and goals. First and foremost is the activity of the recreational collection of trilobites and associated fossils from the Palaeozoic of our region. Through our explorations we hope to understand and add to the knowledge base of this fascinating and still under-explored geological and palaeontological region. One of our goals, with the assistance and encouragement of academics, researchers and scholars is to find new species, add to the knowledge base, and help survey new areas.

Another goal is the accumulation of a western regional web-based trilobite database of photos, specimens, professional research papers etc.

Who we are

We are a group of dedicated amateur trilobite collectors whose focus is on western North America (western U.S.A. and western Canada). States and Provinces included (so far) are Nevada, California, Utah, Montana, Idaho, New Mexico, Washington, Colorado, Arizona, British Columbia and Alberta. Our group includes teachers, academics, professional and non-professional palaeontologists and others from a host of varied backgrounds and careers.

The W.T.A. also has a practical interest in land use policies of the B.L.M. (Bureau of Land Management) and other governmental entities whose interest lies in the use of public lands. Our belief is that public lands of non-national or state park status remain open for the recreational use of amateur fossil collectors.


Ongoing projects such as our trilobite data base are augmented with collecting field trips, lobbying and correspondence with the palaeontological academic and research community.


We try to keep up to date, but it is certainly impossible to keep up with all the taxonomic name changes and new species being discovered. This page is merely a first guide for the trilobites found in many of the Formations. Mistakes will be frequent, but we will try and update any new changes that are sent to us (please include the reference).

Site Evolution

This site cannot evolve without the generous contributions of all amateur and professional palaeontologists involved. Please feel free to comment on the site, point out any additions or corrections, contribute photographs of species not yet up, or add a faunal list for a formation in your area.