Trilobites are extinct sea creatures that were one of the first forms of life on earth. They ruled the world before the time of the dinosaurs, from the Cambrian Period to the Permian Period (between 245 and 600 million years ago). Trilobites are the only extinctform of Arthropods (invertebrate animals having jointed bodies and legs),and were relatedto the lobster and crab. Their closest living relative today is the horseshoe crab.The nameTrilobite refers to it's three lobes, or body sections, consisting of a central axial lobe, andtwo pleural lobes on the sides. The three main parts of it's body are called theCephalon,(head) the Thorax (body), and the Pygidium (tail). There are 8 Main Orders oftrilobites, and over 15,000 different species, many of which have a stratigraphic value,especially those found within Cambrian and Ordovician rocks. It is the fact that trilobitesare found world wide which makes some of them serve as excellent index fossils, and it istheir short life periods which allows precise dates and correlations to be made about therocks in which they are found. Most trilobites were sea floor dwellers, living in the soft sediments, while otherswere free swimming. They appear to have been exclusively marine organisms since thefossilized remains of trilobites are always found in rock containing fossils of othersalt-water animals, e.g. brachiopods, crinoids, and corals. Trilobites ranged in size from1/4 of an inch,and smaller, to almost 3 feet long. Trilobites were also the first creatures todevelop eyes and sensory organs. Trilobites had two compound eyes. In some trilobites,the eyes had densely packed lenses and may have served merely as a light sensitivewarning device to detect movement. In other trilobites, the eyes had fewer and morecomplex lenses and may have been capable of forming images and perceiving depth.Trilobites lived in shelf and slope environments around continental margins and in theshallow continental seas ...
