Trilobites
http://www.rareresource.com

Trilobites are extinct arthropods that appeared in the 2nd Epoch of the Cambrian era and flourished throughout the lower Paleozoic period before beginning a drawn-out decline to extinction when, during the Late Devonian extermination, all trilobite orders, with the sole omission of Proetida, died out. The most recent of the trilobites disappeared in the mass extinction at the end of the Permian about 250 million years back.

Trilobites are very well-known and probably the second-most famous fossil group after the dinosaurs. When trilobites appear in the fossil record of the Lower Cambrian they are already highly diverse and biologically dispersed.

Because of their diversity and an effortlessly fossilized exoskeleton, they left an wide fossil record with some 17,000 known species spanning Paleozoic time.

Different trilobites made their living in dissimilar ways. Some led a benthic existence as predators, scavengers or filter feeders. Some swam (a pelagic way of life) and fed on plankton. Still others (particularly the family Olenidae) are thought to have evolved a symbiotic connection with sulfur-eating bacteria from which they derived food.

The bodies of trilobites are divided into three parts (tagmata): a cephalon (head), a thorax composed of liberally articulating segments; and a pygidium (tail) composed of the final segments fused together with the telson.

Trilobites had a single duo of preoral antennae and otherwise undifferentiated biramous limbs. Every exopodite (walking leg) had six segments. The first section also bore a feather-like epipodite, or gill branch, which was used for respiration and, in some genus, swimming. The limbs were covered by tangential projections of the exoskeleton called pleural lobes, extending superficial from a central axial lobe.

though trilobites were only armoured on top, they still had a moderately heavy exoskeleton, calm of calcite and calcium phosphate minerals in a protein lattice of chitin.

The trilobite had complex, compound eyes with lenses made of calcite, a unique trait of all trilobite eyes. In these compound eyes, the lenses were classically arranged hexagonally.

Some trilobites such as those of the order Lichida evolved intricate spiny forms, from the Ordovician until the end of the Devonian era. These spiny forms could probably have been a defensive response to the evolutionary manifestation of fish.

Trilobites variety in length from one millimetre to 72 cm (1/25 inch to 28 inches), with a characteristic size range of two to seven centimetres (1 to 3½ inches). The world's biggest trilobite, Isotelus rex, was established in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay.

Trilobites appear to have been solely marine organisms, since the fossilized relics of trilobites are always establish in rocks containing fossils of other salt-water animals such as brachiopods, crinoids, and corals. Within the marine paleoenvironment, trilobites were found in a wide range from enormously shallow water to very deep water. The tracks left behind by trilobites crawling on the sea floor are infrequently preserved as trace fossils. Trilobites, like brachiopods, crinoids, and corals, are establish on all modern continents, and occupied every antique ocean from which fossil have been collected.

Scientific Classification:

Kingdom: Animalia

Phylum: Arthropoda

Class: Trilobita