Trilobites - (pronounced TRY-lo-bites)

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

Trilobites are very well-known and possibly 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 geographically dispersed.

 

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

Different trilobites made their living in different ways. Some led a benthic life as predators, scavengers or filter feeders. Some swam (a pelagic lifestyle) and fed on plankton. Still others (particularly the family Olenidae) are thought to have evolved a symbiotic relationship 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 freely articulating segments; and a pygidium (tail) composed of the last segments fused together with the telson.

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

Although trilobites were only armoured on top, they still had a fairly heavy exoskeleton, composed 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 characteristic of all trilobite eyes. In these compound eyes, the lenses were typically arranged hexagonally.

Some trilobites such as those of the order Lichida evolved elaborate spiny forms, from the Ordovician until the end of the Devonian period. These spiny forms could possibly have been a defensive response to the evolutionary appearance of fish.

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

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

Scientific Classification:

Kingdom: Animalia

Phylum: Arthropoda

Class: Trilobita

Orders:
Agnostida
Nectaspida
Redlichiida
Corynexochida
Lichida
Phacopida
Subclass: Librostoma

Proetida
Asaphida
Harpetida
Ptychopariida