Agnostida
Redlichiida
Corynexochida
Lichida
Nektaspida?
Phacopida
Proetida
Asaphida
Harpetida
Ptychopariida

Trilobites are extinct arthropods in the class Trilobita. They appeared in the Cambrian era and flourished throughout the lower Palaeozoic before slowly declining to extinction. The last of the trilobites disappeared in the mass extinction at the end of the Permian. Trilobites are well-known, possibly the second most famous fossil group after the dinosaurs, and are the most diverse group of animal species preserved in the fossil record, consisting of nine orders and over 15,000 species. Most were simple, small marine animals that filtered mud to obtain food.

Physical description

The bodies of trilobites are divided into three parts (tagmata): a cephalon (head), composed of the two preoral and first four postoral segments completely fused together; a thorax composed of freely articulating segments; and a pygidium (tail) composed of the last few segments fused together with the telson. In the most primitive trilobites the pygidia are still fairly rudimentary. The thorax is fairly flexible—trilobites are often found curled up like modern sow bugs for protection.

Trilobites had a single pair of preoral antennae and otherwise undifferentiated biramous limbs. Each exopodite (walking leg) had 6 segments, analogous to those of other early arthropods. The first segment also bore a featherlike epipodite, or gill branch, which was used for respiration and swimming. The limbs were covered by lateral projections called pleural lobes, extending outward from a central axial lobe; it is this tripartite division that gives trilobites their name.

Trilobites were only armored on top, but still had a fairly heavy exoskeleton. In molting it generally split between the head and thorax, which is why so many trilobite fossils are missing one or the other: many trilobite fossils are actually molts, rather than dead individuals. In most groups there were two facial sutures on the cheeks to make shedding easier. The cheeks usually also supported a pair of crescent-shaped compound eyes, which were surprisingly advanced in some species. In fact, trilobites were the first animals to evolve true eyes, about 543 million years ago; the evolutionary appearance of eyes has been postulated as a trigger for the Cambrian Explosion.

Some trilobites evolved into elaborate spiny forms, particularly during the Devonian period. (For example, such specimens are found in the Devonian Hamar Laghdad formation of Alnif in Morocco).

Trilobites range in length from one millimetre to 70 cm (1/25 inch to 20 inches), with a typical size range of two to seven centimetres (1 to 3 1/2 inches).

Sensory organs

Many trilobites had eyes and they had antennae that perhaps were used for taste and smell. Some trilobites were, however, blind, especially those species living too deep in the sea for light to reach them.

The eyes of trilobites were made of calcite (calcium carbonate, CaCO₃). Pure forms of calcite are transparent, and the trilobites used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The trilobite eyes were typically compound, with each lens being an elongated prism. The number of lenses in such an eye varies, however: some trilobites had only one, and some had thousands of lenses in one eye. In these compound eyes, the lenses are arranged hexagonally (nature's way of close packing). A living species with similar eyes is the brittle star Ophiocoma wendtii.

Development

An egg hatched to give a tiny larva called a protaspid, in which only the fused segments of the cephalon are present. Subsequent thoracic segments were added behind the cephalon in successive molts during an intermediate stage called meraspid, until finally the adult number of segments was reached, at which point the animal is called a holaspid. Trilobite larvae are reasonably well known and provide an important aid in their classification.

Extinction

The exact reason for the extinction of the trilobites is not clear, although it would seem to be no coincidence that their numbers began to decrease with the arrival of the first sharks and other early fishes in the Silurian and Devonian periods. Trilobites may have provided a rich source of food for these new arrivals. Their relatively low numbers and diversity at the end of the Permian no doubt contributed to their extinction during that great mass extinction event.

Fossil distribution

Trilobites appear to have been exclusively marine organisms since the fossilized remains of trilobites are always found in rock containing fossils of other salt-water animals such as brachiopods, crinoids, and coral, and they are found in a range of environments 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.

Trilobite fossils

Trilobite fossils are found worldwide, with many thousands of known species. Because they evolved rapidly, trilobites serve as excellent index fossils, enabling geologists to date the age of the rocks in which they are found. They were among the first fossils to attract widespread attention, and new species are being discovered every year. The Native Americans had a name for trilobite which means "little water bug in the rocks" - a name which demonstrates extraordinary zoological acuity.

A famous location for trilobite fossils in the United Kingdom is Wren's Nest , Dudley in the West Midlands, where Calymene blumenbachi is found in the Silurian Wenlock Limestone formation. This trilobite is featured on the town's coat of arms and was named the "Dudley locust" or "Dudley bug" by quarrymen who once worked many of the now abandoned limestone quarries. The other trilobites found here include Dalmanites, Trimerus and Bumastus.

See also

• Phacops rana

Further reading

• Trilobite! - Richard Fortey (ISBN 0002570122)
• Riccardo Levi-Setti. Trilobites. University of Chicago Press, 1993.
• A Guide to the Orders of Trilobite