Asaphiscus wheeleri, a trilobite
from Cambrian-age shale in Utah
Trilobites are extinct arthropods in the class Trilobita. They appeared in the Cambrian period and flourished throughout the lower Paleozoic era before slowly declining to extinction. The last of the trilobites disappeared in the mass extinction at the end of the Permian 250 million years ago (m.y.a.).
Trilobites are very well-known, and possibly the second-most famous fossil group after the dinosaurs. They are the most diverse group of animal species preserved in the fossil record, consisting of nine (or possibly ten) orders and over 15,000 species. The trilobites are currently included along with the Chelicerata in the group Arachnomorpha.
Most were simple, small marine creatures that walked along the seafloor and filtered mud to obtain food.
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. The pygidia are still fairly rudimentary in the most primitive trilobites. The thorax is fairly flexible—fossilised trilobites are often found curled up like modern woodlice for protection.
Trilobites had a single pair of preoral antennae and otherwise undifferentiated biramous limbs. Each exopodite (walking leg) had six segments, analogous to those of other early arthropods. The first segment also bore a feather-like 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. Contrary to popular belief, it is this longitudinal tripartite division into left and right pleural lobes and a central axial lobe that gives trilobites their name, not the division into cephalon, thorax and pygidium.
Although trilobites were only armored on top, they still had a fairly heavy exoskeleton, composed of calcite and calcium phosphate minerals in a protein lattice of chitin. Unlike other groups of armored arthropods, which resorb most of their skeletal minerals prior to molting, a trilobite would cast off a fully mineralized moult. Thus a single trilobite animal could potentially have left multiple well-mineralized skeletons behind -- further enhancing the apparent abundance of trilobites in the fossil record. During moulting, the exoskeleton 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 moulted exoskeletons rather than dead trilobites. In most groups there were two facial sutures on the cephalon to make shedding easier. The cheeks of the cephalon 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 such as those of the order Lichida evolved elaborate spiny forms, from the Ordovician until the end of the Devonian period. Examples of these specimens have been found in the Hamar Laghdad formation of Alnif in Morocco. Collectors of this material should be aware of a serious counterfeiting and fakery problem with much of the Moroccan material that is offered commercially. Spectacular spined trilobites have also been found in western Russia; Oklahoma, USA; and Ontario, Canada. These spiny forms could possibly have been a defensive response to the evolutionary appearance of fish.
According to New Scientist magazine (May 2005), "some... trilobites... had horns on their heads similar to those of modern beetles." Based on the size, location, and shape of the horns, Rob Knell, a biologist at Queen Mary, University of London and Richard Fortey of London's Natural History Museum, concluded that the most likely use of the horns was combat for mates, making trilobites the earliest exemplars of this behavior. While this study only covered the raphiophorid family, the conclusions can be applied to other groups as well, such as the tridentate phacopide trilobite Walliserops trifurcatus.
Trilobites range in length from one millimeter 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.
Many trilobites had eyes; they also had antennae that perhaps were used for taste and smell. Some trilobites were blind, probably living too deep in the sea for light to reach them. Others, such as Phacops rana, had eyes that were quite large.
The eyes of trilobites were made of calcite (calcium carbonate, CaCO3). Pure forms of calcite are transparent, and some trilobites used a single crystallographically oriented, clear calcite crystal to form each lens of each of their eyes. In this, they differ from most other arthropods, which have soft or chitin-supported eyes. The rigid calcite lenses of a trilobite eye would have been unable to accommodate to a change of focus like the soft lens in a human eye would; however, in some trilobites the calcite formed an internal doublet structure, giving superb depth of field and minimal spherical aberration, as rediscovered by Dutch physicist Christiaan Huygens many millions of years later. A living species with similar lenses is the brittle star Ophiocoma wendtii.
The trilobite eyes were typically compound, with each lens being an elongated prism. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in a single eye. In these compound eyes, the lenses were typically arranged hexagonally.
Holochroal eyes had a great number of (tiny) lenses (sometimes over 15,000), and are found in all orders of trilobite. These lenses were packed closely together (hexagonally) and touch each other. A single corneal membrane covered all lenses. These eyes had no sclera, the white layer covering the eyes of most modern arthropods.
Schizochroal eyes typically had fewer (and larger) lenses (to around 700), and are found only in Phacopida. The lenses were separate, with each lens having an individual cornea which extended into a rather large sclera.
Abathochroal eyes had around 70 small lenses, and are found only in Cambrian Eodiscina. Each lens was separate and had an individual cornea. The sclera was separate from the cornea, and did not run as deep as the sclera in schizochroal eyes.
An egg hatched to give a tiny larva called a protaspid, in which all segments are fused into a single carapace. Subsequent thoracic segments were added just ahead of the pygidium ("pygidial release") 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. In many species, molting continued during the holaspid stage with no changes in segment number. Trilobite larvae are reasonably well known and provide an important aid in evaluating high-level phylogenetic relationships among trilobites.
When describing differences between different taxa of trilobites, the presence, size, and shape of the cephalic features above are often mentioned.
Figure 1 shows gross morphology of the cephalon. The cheeks (genae) are the pleural lobes on each side of the axial feature, the glabella. When trilobites molted or died, the librigenae (the so-called "free cheeks") often separated, leaving the cranidium (glabella + fixigenae) exposed. Figure 2 shows a more detailed view of the cephalon.
Based on morphological similarities, it is possible that the trilobites have their ancestors in arthropod-like creatures such as Spriggina, Parvancorina, and other trilobitomorphs of the Ediacaran period of the Precambrian. There are many morphological similarities between early trilobites and other Cambrian arthropods known from the Burgess Shale and other fossiliferous locations. These are investigated further here:  It is reasonable to assume that the trilobites share a common ancestor with these other arthropods prior to the Ediacaran-Cambrian boundary.
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 fish in the Silurian and Devonian periods with their strong, hinged jaw. Trilobites may have provided a rich source of food for these new arrivals.
Additionally, their relatively low numbers and diversity at the end of the Permian no doubt contributed to their extinction during that great mass extinction event. Foreshadowing this, the Ordovician mass extinction, though somewhat less substantial than the Permian one, also seems to have significantly narrowed trilobite diversity.
The closest extant relatives of trilobites may be the cephalocarids.(Lambert, 63)
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.
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. Some Native Americans, recognizing that trilobites were water creatures, had a name for them which means "little water bug in the rocks".
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 Group 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. Other trilobites found there include Dalmanites, Trimerus and Bumastus.
Spectacular trilobite fossils, showing soft body parts like legs, gills and antennae, have been found in British Columbia (Burgess Shale Cambrian fossils, and similar localities in the Canadian Rockies); New York State (Odovician Walcott-Rust Quarry, near Utica, N.Y., and the Beecher Trilobite Beds, near Rome, N.Y.), in China (Burgess Shale-like Lower Cambrian trilobites in the Maotianshan shales near Chengjiang), Germany (the Devonian Hunsrück Slates near Bundenbach, Germany) and, much more rarely, in trilobite-bearing strata in Utah and Ontario.
Trilobites are collected commercially in Russia (especially in the St. Petersburg area), Germany, Morocco's Atlas Mountains, (where a burgeoning trade in faked trilobites is also under way), Utah, Ohio, British Columbia, and in other parts of Canada.
- Trilobite! - Richard Fortey (ISBN 0-00-257012-2)
- David Lambert and the Diagram Group. The Field Guide to Prehistoric Life. New York: Facts on File Publications, 1985. ISBN 0-8160-1125-7
- Riccardo Levi-Setti. Trilobites. University of Chicago Press, 1993.
- A Guide to the Orders of Trilobite by Sam Gon III - an excellent, well-researched site with information covering trilobites from all angles. Includes many line drawings and photographs.
- Earliest combatants in sexual contests revealed from "New Scientist" magazine.
- The Trilobite papers
Other uses of the name
The name "trilobite" is sometimes used for the Moon-shuttles in UFO (TV series), because of their shape.