Trilobites ("three-lobes") are extinct arthropods that form the class Trilobita. They appeared in the Early 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 (m.y.a.).
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. Trilobites have been important in biostratigraphy, paleontology, and plate tectonics research. For example, trilobites have been important in estimating the rate of speciation during the period known as the Cambrian Explosion because they are the most diverse group of metazoans known from the fossil record of the early Cambrian, and are readily distinguishable because of complex and well preserved morphologies. The trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata), although several alternative taxonomies are found in the literature.
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. Most life styles expected of modern marine arthropods are seen, except for parasitism. Some trilobites (particularly the family Olenida) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.
As might be expected for a group of animals comprising 1,500+ genera and 17,000+ species the morphology and description of trilobites can be complex. "Trilobite" (meaning "three-lobed") is named for the three longitudinal lobes: a central axial lobe, and two symmetrical pleural lobes that flank the axis (Fig 1). The trilobite body is divided into three major sections, a cephalon (head) with eyes, mouth parts and antennae, a thorax of multiple similar segments (that in some species allowed enrollment), and a pygidium (tail), see Fig 2. When describing differences between different taxa of trilobites, the presence, size, and shape of the cephalic features are often mentioned and shown in Figs 3 & 4.
Trilobites range in length from 1 mm to 72 cm (1/25 inch to 28 inches), with a typical size range of 3 to 10 cm (1 to 4 inches). The world's largest trilobite, Isotelus rex, was found in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay.
The exoskeleton is composed of calcite and calcium phosphate minerals in a protein lattice of chitin that covers the upper surface (dorsal) of the trilobite and curled round to produce a small fringe beneath called the doublure. The exoskeleton of trilobites are divided into three parts (tagmata): a cephalon, composed of the two preoral and first four postoral segments completely fused together; a thorax composed of freely articulating segments; and a pygidium composed of the last segments fused together with the telson.
During molting, the exoskeleton generally split between the head and thorax, which is why so many trilobite fossils are missing one or the other. In most groups there were facial sutures on the cephalon to facilitate molting.
Fossilised trilobites are often found enrolled (curled up) like modern woodlice for protection. Some trilobites achieved a fully closed capsule (e.g. Phacops) while others with long pleural spines (e.g. Selenopeltis) or a small pygidium (e.g. Paradoxides) left a gap at the sides or between the cephalon and pygidium . Even in an Agnostid, with only 2 articulating thoracic segments, the process of enrollment required a complex musculature to contract the exoskeleton and return to the flat condition. In Phacops the pleurae overlap a smooth bevel (facet) allowing a close seal with the doublure. The doublure carries a panderian notch or protuberance on each segment to prevent over rotation of each segment. Long lateral muscles extended from the cephalon to mid way down the pygidium, attaching to the axial rings allowing enrollment while separate muscles on the legs tucked them out of the way.
Some trilobites had horns on their heads similar to those of modern beetles. Based on the size, location, and shape of the horns the most likely use of the horns was combat for mates, making the Asaphida family Raphiophoridae the earliest exemplars of this behavior. A conclusion likely to be applicable to other trilobites as well, such as in the Phacopid trilobite genus Walliserops that developed spectacular tridents.
The trilobite eyes were typically compound, with each lens being an elongated prism. The number of lenses in such an eye varied: some trilobites had only one, while some had thousands of lenses in a single eye. In these compound eyes, the lenses were typically arranged hexagonally. The fossil record of trilobite eyes is complete enough that their evolution can be studied through time, which compensates to some extent the lack of preservation of soft internal parts.
The lenses of trilobites eyes were made of calcite (calcium carbonate, CaCO3). Pure forms of calcite are transparent, and some trilobites used crystallographically oriented, clear calcite crystals 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 French scientist René Descartes and Dutch physicist Christiaan Huygens many millions of years later. A living species with similar lenses is the brittle star Ophiocoma wendtii. In other trilobites, with a Huygens interface apparently missing, a gradient index lens is invoked with the refractive index of the lens changing towards the center.
Holochroal eyes had a great number (sometimes over 15,000) of small (30-100μm, rarely larger lenses.) Lenses were hexagonally close packed, touching each other, with a single corneal membrane covering all lenses. Holochroal eyes had no sclera, the white layer covering the eyes of most modern arthropods. Holochroal eyes are by far the commonest amongst trilobites, found in all orders and all ages from Cambrian to Permian. Little is known of the early history of holochroal eyes; adult Lower and Middle Cambrian trilobites rarely preserve the visual surface.
Schizochroal eyes typically had fewer (to around 700), larger lenses, and are found only in Phacopida. Lenses were separate, with each lens having an individual cornea which extended into a rather large sclera. Schizochroal eyes appear quite suddenly in the early Ordovician, and were presumably derived from a holochroal ancestor. Field of view (all around vision) and coincidental development of more efficient enrollment mechanisms point to the eye as a more defensive "early warning" system than directly aiding in the hunt for food.
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.
Secondary blindness is not uncommon, particularly in long lived groups such as the Agnostida and Trinucleioidea. In Proetida, Phacopina and Tropidocoryphinae there are well studied trends showing progressive eye reduction between closely related species that eventually leads to blindness.
Several other structures on trilobites have been explained as photo-receptors. Of particular interest are the small areas of thinned cuticle on the underside of the hypostome (macula) which, in some trilobites, are suggested to be simple ventral eyes that could have detected night and day or allowed a trilobite to navigate while swimming (or turned) upside down.
Trilobite development was unusual in the way in which articulations developed between segments, and changes in the development of articulation gave rise to the conventionally recognized developmental phases of the trilobite life cycle (divided into 3 stages), which are not readily compared with those of other arthropods. Actual growth and change in external form of the trilobite would have occurred when the trilobite was soft shelled, following molting and before the next hard exoskeleton.
Trilobite larvae are known from the Cambrian to the Carboniferous and from all sub-orders. As instars from closely related taxa are more similar than instars from distantly related taxa, trilobite larvae provide morphological information important in evaluating high-level phylogenetic relationships among trilobites.
Trilobites are thought to have reproduced sexually, producing eggs, albeit without undoubted examples in the fossil record. Some species may have kept eggs or larvae in a brood pouch forward of the glabella, particularly when the ecological niche was particularly challenging to larvae. Size and morphology of the first calcified stage are highly variable between (but not within) trilobite taxa, suggesting some trilobites passed through more growth within the egg than others. Early developmental stages prior to calcification of the exoskeleton are a possibility, but so is calcification and hatching coinciding.
The earliest post-embryonic trilobite growth stage known with certainty are the protaspid stages. Starting with an indistinguishable proto-cephalon and proto-pygidium (anaprotaspid) a number of changes occur ending with a transverse furrow separating the proto-cephalon and proto-pygidium (metaprotaspid) that can continue to add segments. Segments are added at the posterior part of the pygidium but, all segments remain fused together. The meraspid phase of development is marked by the appearance of an articulation between the head and the fused trunk. At the onset of the meraspid phase the animal had a two-part structure - the head and the plate of fused trunk segments, the pygidium. During the meraspid phase, new segments appeared near the rear of the pygidium as additional articulations developed at the anterior of the pygidium, releasing freely articulating thoracic segments. Segments are generally added one per molt (although two per molt and one every alternate molt are also recorded), with number of stages equal to the number of thoracic segments. A substantial amount of growth, from less than 25% up to 30-40%, probably took place in the meraspid stages.
The holaspid phase of growth commenced when a stable, mature number of segments had been released into the thorax. Molting continued during the holaspid stage, with no changes in thoracic segment number. Onset of the holaspid phase and the epimorphic phase was coincident in some, but not all, trilobites.
Some trilobites showed a marked transition in morphology at one particular instar, which has been called trilobite metamorphosis. Radical change in morphology is linked to the loss or gain of distinctive features that mark a change in mode of life. A change in lifestyle during development has significance in terms of evolutionary pressure, as the trilobite could pass through several ecological niches on the way to adult development and changes would strongly affect survivor-ship and dispersal of trilobite taxa. It is worth noting that trilobites with all protaspid stages planktonic and meraspid stages benthic (e.g. Asaphids) failed to last through the Ordovician extinctions, while trilobites that were planktonic for only the first protaspid stage before metamorphosing into benthic forms survived (e.g. Lichids, Phacopids).
After the mid-Cambrian extinction event, the next great extinction event occurred at the Frasnian - Famennian boundary at the end of the Devonian period. All orders (except one) of Trilobites became extinct. Trilobites were bottlenecked into one single order, the Proetida. This single order survived for millions of years, continued through the Carboniferous period and lasted to the great extinction event at the end of the Permian (where the vast majority of species on earth were wiped out). It is unknown why Order Proedita alone, survived. It may have been a deeper water order that was able to avoid rapid changes that would affect species along the continental shelves. For many millions of years, the Proetida found a perfect niche. An anology would be today's crinoids which exist as deep water species only. In the Paleozoic era, vast 'forests' of crinoids lived in shallow near shore environments.
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.
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 often preserved as trace fossils. These same trace fossils are also occasionally found in freshwater environments, suggesting either that some freshwater trilobites existed, or that the tracks are also made by other organisms. Trilobites, like brachiopods, crinoids, and corals, are found on all modern continents, and occupied every ancient ocean from which Paleozoic fossils have been collected.
Trilobite fossils are found worldwide, with many thousands of known species. Because they appeared quickly in geological time, and moulted like other arthropods, 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. This trilobite is featured on the town's coat of arms and was named the Dudley Bug or Dudley Locust by quarrymen who once worked the now abandoned limestone quarries. Other trilobites found there include Dalmanites, Trimerus, Bumastus and Balizoma. Llandrindod Wells, Powys, Wales, is another famous trilobite location. The well known Elrathia kingi trilobite is found in spectacular abundance in the Wheeler Shale (Cambrian) of west-central Utah.
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.