Trilobites (; meaning "three-lobed entities") are extinction marine that form the class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most successful of all early animals, existing in oceans for almost 270million years, with over 22,000 species having been described. Because trilobites had wide diversity and an easily mineralised exoskeleton made of calcite, they left an extensive fossil record. The study of their fossils has facilitated important contributions to biostratigraphy, paleontology, evolution, and plate tectonics. Trilobites are placed within the clade Artiopoda, which includes many organisms that are morphologically similar to trilobites, but are largely unmineralised. The relationship of Artiopoda to other arthropods is uncertain.
Trilobites evolved into many ecological niches; some moved over the seabed as , , or , and some swam, feeding on plankton. Some even crawled onto land. Most lifestyles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where scientific debate continues). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiosis relationship with sulfur-eating bacteria from which they derived food. The largest trilobites were more than long and may have weighed as much as .
The first appearance of trilobites in the fossil record defines the base of the Atdabanian time period of the Early Cambrian around . Trilobites were already diverse and globally dispersed shortly after their origination, with trilobites reaching an apex of diversity during the late Cambrian–Ordovician, and remained diverse during the following Silurian and early Devonian. During the mid-late Devonian, their diversity strongly declined, being impacted by successive extinction events, including the Taghanic event, the Late Devonian mass extinction/Kellwasser event and the Hangenberg event, wiping out most trilobite diversity and leaving Proetida as the only surviving order. Their diversity moderately recovered during the Early Carboniferous, before dropping to persistently low levels during the late Carboniferous and Permian periods, though they remained widespread until the end of their existence. The last trilobites disappeared in the end-Permian mass extinction event about 251.9million years ago, by which time only a handful of species remained.
The exact relationships of artiopods to other arthropods is uncertain. Some scholars consider them closely related to chelicerates (which include horseshoe crabs, Sea spider, and arachnids) as part of a clade called Arachnomorpha, while others consider them to be more closely related to Mandibulata (which contains insects, and myriapods) as part of a clade called Antennulata.
Cladogram of Artiopoda including trilobites after Berks et al. 2023.
All Olenellina lack facial sutures (see below), and this is thought to represent the original state. The earliest sutured trilobite found so far ( Lemdadella), occurs almost at the same time as the earliest Olenellina, suggesting the trilobites origin lies before the start of the Atdabanian, but without leaving fossils. Other groups show secondary lost facial sutures, such as all Agnostina and some Phacopina. Another common feature of the Olenellina also suggests this suborder to be the ancestral trilobite stock: early protaspid stages have not been found, supposedly because these were not calcified, and this also is supposed to represent the original state. Earlier trilobites may be found and could shed more light on their origins.
Three specimens of a trilobite from Morocco, Megistaspis hammondi, dated 478million years old contain fossilized soft parts. In 2024, researchers discovered soft tissues and other structures including the labrum in well-preserved trilobite specimens from Cambrian Stage 4 of Morocco, providing new anatomical information regarding the external and internal morphology of trilobites, and the cause of such extraordinary preservation is probably due to their rapid death after an underwater pyroclastic flow.
Effacement, the loss of surface detail in the cephalon, pygidium, or the thoracic furrows, is also a common evolutionary trend. Notable examples of this were the orders Agnostida and Asaphida, and the suborder Illaenina of the Corynexochida. Effacement is believed to be an indication of either a burrowing lifestyle or a pelagic one. Effacement poses a problem for since the loss of details (particularly of the glabella) can make the determination of phylogenetics relationships difficult.
Notable trilobite genera appearing in the Cambrian include:
Some of the genera of Trilobites appearing in the Ordovician include:
The Middle-Late Devonian was a decisive turning point in trilobite history, with the Taghanic event during the Givetian sharply decreasing trilobite diversity, particularly in shallow water environments, which was followed by the Late Devonian mass extinction/Kellwasser event (involving a combination of sea level change and Anoxic event) at the Frasnian–Famennian boundary, widely regarded as one of the most significant mass extinction events in Earth's history, decimating the groups diversity including the extinction of the orders Corynexochida, Harpetida and Odontopleurida, with the low trilobite diversity in its aftermath in the Famennian, consisting only of the orders Phacopida and Proetida, being again strongly impacted by the Hangenberg event (also called the end-Devonian mass extinction) at the end of the Devonian, with both shallow water and deep water trilobites being affected. Only a single order, the Proetida, survived into the Carboniferous.
Genera of trilobites during the Silurian and Devonian periods include:
By the end of the Carboniferous, the diversity of trilobites had dropped to only 1.8–2.2% (around 7 genera) of the peak diversity it had had during the early Paleozoic, with this low diversity continuing into the Permian. During the Permian period, while trilobites were widespread and occurred in a variety of environments, they were typically rare components of local faunas, in sharp contrast to their often great abundance earlier in the Paleozoic. Permian trilobite diversity reached a peak during the Guadalupian with diversity sharply dropping by the beginning of the following Lopingian.
Some of the genera of trilobites during the Carboniferous and Permian periods include:
Exactly why the trilobites became extinct is not clear; with repeated extinction events (often followed by apparent recovery) throughout the trilobite fossil record, a combination of causes is likely. After the extinction event at the end of the Devonian period, what trilobite diversity remained was bottlenecked into the order Proetida. Decreasing diversity of genera limited to shallow-water shelf habitats coupled with a drastic lowering of sea level (regression) meant that the final decline of trilobites happened shortly before the end Permian mass extinction event. With so many marine species involved in the Permian extinction, the end of nearly 300million successful years for the trilobites would not have been unexpected at the time.
There are three main forms of trace fossils associated with trilobites: Rusophycus, Cruziana and Diplichnites—such trace fossils represent the preserved life activity of trilobites active upon the sea floor. Rusophycus, the resting trace, are trilobite excavations involving little or no forward movement and ethological interpretations suggest resting, protection and hunting. Cruziana, the feeding trace, are furrows through the sediment, which are believed to represent the movement of trilobites while deposit feeding. Many of the Diplichnites fossils are believed to be traces made by trilobites walking on the sediment surface. Care must be taken as similar trace fossils are recorded in freshwater and post-Paleozoic deposits, representing non-trilobite origins.
Trilobite fossils are found worldwide, with 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.
In the United States, the best open-to-the-public collection of trilobites is located in Hamburg, New York. The shale quarry, informally known as Penn Dixie, stopped mining in the 1960s. The large amounts of trilobites were discovered in the 1970s by Dan Cooper. As a well-known rock collector, he incited scientific and public interest in the location. The fossils are dated to the Givetian (387.2–382.7million years ago) when the Western New York Region was 30 degrees south of the equator and completely covered in water. The site was purchased from Vincent C. Bonerb by the Town of Hamburg with the cooperation of the Hamburg Natural History Society to protect the land from development. In 1994, the quarry became Penn Dixie Fossil Park & Nature Reserve when they received 501(c)3 status and was opened for visitation and collection of trilobite samples. The two most common found samples are Eldredgeops rana and Greenops.
A famous location for trilobite fossils in the United Kingdom is Wren's Nest, Dudley, in the West Midlands, where Calymene blumenbachii 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. Llandrindod Wells, Powys, Wales, is another famous trilobite location. The well-known Elrathia trilobite is found in abundance in the Cambrian Wheeler Shale of Utah.
Spectacularly preserved trilobite fossils, often showing soft body parts (legs, gills, antennae, etc.) have been found in British Columbia, Canada (the Cambrian Burgess Shale and similar localities); New York, US (Ordovician Walcott–Rust quarry, near Russia, New York, and Beecher's Trilobite Bed, near Rome, New York); China (Lower Cambrian Maotianshan Shales near Chengjiang); Germany (the Devonian Hunsrück Slates near Bundenbach) and, much more rarely, in trilobite-bearing strata in Utah (Wheeler Shale and other formations), Ontario, and Manuels River, Newfoundland and Labrador.
Sites in Morocco also yield very well-preserved trilobites, many buried in mudslides alive and so perfectly preserved. An industry has developed around their recovery, leading to controversies about practices in restoral. The variety of eye and upper body forms and fragile protuberances is best known from these samples preserved similarly to bodies in Pompeii.
The French palaeontologist Joachim Barrande (1799–1883) carried out his landmark study of trilobites in the Cambrian, Ordovician and Silurian of Bohemia, publishing the first volume of Système silurien du centre de la Bohême in 1852.
Identification of the 'Atlantic' and 'Pacific' trilobite faunas in North America and Europe implied the closure of the Iapetus Ocean (producing the Iapetus suture), thus providing important supporting evidence for the theory of continental drift.
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 animal known from the fossil record of the early Cambrian.
Trilobites are excellent stratigraphic markers of the Cambrian period: researchers who find trilobites with alimentary prosopon, and a micropygium, have found Early Cambrian strata. Most of the Cambrian stratigraphy is based on the use of trilobite marker fossils.
Trilobites are the state fossils of Ohio ( Isotelus), Wisconsin ( Calymene celebra) and Pennsylvania ( Phacops rana).
Over 22,000 species of trilobite have been described.
Despite their rich fossil record with thousands of described genus found throughout the world, the taxonomy and phylogeny of trilobites have many uncertainties. Except possibly for the members of the orders Phacopida and Lichida (which first appear during the early Ordovician), nine of the eleven trilobite orders appear prior to the end of the Cambrian. Most scientists believe that order Redlichiida, more specifically its suborder Redlichiina, contains a common ancestor of all other orders, with the possible exception of the Agnostina. While many potential phylogenies are found in the literature, most have suborder Redlichiina giving rise to orders Corynexochida and Ptychopariida during the Lower Cambrian, and the Lichida descending from either the Redlichiida or Corynexochida in the Middle Cambrian. Order Ptychopariida is the most problematic order for trilobite classification. In the 1959 Treatise on Invertebrate Paleontology, what are now members of orders Ptychopariida, Asaphida, Proetida and Harpetida were grouped together as order Ptychopariida; subclass Librostoma was erected in 1990 to encompass all of these orders, based on their shared ancestral character of a natant (unattached) hypostome. The most recently recognized of the nine trilobite orders, Harpetida, was erected in 2002. The progenitor of order Phacopida is unclear.
Trilobites range in length from minute at less than to very large at over , with an average size range of . Supposedly the smallest species is Acanthopleurella with a maximum of . The world's largest-known trilobite specimen, assigned to Isotelus is in length. It was found in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay. However, a partial specimen of the Ordovician trilobite Hungioides bohemicus found in 2009 in Arouca, Portugal is estimated to have measured when complete in length.
Only the upper (dorsal) part of their exoskeleton is mineralized, composed of calcite and calcium phosphate minerals in a lattice of chitin, and is curled round the lower edge to produce a small fringe called the "doublure". Their appendages and soft underbelly were non-mineralized. Cambrian Ocean World: Ancient Sea Life of North America Dynamic Paleontology: Using Quantification and Other Tools to Decipher the History of Life Three distinctive tagmata (sections) are present: cephalon (head); thorax (body) and pygidium (tail).
During ecdysis, the exoskeleton generally splits between the head and thorax, which is why so many trilobite fossils are missing one or the other. In most groups facial sutures on the cephalon helped facilitate moulting. Similar to and , trilobites would have physically "grown" between the moult stage and the hardening of the new exoskeleton.
Hypostome morphology is highly variable; sometimes supported by an un-mineralised membrane (natant), sometimes fused onto the anterior doublure with an outline very similar to the glabella above (conterminant) or fused to the anterior doublure with an outline significantly different from the glabella (impendent). Many variations in shape and placement of the hypostome have been described. The size of the glabella and the lateral fringe of the cephalon, together with hypostome variation, have been linked to different lifestyles, diets and specific .
The anterior and lateral fringe of the cephalon is greatly enlarged in the Harpetida, in other species a bulge in the pre-glabellar area is preserved that suggests a brood pouch. Highly complex compound eyes are another obvious feature of the cephalon.
All species assigned to the suborder Olenellina, that became extinct at the very end of the Early Cambrian (like Fallotaspis, Nevadia, Judomia, and Olenellus) lacked facial sutures. They are believed to have never developed facial sutures, having pre-dated their evolution. Because of this (along with other primitive characteristics), they are thought to be the earliest ancestors of later trilobites.
Some other later trilobites also lost facial sutures secondarily. The type of sutures found in different species are used extensively in the taxonomy and phylogeny of trilobites.
Trilobite facial sutures on the dorsal side can be roughly divided into five main types according to where the sutures end relative to the angle (the edges where the side and rear margins of the cephalon converge).
The primitive state of the dorsal sutures is proparian. Opisthoparian sutures have developed several times independently. There are no examples of proparian sutures developing in Taxon with opisthoparian ancestry. Trilobites that exhibit opisthoparian sutures as adults commonly have proparian sutures as instars (known exceptions being Yunnanocephalus and Duyunaspis). Hypoparian sutures have also arisen independently in several groups of trilobites.
The course of the facial sutures from the front of the visual surface varies at least as strongly as it does in the rear, but the lack of a clear reference point similar to the genal angle makes it difficult to categorize. One of the more pronounced states is that the front of the facial sutures do not cut the lateral or frontal border on its own, but coincide in front of the glabella, and cut the frontal border at the midline. This is, inter alia, the case in the Asaphida. Even more pronounced is the situation that the frontal branches of the facial sutures end in each other, resulting in yoked free cheeks. This is known in Triarthrus, and in the Phacopidae, but in that family the facial sutures are not functional, as can be concluded from the fact that free cheeks are not found separated from the cranidium.
There are also two types of sutures in the dorsal surface connected to the of trilobites. They are:
During molting in trilobites like Paradoxides, the rostrum is used to anchor the front part of the trilobite as the cranidium separates from the librigena. The opening created by the arching of the body provides an exit for the molting trilobite.
It is absent in some trilobites like Lachnostoma.
Each segment consists of the central axial ring and the outer pleurae, which protected the limbs and gills. The pleurae are sometimes abbreviated or extended to form long spines. Apodemes are bulbous projections on the ventral surface of the exoskeleton to which most leg muscles attached, although some leg muscles attached directly to the exoskeleton. Determining a junction between thorax and pygidium can be difficult and many segment counts suffer from this problem.
Some trilobites achieved a fully closed capsule (e.g. Phacops), while others with long pleural spines (e.g. Selenopeltis) left a gap at the sides or those with a small pygidium (e.g. Paradoxides) left a gap between the cephalon and pygidium. 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 and achieve a good seal. 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.
Some trilobites had horns on their heads similar to several modern beetles. Based on the size, location, and shape of the horns it has been suggested that these horns may have been used to combat for mates. Horns were widespread in the family Raphiophoridae (Asaphida). Another function of these spines was protection from predators. When enrolled, trilobite spines offered additional protection. This conclusion is likely to be applicable to other trilobites as well, such as in the Phacopida trilobite genus Walliserops, that developed spectacular tridents.
The fragments are indicative of durophagous predation (shell crushing). As the composition of the shells found were not taxonomically significant, rather based on physical properties regarding the shell strength and size, B. incola was opportunistic for food classifying feeding habits to be similar to scavengers. The remains of shells address another digestive aspect of B. incola, in the enzymatic ways in which these indigestible shells were siphoned out of little nutrition leaving only fragments behind. These remnants build on the concept of early Trilobites potentially having glands that secrete enzymes that aid in the digestive process.
Trilobite eyes were typically compound eye, 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 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 for the lack of preservation of soft internal parts.
Lenses of trilobites' Arthropod eye 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 eye. Rigid calcite lenses would have been unable to accommodate to a change of focus like the soft lens in a human eye would; in some trilobites, the calcite formed an internal doublet structure, giving superb depth of field and minimal spherical aberration, according to optical principles discovered by French scientist René Descartes and Dutch physicist Christiaan Huygens in the 17th century. 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 toward the center.
Sublensar sensory structures have been found in the eyes of some Phacopida trilobites. The structures consist of what appear to be several sensory cells surrounding a rhadomeric structure, resembling closely the sublensar structures found in the eyes of many modern arthropod , especially Limulus, a genus of horseshoe crabs.
Secondary blindness is not uncommon, particularly in long lived groups such as the Agnostida and Trinucleioidea. In Proetida and Phacopina from western Europe and particularly Tropidocoryphinae from France (where there is good stratigraphic control), 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 "macula", the small areas of thinned cuticle on the underside of the hypostome. In some trilobites macula are suggested to function as 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 three stages), which are not readily-comparable 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 moulting and before the next exoskeleton hardened. 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.
Despite the absence of supporting fossil evidence, their similarity to living arthropods has led to the belief that trilobites multiplied sexually and produced eggs. Some species may have kept eggs or larvae in a brood pouch forward of the glabella, particularly when the ecological niche was 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 (suggested for fallotaspids), but so is calcification and hatching coinciding.
The earliest post-embryonic trilobite growth stage known with certainty are the "protaspid" stages (anamorphic phase). 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" stages (anamorphic phase) are marked by the appearance of an articulation between the head and the fused trunk. Prior to the onset of the first meraspid stage the animal had a two-part structure—the head and the plate of fused trunk segments, the pygidium. During the meraspid stages, new segments appeared near the rear of the pygidium as well as additional articulations developing at the front of the pygidium, releasing freely articulating segments into the thorax. Segments are generally added one per moult (although two per moult and one every alternate moult 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" stages (epimorphic phase) commence when a stable, mature number of segments has been released into the thorax. Moulting continued during the holaspid stages, with no changes in thoracic segment number. Some trilobites are suggested to have continued moulting and growing throughout the life of the individual, albeit at a slower rate on reaching maturity.
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 on the way to adult development and changes would strongly affect survivorship and dispersal of trilobite taxa. It is worth noting that trilobites with all protaspid stages solely planktonic and later 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). Pelagic larval life-style proved ill-adapted to the rapid onset of global climatic cooling and loss of tropical shelf habitats during the Ordovician.
There is no evidence that trilobites reabsorbed their exoskeletons during moulting. Some authors have argued that the failure of trilobites to reabsorb their mineralised exoskeletons when they moulted was a functional disadvantage when compared to modern arthropods that generally do reabsorb their cuticles, as it took substantially longer to reconstruct their exoskeletons, making them more vulnerable to predators.
The discovery of Calymene blumenbachii (the Dudley locust) in 1749 by Charles Lyttleton, could be identified as the beginning of trilobite research. Lyttleton submitted a letter to the Royal Society of London in 1750 concerning a "petrified insect" he found in the "limestone pits at Dudley". In 1754, Manuel Mendez da Costa proclaimed that the Dudley locust was not an insect, but instead belonged to "the crustaceous tribe of animals". He proposed to call the Dudley specimens Pediculus marinus major trilobos (large trilobed marine louse), a name which lasted well into the 19th century. German naturalist Johann Walch, who executed the first inclusive study of this group, proposed the use of the name "trilobite". He considered it appropriate to derive the name from the unique three-lobed character of the central axis and a pleural zone to each side. Written descriptions of trilobites date possibly from the third century BC and definitely from the fourth century AD. The Spanish geologists Eladio Liñán and Rodolfo Gozalo argue that some of the fossils described in Greek and Latin Lapidary as scorpion stone, beetle stone, and ant stone, refer to trilobite fossils. Less ambiguous references to trilobite fossils can be found in Chinese sources. Fossils from the Kushan formation of northeastern China were prized as inkstones and decorative pieces.
In the New World, American fossil hunters found plentiful deposits of Elrathia in western Utah in the 1860s. Until the early 1900s, the Ute people of Utah wore these trilobites, which they called pachavee (little water bug), as . A hole was bored in the head and the fossil was worn on a string. According to the Ute themselves, trilobite necklaces protect against bullets and diseases such as diphtheria. In 1931, Frank Beckwith uncovered evidence of the Ute use of trilobites. Travelling through the badlands, he photographed two that most likely represent trilobites. On the same trip he examined a burial, of unknown age, with a drilled trilobite fossil lying in the chest cavity of the interred. Since then, trilobite amulets have been found all over the Great Basin, as well as in British Columbia and Australia.
In the 1880s, archaeologists discovered in the Grotte du Trilobite (Caves of Arcy-sur-Cure, Yonne, France) a much-handled trilobite fossil that had been drilled as if to be worn as a pendant. The occupation stratum in which the trilobite was found has been dated as 15,000 years old. Because the pendant was handled so much, the species of trilobite cannot be determined. This type of trilobite is not found around Yonne, so it may have been highly prized and traded from elsewhere.
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