Trilobites are extinct arthropods that form the class Trilobita. They appeared in the 2nd Epoch (Series 2) 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 (m.y.a.). Superfamilies Aulacopleuroidea Bathyuroidea Proetoidea Proetida is an order of trilobite that lived from the Ordovician to the Permian. ... Superfamilies Anomocaroidea Asaphoidea Dikelokephaloidea Remopleuridoidea Cyclopygoidea Trinucleioidea Asaphida is a large, morphologically diverse order of trilobites that are found in strata dated from the Middle Cambrian boundary to the Upper Ordovician. ... Families Entomaspididae Harpetidae Harpididae Harpetida is one of the nine orders of the extinct arthropod class Trilobita. ... Suborders Suborder Ptychopariina Superfamily Ellipsocephaloidea Superfamily Ptychoparioidea Suborder Olenina Ptychopariida is a large, heterogenous order of trilobite containing some of the most primative species known. ... For other uses, see Extinction (disambiguation). ... Subphyla and Classes Subphylum Trilobitomorpha Trilobita - trilobites (extinct) Subphylum Chelicerata Arachnida - spiders,scorpions, etc. ... A class is the rank in the scientific classification of organisms in biology below Phylum and above Order. ... For other uses, see Cambrian (disambiguation). ... The Paleozoic Era (from the Greek palaio, old and zoion, animals, meaning ancient life) is the earliest of three geologic eras of the Phanerozoic eon. ... Comparison of the three episodes of extinction in the Late Devonian (Late D) to other mass extinction events in Earths history. ... Superfamilies Aulacopleuroidea Bathyuroidea Proetoidea Proetida is an order of trilobite that lived from the Ordovician to the Permian. ... The Permian-Triassic (P-T or PT) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251 million years ago (mya), forming the boundary between the Permian and Triassic geologic periods. ... The Permian is a geologic period that extends from about 299. ... For other uses of mya, see mya (disambiguation). ...

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 (Lieberman, 1999), 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) (e.g., Cotton & Braddy 2004), although several alternative taxonomies are found in the literature. Orders & Suborders Saurischia Sauropodomorpha Theropoda Ornithischia Thyreophora Ornithopoda Marginocephalia Dinosaurs were vertebrate animals that dominated the terrestrial ecosystem for over 160 million years, first appearing approximately 230 million years ago. ... It has been suggested that this article or section be merged into Fossil. ... For other uses, see Cambrian (disambiguation). ... For other uses of the term, see Fossil (disambiguation) Fossils are the mineralized remains of animals or plants or other artifacts such as footprints. ... The Paleozoic Era (from the Greek palaio, old and zoion, animals, meaning ancient life) is the earliest of three geologic eras of the Phanerozoic eon. ... This article or section does not cite any references or sources. ... Paleontology, palaeontology or palæontology (from Greek: paleo, ancient; ontos, being; and logos, knowledge) is the study of prehistoric life forms on Earth through the examination of plant and animal fossils. ... The tectonic plates of the world were mapped in the second half of the 20th century. ... Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ... The Cambrian explosion is the geologically kukko sudden appearance in the fossil record of the ancestors of familiar animals, starting about 542 million years ago (Mya). ... Animalia redirects here. ... Subphyla and Classes Subphylum Trilobitomorpha Trilobita - trilobites (extinct) Subphylum Chelicerata Arachnida - spiders,scorpions, etc. ... Arachnomorpha Lameere 1890 is a subdivision of Arthropoda, containing the monophyletic group formed by the trilobites and the chelicerates. ... Taxonomy (from Greek ταξινομία (taxinomia) from the words taxis = order and nomos = law) may refer to either the classification of things, or the principles underlying the classification. ...

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. In marine geology and biology, benthos are the organisms and habitats of the sea floor; in freshwater biology they are the organisms and habitats of the bottoms of lakes, rivers, and creeks. ... This snapping turtle is trying to make a meal of a Canada goose, but the goose is too wary. ... Categories: Stub ... Scale diagram of the layers of the pelagic zone. ... This article is about the real-life under-sea organisms. ... Common Clownfish (Amphiprion ocellaris) in their Magnificent Sea Anemone (Heteractis magnifica) home. ...

The name "trilobite" (meaning "three-lobed") is not based on the body sections cephalon, thorax and pygidium, but rather on the three longitudinal lobes: a central axial lobe, and two symmetrical pleural lobes that flank the axis.

Image File history File links SamGonIII_3lobes. ...

Phylogeny

Despite their rich fossil record with thousands of genera found throughout the world, the taxonomy and phylogeny of trilobites have many uncertainties. Nonetheless, the systematic division of trilobites into nine distinct orders is represented by a widely held view that will inevitably change as new data emerge. Except possibly for the members of order Phacopida, all trilobite orders appeared prior to the end of the Cambrian. Most scientists believe that order Redlichiida, and 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 by Fortey (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. For the science of classifying living things, see alpha taxonomy. ... In biology, phylogenetics (Greek: phylon = tribe, race and genetikos = relative to birth, from genesis = birth) is the study of evolutionary relatedness among various groups of organisms (e. ... Phacopida is an order of trilobite. ... Suborders Olenellina Redlichiina Redlichiida is an order within the major extinct arthropod class Trilobita. ... Superfamilies Superfamily Emuelloidea Superfamily Redlichioidea Abadiellidae Chengkouaspidae Dolerolenidae Gigantopygidae Kuechowiidae Mayiellidae Menneraspididae Metadoxididae Redlichiidae Redlichiidae Redlichia takooensis Saukiandidae Yinitidae Superfamily Paradoxidoidea Centropleuridae Paradoxididae Paradoxides Xystriduridae Redlichiina is a suborder of the order Redlichiida of Trilobites. ... Suborders Suborder Corynexochina Suborder Illaenina Suborder Leiostegiina Corynexochida is an order of trilobite that lived from the Lower Cambrian to the Middle Devonian. ... Suborders Suborder Ptychopariina Superfamily Ellipsocephaloidea Superfamily Ptychoparioidea Suborder Olenina Ptychopariida is a large, heterogenous order of trilobite containing some of the most primative species known. ... Superfamilies Superfamily Dameselloidea Superfamily Lichoidea Superfamily Odontopleuroidea Lichida is an order of typically spiny trilobite that lived from the Ordovician to the Devonian period. ... Suborders Suborder Ptychopariina Superfamily Ellipsocephaloidea Superfamily Ptychoparioidea Suborder Olenina Ptychopariida is a large, heterogenous order of trilobite containing some of the most primative species known. ... Superfamilies Anomocaroidea Asaphoidea Dikelokephaloidea Remopleuridoidea Cyclopygoidea Trinucleioidea Asaphida is a large, morphologically diverse order of trilobites that are found in strata dated from the Middle Cambrian boundary to the Upper Ordovician. ... Superfamilies Aulacopleuroidea Bathyuroidea Proetoidea Proetida is an order of trilobite that lived from the Ordovician to the Permian. ... Families Entomaspididae Harpetidae Harpididae Harpetida is one of the nine orders of the extinct arthropod class Trilobita. ... A hypostome (also called the maxilla, radula, labium or Unterkiefer), is a calcified harpoon-like structure near the mouth area of certain parasitic arthropods including ticks, that allows them to anchor themselves firmly in place on a host mammal while sucking blood. ...

The trilobite body is divided into three major sections, a cephalon with eyes, mouthparts and sensory organs such as antennae, a thorax of multiple similar segments (that in some species allowed enrollment), and a pygidium, or tail section.

Image File history File links Summary Diagram showing the location of the cephalon, thorax and pygidium of a trilobite. ...

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 segments fused together with the telson. The pygidia are fairly rudimentary in the most primitive trilobites. The thorax is fairly flexible—fossilised trilobites are often found enrolled (curled up) like modern woodlice for protection. Trilobites are described based on the pydigium being micropygous (pydigium smaller than cephalon), isopygous (pydigium equal in size to cephalon), or macropygous (pydigium larger than cephalon). In invertebrate biology, a tagma (plural tagmata) is a specialized grouping of arthropodan segments, such as head, body, and tail. ... Diagram of a tsetse fly, showing the head, thorax and abdomen The thorax is a division of an animals body that lies between the head and the abdomen. ... The pygidium is the posterior body part or shield of crustaceans, some insects, and trilobites. ... The telson is the last division of the body of a crustacean. ... Infraorders and Families Not necessarily a complete list Infraorders: Ligiamorpha Tylomorpha Families: Dubioniscidae Irmaosidae Pseudarmadillidae Scleropactidae Armadillidium vulgare A woodlouse, also known as a pill bug (genus Armadillidium only), armadillo bug, sow bug, slater, ball bug, or roley-poley, is a terrestrial crustacean with a rigid, segmented, calcareous exoskeleton and...

Trilobite exoskeletons bear a variety of small-scale structures, such as nodes, ridges, tubercles and spines, collectively called prosopon. Alimentary ridge networks may have been either digestive or respiratory tubes in the cephalon and other regions (Clarkson, 1979). Early Cambrian trilobites have thin cuticles in which the alimentary networks can easily be seen. Trilobites had a single pair of preoral antennae and otherwise undifferentiated biramous limbs. Each exopodite (walking leg) had six segments, homologous to other early arthropods. 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 the lateral projections of the dorsal exoskeleton called pleural lobes, extending outward from a central axial lobe. Insects display a wide variety of antennal shapes. ... Biramous is a term used for branched arthropod appendages. ... For other uses, see Gill (disambiguation). ...

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 molt. Thus a single trilobite animal could potentially have left multiple well-mineralized skeletons behind -- contributing to the abundance of trilobites in the fossil record. 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. The cheeks (genae) of the cephalon of trilobites, except some sightless species, supported a pair of compound eyes. The earliest trilobite known from the fossil record is the genus Fallotaspis within Order Redlichiida, dated to some 543 million years ago (Fortey, 2000)^{[verification needed]}.<--- Laudo Correctum 12/15/07 BLS. Other early genera include Profalloptaspis and Eofallotaspis, all appearing about the same time. An exoskeleton is an external anatomical feature that supports and protects an animals body, in contrast to the internal endoskeleton of, for example, a human. ... Doubly refracting Calcite from Iceberg claim, Dixon, New Mexico. ... Structure of the chitin molecule, showing two of the N-Acetylglucosamine units that repeat to form long chains in beta-1,4 linkage. ... Ecdysis is the molting of the cuticula in arthropods and related groups (Ecdysozoa). ... It has been suggested that this article or section be merged into Fossil. ... For other uses, see Genus (disambiguation). ... In scientific classification used in biology, the order (Latin: ordo, plural ordines) is a rank between class and family (termed a taxon at that rank). ... Suborders Olenellina Redlichiina Redlichiida is an order within the major extinct arthropod class Trilobita. ...

Even the earliest trilobites had complex, compound eyes with lenses made of calcite, a unique characteristic of all trilobite eyes. This confirms that eyes of arthropods and probably other animals were already quite developed at the beginning of the Cambrian. Improving eyesight of both predator and prey in marine environments probably provided one of the evolutionary pressures furthering an apparent rapid development of new life forms during what is known as the Cambrian Explosion. Evolutionary pressure or selection pressure can be formalized as an external pressure applied to a process, thereby pushing that process in a distinct direction. ... The Cambrian explosion is the geologically kukko sudden appearance in the fossil record of the ancestors of familiar animals, starting about 542 million years ago (Mya). ...

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. Superfamilies Superfamily Dameselloidea Superfamily Lichoidea Superfamily Odontopleuroidea Lichida is an order of typically spiny trilobite that lived from the Ordovician to the Devonian period. ... Artist impression of the Ordovician Sea. ... For the Celtic language, see Southwestern Brythonic language; for the residents of the English county, see Devon. ... For other uses, see Fish (disambiguation). ...

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 considered members of the Asaphida family Raphiophoridae, the conclusions are likely to be applicable to other trilobites as well, such as in the Phacopid trilobite Walliserops trifurcatus that had prominent horn-like spines on its cephalon. New Scientist is a weekly international science magazine covering recent developments in science and technology for a general English-speaking audience. ... For other uses, see Beetle (disambiguation). ... Affiliations: University of London Association of Commonwealth Universities 1994 Group Website: http://www. ... Professor Richard A. Fortey FRS (born 1946 in London) is a British paleontologist and writer, formerly a Merit Researcher at the Natural History Museum in London. ... For other similarly-named museums see Museum of Natural History. ... Superfamilies Anomocaroidea Asaphoidea Dikelokephaloidea Remopleuridoidea Cyclopygoidea Trinucleioidea Asaphida is a large, morphologically diverse order of trilobites that are found in strata dated from the Middle Cambrian boundary to the Upper Ordovician. ... For other uses, see Family (disambiguation). ... Suborders Suborder Phacopina Suborder Calymenina Suborder Cheirurina Phacopida (Lens-face) is an order of trilobite that lived from the Ordovician to the Devonian. ... Binomial name Walliserops trifurcates Walliserops trifurcates was a phacopid trilobite found from Devonian Morocco. ...

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 centimeters (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. An asaphid trilobite from the middle and upper Ordovician Period fairly common in the Northeast U.S., northwest Manitoba, southwestern Quebec and southeastern Ontario. ... Year 1998 (MCMXCVIII) was a common year starting on Thursday (link will display full 1998 Gregorian calendar). ... New York Harbor, the outflow for Hudson River, is sometimes called Hudsons Bay. Hudson Bay, Canada. ...

Sensory organs

An exceptionally well preserved trilobite from the Burgess Shale. The antennæ and legs are preserved as reflective carbon films.

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. As such, they became secondarily blind in this branch of trilobite evolution. Others, such as Phacops rana, had eyes that were quite large for use in more well lit, predator-filled waters. Image File history File linksMetadata Size of this preview: 581 × 600 pixel Image in higher resolution (1114 × 1150 pixel, file size: 881 KB, MIME type: image/jpeg) A cropped and enhanced version of LeggedTrilobite. ... Image File history File linksMetadata Size of this preview: 581 × 600 pixel Image in higher resolution (1114 × 1150 pixel, file size: 881 KB, MIME type: image/jpeg) A cropped and enhanced version of LeggedTrilobite. ... Hallucigenia sparsa, one of the organisms unique to the Burgess Shale. ... Insects display a wide variety of antennal shapes. ... Binomial name Phacops rana Phacops rana is a species of trilobite from the middle Devonian period. ...

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. For other uses, see Eye (disambiguation). ... Doubly refracting Calcite from Iceberg claim, Dixon, New Mexico. ... Calcium carbonate is a chemical compound, with the chemical formula CaCO3. ... Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens. ... For other uses, see Doublet. ... In optics, particularly film and photography, the depth of field (DOF) is the distance in front of and beyond the subject that appears to be in focus. ... Focal plane Longitudinal sections In optics, spherical aberration is an image imperfection that occurs due to the increased refraction of light rays that occurs when rays strike a lens or mirror near its edge, in comparison with those that strike nearer the center. ... Christiaan Huygens (pronounced in English (IPA): ; in Dutch: ) (April 14, 1629 – July 8, 1698), was a Dutch mathematician, astronomer and physicist; born in The Hague as the son of Constantijn Huygens. ... Orders ME Oegophiurida Ophiurida Phrynophiurida Brittle stars are echinoderms, closely related to starfish. ... Binomial name Ophiocoma wendtii The brittlestar Ophiocoma wendtii inhabits coral reefs from Bermuda to Brazil. ...

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. Compound eye of a dragonfly A compound eye is a visual organ found in arthropods such as insects and crustaceans. ...

Holochroal eyes

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. The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eyes optical power [1]. Together with the lens, the cornea refracts light and, as a result, helps the eye to focus. ...

Schizochroal eyes

Phacopid trilobite Eldredgeops rana crassituberculata. Schizochroal eyes were instrumental in the formulation of the theory of punctuated equilibrium.

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. Image File history File links Eldredgeops-rana-crassituberculata. ... Image File history File links Eldredgeops-rana-crassituberculata. ... Punctuated equilibrium (or punctuated equilibria) is a theory in evolutionary biology which states that most sexually reproducing species will show little to no evolutionary change throughout their history. ... Phacopida is an order of trilobite. ...

Abathochroal eyes

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. Families Suborder Agnostina Superfamily Agnostoidea Agnostidae Ammagnostidae Clavagnostidae Diplagnostidae Doryagnostidae Glyptagnostidae Metagnostidae Peronopsidae Ptychagnostidae Superfamily Condylopygoidea Condylopygidae Suborder Eodiscina Superfamily Eodiscoidea Calodiscidae Eodiscidae Hebediscidae Tsunyidiscidae Weymouthiidae Yukoniidae Agnostida (the agnostids) is an order of trilobite. ...

Development

Trilobites grew through successive molt stages called "instars", in which existing segments increased in size and new trunk segments appeared at a sub-terminal generative zone during the "anamorphic" phase of development. The molt itself, is called ecdysis. This was followed by the "epimorphic" developmental phase, in which the animal continued to grow and molt, but no new trunk segments were expressed in the exoskeleton. The combination of anamorphic and epimorphic growth consistutes the "hemianamorphic" developmental mode that is common among many living arthropods. 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, which are not readily compared with those of other arthropods. The earliest trilobite growth stages known with certainty are of the protaspid stage, in which all segments were fused into a single shield comprising a cephalic (head) and trunk regions. In subsequent molt stages an articulation appeared between the head and the trunk, which marked the onset of the "meraspid" phase of development. At the onset of the meraspid phase the animal had a two-part structure - the head and the plate of fused trunk segments, called 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. The "holaspid' phase of grow 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. Trilobite juveniles are reasonably well known and provide an important aid in evaluating high-level phylogenetic relationships among trilobites.

Terminology

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.

Fig. 1 - Morphology of the cephalon

Fig. 2 - Detailed morphology of the cephalon

Image File history File links SamGonIII_cranidium. ... Image File history File links SamGonIII_cephareas. ...

Origins

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, the Maotianshan shales at Chengjiang and other fossiliferous locations. These are investigated further here: [1] It is reasonable to assume that the trilobites share a common ancestor with these other arthropods prior to the Ediacaran-Cambrian boundary. Ancestral trilobites may have been somewhat soft bodied and developed their thick carapaces through Cuticularisation. As with other forms of trilobite body evolution, this was a defensive measure. Binomial name Glaessner, 1958 Digitally enhanced specimen photo Spriggina was an organism of the Ediacaran period, fossils of which have been found in the Ediacara Hills of Australia. ... Species Parvancorina is a genus of shield-shaped Ediacaran fossils. ... The Ediacaran[5][6]  â€¢  â€¢  | Neoproterozoic (last æon of the Precambrian) Phanerozoic Axis scale: millions of years ago. ... The Precambrian (Pre-Cambrian) is an informal name for the supereon comprising the eons of the geologic timescale that came before the current Phanerozoic eon. ... For other uses, see Cambrian (disambiguation). ... Hallucigenia sparsa, one of the organisms unique to the Burgess Shale. ... The Maotianshan shale is a lower Cambrian (Atdabanian) rock formation, of ca 522 Mya, now lying exposed in the Yunnan Province of China in the villages of Ercaicun and Chengjiang near the city of Kunming. ... For other uses, see Fossil (disambiguation). ...

Extinction

The reason for the extinction of the trilobites is not clear, although it may be no coincidence that their numbers began to decrease with the appearance of the first sharks and other early gnathostomes in the Silurian and their subsequent rise in diversity during the Devonian period. Trilobites may have provided a rich source of food for these new animals. A smaller extinction event in the Middle Cambrian of trilobite orders possessing alimentary prosopon and a micropygidium may have been linked to the rise of cephalopods. Trilobites were under great selective pressure to develop defensive bodies quickly. The most radical change in body form occurred in the Middle Cambrian. As a means of defense, surviving orders developed isopygidius or macropygius bodies. This enabled trilobites to curl their bodies into a ball as a means of defense. A micropygidius trilobite cannot completely protect itself in a curled position with a pygidium smaller than the cephalon. It is analogous to pleurodirian (side-necked) turtles of the present day (Holocene). A terrestrial side neck could never evolve because the exposed neck in a side withdraw state would be vulnerable to a predator. Surviving trilobites developed thicker cuticles (as mentioned earlier) and as such, the alimentary prosopon are no longer visible due to the thickness. This makes an excellent fossil stratigraphic marker of the Cambrian period in that if a researcher finds trilobites with alimentary prosopon and a micropygium, she or he has found Early Cambrian strata (Schnirel, 2001). For other uses, see Shark (disambiguation). ... Classes Placodermi Chondrichthyes Acanthodii Osteichthyes Gnathostomata is the group of vertebrates with jaws. ... For other uses, see Silurian (disambiguation). ... For the Celtic language, see Southwestern Brythonic language; for the residents of the English county, see Devon. ...

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.

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 Permian-Triassic extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 252 million years ago (mya), forming the boundary of the Permian and Triassic geologic periods. ... The Ordovician-Silurian extinction event, labeled End O here. ...

The closest extant relatives of trilobites may be the horseshoe crabs, according to Fortey (2000), or the cephalocarids, according to Lambert (1985). Binomial name Linnaeus, 1758 The horseshoe crab, horsefoot, king crab, or sauce-pan (Limulus polyphemus, formerly known as Limulus cyclops, Xiphosura americana, Polyphemus occidentalis) is a chelicerate arthropod. ... Genera Chiltonella Hampsonellus Hutchinsoniella Lightiella Sandersiella Cephalocarida is a class inside the subphylum Crustacea that comprises only about 9 shrimp-like benthic species. ...

Fossil distribution

Cruziana, fossil track of Trilobites

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. Image File history File links Download high-resolution version (1024x954, 319 KB) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Trilobite ... Image File history File links Download high-resolution version (1024x954, 319 KB) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Trilobite ...

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". Index fossils (or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). ... For other uses, see Native Americans (disambiguation). ...

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 locust" or "Dudley bug" by quarrymen who once worked many of the now abandoned limestone quarries. Other trilobites found there include Dalmanites, Trimerus, Bumastus and Balizoma. Llandrindod Wells, Powys, Wales, is another famous trilobite location. Map sources for Dudley at grid reference SO9390 Dudley is a town in the West Midlands, England. ... The County of West Midlands is a metropolitan county in western central England with a population of around 2,600,000 people. ... For other uses, see Silurian (disambiguation). ... Wenlock Group (Wenlockian), in geology, is the middle series of strata in the Silurian (Upper Silurian) of Great Britain. ... A modern coat of arms is derived from the medi val practice of painting designs onto the shield and outer clothing of knights to enable them to be identified in battle, and later in tournaments. ... For other uses, see Limestone (disambiguation). ...

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. Hallucigenia sparsa, one of the organisms unique to the Burgess Shale. ... The Maotianshan shale is a lower Cambrian (Atdabanian) rock formation, of ca 522 Mya, now lying exposed in the Yunnan Province of China in the villages of Ercaicun and Chengjiang near the city of Kunming. ... The Hunsrück Slate (Hunsrückschiefer) is a Devonian Lagerstätte famous for exceptional preservation of a highly diverse fossil fauna assemblage. ...

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.

Gallery

Trilobite fossils Picture by: John Mittler Original at: 777Life. ...	Fossil trilobite Ductina vietnamica from the Devonian of China Photograph of the fossil trilobite Ductina taken by Dlloyd. ...	Balizoma variolaris Brongniart, 1822, from Dudley, UK Image File history File links Size of this preview: 355 × 599 pixel Image in higher resolution (480 × 810 pixel, file size: 88 KB, MIME type: image/jpeg) Neotype (BU55) of the encrinurine trilobite Balizoma variolaris (Brongniart, 1822); Wenlock Series; Much Wenlock Limestone Formation; Dudley, West Midlands, UK I, the creator...	Kolihapeltis sp. Image File history File linksMetadata Size of this preview: 512 × 600 pixelsFull resolution (2424 × 2840 pixel, file size: 1. ...
Crotalocephalus sp. Image File history File linksMetadata Download high-resolution version (1005x392, 89 KB) 3D Fossil of trilobite, photo taken in Quebec City store, Quebec, Canada, by Yannick Trottier in 2006. ...	Asaphiscus wheeleri, Cambrian shale,Utah Download high resolution version (878x1041, 124 KB) Wikipedia does not have an article with this exact name. ...	Cyphaspis tafilalet - Proetid trilobites from Morocco Download high resolution version (777x1010, 373 KB) Wikipedia does not have an article with this exact name. ... Superfamilies Aulacopleuroidea Bathyuroidea Proetoidea Proetida is an order of trilobite that lived from the Ordovician to the Permian. ...	Cheirurus middle Ordovician (Volchow River, Russia)

See also

Wikimedia Commons has media related to:

Trilobite

Wikispecies has information related to:

Trilobita

• Prehistoric life
• List of trilobites

Image File history File links Commons-logo. ... Image File history File links Wikispecies-logo. ... Wikispecies is a wiki-based online project supported by the Wikimedia Foundation that aims to create a comprehensive free content catalogue of all species (including animalia, plantae, fungi, bacteria, archaea, and protista). ... Prehistoric life is a term used to refer to diverse organisms that inhabited Earth from the origin of life about 3. ... Asaphiscus wheeleri, a trilobite from the Cambrian shale of Utah. ...

References

• Clarkson, E.N. (1993), Invertebrate Paleontology and Evolution. 4th Edition. Chapman/Hall, N.Y..
• Cotton, T.J., and S.J. Braddy. 2004. The phylogeny of arachnomorph arthropods and the origins of the Chelicerata. Transactions of the Royal Society of Edinburgh: Earth Sciences, 94: 169–193
• Trilobite! - Richard Fortey (2000) (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
• Lieberman, BS, (1999) Testing the Darwinian Legacy of the Cambrian Radiation Using Trilobite Phylogeny and Biogeography Journal of Paleontology, 73(2)
• Tripp, RP, JT Temple, and KC Gass, (1977) The Silurian Trilobite Encrinurus variolaris and Allied Species, with Notes on Frammia, Palaeontology, 20(4)
• Fortey RA. 2001. Trilobite systematics: The last 75 years. Journal of Paleontology 75:1141–1151.
• Ebach, M.C. & K.J. McNamara. 2002. A systematic revision of the family Harpetidae (Trilobita). Records of the Western Australian Museum 21:135-67.
• The Virtual Fossil Museum - Class Trilobita - Including extensive photographs organized by taxonomy and locality.
• Kaesler RL, ed. 1997. Treatise on Invertebrate Paleontology, Part O, Volume 1, revised, Trilobita. Geological Society of America and University of Kansas Press, Lawrence, Kansas.
• Schnirel, B.L.(2001). Trilobite Evolution and Extinction., Graves Museum of Natural History, Dania, Florida.

External links

• Western Trilobite Association
• Kevin's Trilobite Gallery - a collection of photographs of trilobite fossils
• A Guide to the Orders of Trilobites
• Canadian trilobite web site: photographs of trilobite fossils
• The Paleontological Society