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Cephalon (arthropod Head)
The cephalon is the head section of an arthropod. It is a tagma, i.e., a specialized grouping of arthropod segments. The word cephalon derives from the Greek κεφαλή (cephale), meaning "head".Contents1 Insects 2 Chelicerates and crustaceans 3 Proarticulata 4 Thylacocephala 5 Trilobites5.1 Facial sutures6 See also 7 ReferencesInsects[edit]Left: Peronopsis interstrictus, an Agnostida
Agnostida
from Cambrian
Cambrian
age strata of Utah. Center:Praecambridium sigillum Right:The enigmatic creature, Ainiktozoon loganese, a Thylacocephala, which was once thought to be an ancestral chordate, but is now thought to be a peculiar-looking crustacean. The trilobite in the same picture is Calymene blumenbachii
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Silurian
The Silurian
Silurian
is a geologic period and system spanning 24.6 million years from the end of the Ordovician
Ordovician
Period, at 443.8 million years ago (Mya), to the beginning of the Devonian
Devonian
Period, 419.2 Mya.[8] As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian
Silurian
is set at a series of major Ordovician–Silurian extinction events
Ordovician–Silurian extinction events
when 60% of marine species were wiped out. A significant evolutionary milestone during the Silurian
Silurian
was the diversification of jawed and bony fish
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Cambrian
The Cambrian
Cambrian
Period ( /ˈkæmbriən/ or /ˈkeɪmbriən/) was the first geological period of the Paleozoic
Paleozoic
Era, of the Phanerozoic
Phanerozoic
Eon.[6] The Cambrian
Cambrian
lasted 55.6 million years from the end of the preceding Ediacaran
Ediacaran
Period 541 million years ago (mya) to the beginning of the Ordovician
Ordovician
Period 485.4 mya.[7] Its subdivisions, and its base, are somewhat in flux
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Suborder
In biological classification, the order (Latin: ordo) isa taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family. An immediately higher rank, superorder, may be added directly above order, while suborder would be a lower rank. a taxonomic unit, a taxon, in that rank. In that case the plural is orders (Latin ordines).Example: All owls belong to the order Strigiformes.What does and does not belong to each order is determined by a taxonomist, as is whether a particular order should be recognized at all. Often there is no exact agreement, with different taxonomists each taking a different position. There are no hard rules that a taxonomist needs to follow in describing or recognizing an order
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Olenellina
Olenellina
Olenellina
is a suborder of the order Redlichiida
Redlichiida
of Trilobites that occurs about halfway during the Lower Cambrian, at the start of the stage called the Atdabanian. The earliest trilobites in the fossil record are arguably Olenellina, although the earliest Redlichiina and Eodiscina
Eodiscina
follow quickly.[2] The suborder died out when the Lower passed into the Middle Cambrian, at the end of the stage called Toyonian
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Ecological Niche
In ecology, a niche (CanE, UK: /ˈniːʃ/ or US: /ˈnɪtʃ/)[1] is the fit of a species living under specific environmental conditions.[2][3] The ecological niche describes how an organism or population responds to the distribution of resources and competitors (for example, by growing when resources are abundant, and when predators, parasites and pathogens are scarce) and how it in turn alters those same factors (for example, limiting access to resources by other organisms, acting as a food source for predators and a consumer of prey). "The type and number of variables comprising the dimensions of an environmental niche vary from one species to another [and] the relative importance of particular environmental variables for a species may vary according to the geographic and biotic contexts".[4] A Grinnellian niche is determined by the habitat in which a species lives and its accompanying behavioral adaptations
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Hypostome (trilobite)
The hypostome is the hard mouthpart of trilobites found on the ventral side of the cephalon (head). The hypostome can be classified into three types based on whether they are permanently attached to the rostrum or not and whether they are aligned to the anterior dorsal tip of the glabella.Contents1 Morphology1.1 Types1.1.1 Natant 1.1.2 Conterminant 1.1.3 Impendent2 See also 3 ReferencesMorphology[edit] The center of the hypostome is an ovoid, typically convex part called the median body, often divided into an anterior lobe and a posterior lobe. Either side of the median body is a border with various extensions, including anterior and posterior wings, sometimes bearing knob-like processes. The hypostome is hollow, and encloses the mouthparts, the anterior digestive tract, and the bases of the antennae. Trilobite
Trilobite
antennae pass through notches between the anterior and posterior wings, then forward
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Devonian
The Devonian
Devonian
is a geologic period and system of the Paleozoic, spanning 60 million years from the end of the Silurian, 419.2 million years ago (Mya), to the beginning of the Carboniferous, 358.9 Mya.[9] It is named after Devon, England, where rocks from this period were first studied. The first significant adaptive radiation of life on dry land occurred during the Devonian. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods also became well-established. Fish
Fish
reached substantial diversity during this time, leading the Devonian
Devonian
to often be dubbed the "Age of Fish"
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Proarticulata
Proarticulata
Proarticulata
is an extinct phylum of very early, superficially bilaterally symmetrical animals known from fossils found in the Ediacaran
Ediacaran
(Vendian) marine deposits, and dates to approximately 558 to 555 million years ago. The name from Greek προ (pro-) = "before" and Articulata, i.e. prior to animals with true segmentation such as annelids and arthropods. This phylum was established by Mikhail A. Fedonkin
Mikhail A

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Malacostraca
See text for orders. Malacostraca
Malacostraca
is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobsters, crayfish, shrimp, krill, woodlice, amphipods, mantis shrimp and many other, less familiar animals. They are abundant in all marine environments and have colonised freshwater and terrestrial habitats
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Phylogeny Of Malacostraca
Although the class Malacostraca is united by a number of well-defined and documented features, which were recognised a century ago by William Thomas Calman in 1904,[1] the phylogenetic relationship (the evolutionary tree) of the orders which compose this class is unclear due to the vast diversity present in their morphology
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Arthropod Head Problem
The arthropod head problem is a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, and how they are evolutionarily related to each other. While the dispute has historically centered on the exact make-up of the insect head, it has been widened to include other living arthropods such as the crustaceans and chelicerates; and fossil forms, such as the many arthropods known from exceptionally preserved Cambrian faunas. While the topic has classically been based on insect embryology, in recent years a great deal of developmental molecular data has become available. Dozens of more or less distinct solutions to the problem, dating back to at least 1897,[2] have been published, including several in the 2000s. The arthropod head problem is popularly known as the endless dispute, the title of a famous paper on the subject by Jacob G. Rempel in 1975,[3] referring to its seemingly intractable nature
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Tagma (biology)
In biology a tagma (Greek: τάγμα, plural tagmata – τάγματα) is a specialized grouping of multiple segments or metameres into a coherently functional morphological unit. Familiar examples are the head, the thorax, and the abdomen of insects.[1] The segments within a tagma may be either fused (such as in the head of an insect) or so jointed as to be independently moveable (such as in the abdomen of most insects). Usually the term is taken to refer to tagmata in the morphology of members of the phylum Arthropoda, but it applies equally validly in other phyla, such as the Chordata. In a given taxon the names assigned to particular tagmata are in some sense informal and arbitrary; for example, not all the tagmata of species within a given subphylum of the Arthropoda are homologous to those of species in other subphyla; for one thing they do not all comprise corresponding somites, and for another, not all the tagmata have closely analogous functions or anatomy
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Cephalothorax
The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind.[1] (The terms prosoma and opisthosoma are equivalent to cephalothorax and abdomen in some groups.) The word cephalothorax is derived from the Greek words for head (κεφαλή, kephalé) and thorax (θώραξ, thórax).[2] This fusion of the head and thorax is seen in chelicerates and crustaceans; in other groups, such as the Hexapoda
Hexapoda
(including insects), the head remains free of the thorax.[1] In horseshoe crabs and many crustaceans, a hard shell called the carapace covers the cephalothorax.[3]Contents1 Arachnid anatomy1.1 Fovea 1.2 Clypeus 1.3 Ocularium 1.4 Trident2 ReferencesArachnid anatomy[edit] Fovea[edit] The fovea is the centre of the cephalothorax and is located behind the head (only in spiders).[4] It is often important in identification
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Crustacean
Thylacocephala? † BranchiopodaPhyllopoda SarsostracaRemipedia Cephalocarida MaxillopodaThecostraca Tantulocarida Branchiura Pentastomida Mystacocarida CopepodaOstracodaMyodocopa PodocopaMalacostracaPhyllocarida Hoplocarida EumalacostracaCladistically included but traditionally excluded groupsHexapodsCrustaceans (Crustacea /krʌˈsteɪʃə/) form a large, diverse arthropod taxon which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill, woodlice, and barnacles.[1] The crustacean group is usually treated as a subphylum, and thanks to recent molecular studies it is now well accepted that the crustacean group is paraphyletic, and comprises all animals in the Pancrustacea clade other than hexapods.[2] Some crustaceans are more closely related to insects and other hexapods than they are to certain other crustaceans. The 67,000 described species range in size from
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Chelicerata
The subphylum Chelicerata
Chelicerata
(New Latin, from French chélicère, from Greek khēlē "claw, chela" and kéras "horn") constitutes one of the major subdivisions of the phylum Arthropoda. It contains the horseshoe crabs, sea spiders, and arachnids (including scorpions and spiders). The chelicerata originated as marine animals, possibly in the Cambrian period, but the first confirmed chelicerate fossils, eurypterids, date from 445 million years ago in the Late Ordovician
Ordovician
period. The surviving marine species include the four species of xiphosurans (horseshoe crabs), and possibly the 1,300 species of pycnogonids (sea spiders), if the latter are indeed chelicerates
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