36-684: Tetraconata Dohle, 2001 Pancrustacea is the clade that comprises all crustaceans , and all hexapods ( insects and relatives). This grouping is contrary to the Atelocerata hypothesis, in which Hexapoda and Myriapoda are sister taxa , and Crustacea are only more distantly related. As of 2010, the Pancrustacea taxon was considered well accepted, with most studies recovering Hexapoda within Crustacea. The clade has also been called Tetraconata , referring to having four cone cells in
72-550: A carapace , which may be a shell of two valves enclosing the trunk (as in most Cladocera), broad and shallow (as in the Notostraca), or entirely absent (as in the Anostraca). In the groups where the carapace prevents the use of the trunk limbs for swimming (Cladocera and clam shrimp), the antennae are used for locomotion, as they are in the nauplius . Male fairy shrimp have an enlarged pair of antennae with which they grasp
108-534: A clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group , is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree . In the taxonomical literature, sometimes the Latin form cladus (plural cladi ) is used rather than the English form. Clades are
144-479: A "ladder", with supposedly more "advanced" organisms at the top. Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming , this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a rank can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, phylogenetic nomenclature has been developed; it
180-402: A 2005 study of nuclear genomes Regier et al. suggest that Hexapoda is most closely related to Branchiopoda and Cephalocarida + Remipedia , thereby hexapods are "terrestrial crustaceans", thus supporting the Pancrustacea hypothesis that maxillopods are not monophyletic (in the following cladograms Maxillopoda subclasses are highlighted ). In addition, there appeared some evidence against
216-623: A clade can be described based on two different reference points, crown age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils , or from molecular clock estimates. Viruses , and particularly RNA viruses form clades. These are useful in tracking
252-422: A revised taxonomy based on a concept strongly resembling clades, although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley . German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics . He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on
288-429: A suffix added should be e.g. "dracohortian". A clade is by definition monophyletic , meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics ,
324-481: Is a single median compound eye . Most species show cyclical parthenogenesis , where asexual reproduction is occasionally supplemented by sexual reproduction , which produces resting eggs that allow the species to survive harsh conditions and disperse to distant habitats. In the water bodies of the world, a lot of Cladocera are non-native species, many of which pose a great threat to aquatic ecosystems. The fossil record of branchiopods extends back at least into
360-399: Is a sister group to Hexapoda , and Branchiopoda is a sister group to (Remipedia + Hexapoda). Thus, their data strongly suggest that Branchiopoda is more closely related to Hexapoda and Remipedia than to Multicrustacea. Based on these data, they propose the following scenario of evolution of Branchiopoda, Remipedia and Hexapoda: under the impact of predatory fishes their common ancestors go to
396-499: Is also used with a similar meaning in other fields besides biology, such as historical linguistics ; see Cladistics § In disciplines other than biology . The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis , the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch . Many commonly named groups – rodents and insects , for example – are clades because, in each case,
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#1732772975494432-469: Is divided into four lineages: Oligostraca ( Ostracoda , Mystacocarida , Branchiura , Pentastomida ), Vericrustacea ( Malacostraca , Thecostraca , Copepoda , Branchiopoda ), Xenocarida ( Cephalocarida , Remipedia ) and Hexapoda , with Xenocarida as a sister group to the Hexapoda (comprising "Miracrustacea"). New clades proposed by Regier et al. are: Of these proposed clades, only Multicrustacea
468-476: Is in turn included in the mammal, vertebrate and animal clades. The idea of a clade did not exist in pre- Darwinian Linnaean taxonomy , which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus's original Systema Naturae (mostly vertebrate groups) do represent clades. The phenomenon of convergent evolution
504-533: Is probably closely related, although its relationships to other orders remain unclear. The body is 3 mm (0.12 in) long, with 23 body segments and 19 pairs of appendages , but no carapace . It occurred chiefly among charophytes , probably in alkaline temporary pools . The order Notostraca comprises the single family Triopsidae, containing the tadpole shrimp or shield shrimp. The two genera, Triops and Lepidurus , are considered living fossils , having not changed significantly in outward form since
540-515: Is responsible for many cases of misleading similarities in the morphology of groups that evolved from different lineages. With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary tree of life . The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley , an early advocate of evolutionary theory, proposed
576-489: Is still controversial. As an example, see the full current classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand". The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade Dracohors , which was made by haplology from Latin "draco" and "cohors", i.e. "the dragon cohort "; its form with
612-564: The Ostracoda monophyly: that Ostracoda subclass Podocopa may form a clade with Branchiura . Chelicerata Ostracoda Branchiura Copepod Thecostraca and Tantulocarida Malacostraca Remipedia Cephalocarida Branchiopoda Hexapoda A 2010 study of nuclear genomes (Regier et al. ) strongly supports Pancrustacea and strongly favour Mandibulata ( Myriapoda + Pancrustacea) over Paradoxopoda (Myriapoda + Chelicerata ). According to this study, Pancrustacea
648-682: The Triassic . They have a broad, flat carapace, which conceals the head and bears a single pair of compound eyes. The abdomen is long, appears to be segmented and bears numerous pairs of flattened legs. The telson is flanked by a pair of long, thin caudal rami. Phenotypic plasticity within taxa makes species-level identification difficult, and is further compounded by variation in the mode of reproduction. The evidence of phenotypic plasticity of Arctic tadpole shrimp ( Lepidurus arcticus , Notostraca) has been observed in Svalbard. Notostracans are
684-557: The Upper Cambrian and possibly further. The group is thought to be monophyletic , with the Anostraca having been the first group to branch off . It is thought that the group evolved in the seas, but was forced into temporary pools and hypersaline lakes by the evolution of bony fishes . Although they were previously considered the sister group to the remaining crustaceans, it is now widely accepted that crustaceans form
720-562: The littoral zone , then ancestors of Branchiopoda go to the ephemeral freshwater habitat , whereas ancestors of Remipedia go to the anchialine cave , and ancestors of Hexapoda go to the land . Chelicerata Ostracoda Copepod Thecostraca and Tantulocarida Malacostraca Branchiopoda Remipedia Hexapoda Chelicerata Ostracoda Copepod Thecostraca and Tantulocarida Malacostraca Branchiopoda Remipedia Hexapoda Chelicerata Ostracoda Clade In biological phylogenetics ,
756-559: The ommatidia . The term "Tetraconata" is preferred by some scientists in order to avoid confusion with the use of "pan-" to indicate a clade that includes a crown group and all of its stem group representatives. A monophyletic Pancrustacea has been supported by several molecular studies, in most of which the subphylum Crustacea is paraphyletic with regard to hexapods (that is, that hexapods, including insects, are derived from crustacean ancestors). This means that within pancrustacea, only some members are actually crustaceans, hexapods being
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#1732772975494792-582: The Branchiopoda were all placed in a single genus, Monoculus . The taxon Branchiopoda was erected by Pierre André Latreille in 1817, initially at the rank of order . The current upper-level classification of Branchiopoda, according to the World Register of Marine Species (2021), is as follows: Class Branchiopoda Latreille, 1817 In addition, the extinct genus Lepidocaris is generally placed in Branchiopoda. The fairy shrimp of
828-697: The Devonian Lepidocaris and possibly the Cambrian Rehbachiella . They are mostly small, freshwater animals that feed on plankton and detritus. Members of the Branchiopoda are unified by the presence of gills on many of the animals' appendages , including some of the mouthparts . This is also responsible for the name of the group (from the Ancient Greek : βράγχια , gills, akin to βρόγχος , windpipe ; Greek : πούς , foot). They generally possess compound eyes and
864-593: The female during mating, while the bottom-feeding Notostraca, the antennae are reduced to vestiges . The trunk limbs are beaten in a metachronal rhythm , causing a flow of water along the midline of the animal, from which it derives oxygen , food and, in the case of the Anostraca and Notostraca, movement. Branchiopods are found in continental fresh water , including temporary pools and in hypersaline lakes , and some in brackish water . Only two groups of water fleas include marine species: Family Podonidae in
900-451: The fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual, a population , or a species ( extinct or extant ). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups. Over
936-546: The group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant , each of which consists of even smaller clades. The clade "rodent"
972-462: The largest branchiopodans and are omnivores living on the bottom of temporary pools , ponds and shallow lakes. Clam shrimp are bivalved animals which have lived since at least the Devonian . The three groups are not believed to form a clade . They have 10–32 trunk segments, decreasing in size from front to back, and each bears a pair of legs which also carry gills . A strong muscle can close
1008-590: The last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic . Some of the relationships between organisms that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria , and multicellular organisms may have evolved from archaea. The term "clade"
1044-518: The latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms ; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature : node-, stem-, and apomorphy-based (see Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions). The relationship between clades can be described in several ways: The age of
1080-442: The main exception. The evidence for this clade derives from molecular data and morphological characteristics. The molecular data consists of comparisons of nuclear ribosomal RNA genes , mitochondrial ribosomal RNA genes, and protein coding genes. The morphological data consists of ommatidial structures (see arthropod eye ), the presence of neuroblasts , and the form and style of axonogenesis by pioneer neurons . In
1116-533: The order Anostraca are usually 6–25 mm (0.24–0.98 in) long (exceptionally up to 170 mm or 6.7 in). Most species have 20 body segments, bearing 11 pairs of leaf-like phyllopodia (swimming legs), and the body lacks a carapace . They live in vernal pools and hypersaline lakes across the world, including pools in deserts , in ice-covered mountain lakes and in Antarctica . They swim "upside-down" and feed by filtering organic particles from
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1152-402: The order Diplostraca , and family Sididae in the order Diplostraca . Most branchiopodans eat floating detritus or plankton , which they take using the setae on their appendages. But notostracans are omnivorous and very opportunistic feeders and will eat algae and bacteria in addition to animals as both predators and scavengers. In early taxonomic treatments, the current members of
1188-522: The spread of viral infections . HIV , for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. Branchiopoda Branchiopoda is a class of crustaceans . It comprises fairy shrimp , clam shrimp , Diplostraca (or Cladocera), Notostraca ,
1224-433: The two halves of the shell together. These four orders make up a group of small crustaceans commonly called water fleas. Around 620 species have been recognised so far, with many more undescribed . They are ubiquitous in inland aquatic habitats, but rare in the oceans. Most are 0.2–6.0 mm (0.01–0.24 in) long, with a down-turned head, and a carapace covering the apparently unsegmented thorax and abdomen. There
1260-459: The water or by scraping algae from surfaces. They are an important food for many birds and fish, and are cultured and harvested for use as fish food. There are 300 species spread across 8 families . Lipostraca contains a single extinct Early Devonian species, Lepidocaris rhyniensis , which is the most abundant animal in the Rhynie chert deposits. It resembles modern Anostraca, to which it
1296-431: Was confirmed in later molecular studies. Myriapoda Ostracoda Branchiura Pentastomida Mystacocarida Branchiopoda Copepod Thecostraca Malacostraca Remipedia Cephalocarida Hexapoda In a 2012 molecular study, von Reumont et al. challenge the monophyly of Vericrustacea: they present four versions of Pancrustacea cladogram (figures 1–4), and in all four figures Remipedia
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