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The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids, as well as a number of extinct lineages, such as the eurypterids and chasmataspidids.
Arachnida is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaroons.
Ecdysozoa is a group of protostome animals, including Arthropoda, Nematoda, and several smaller phyla. The grouping of these animal phyla into a single clade was first proposed by Eernisse et al. (1992) based on a phylogenetic analysis of 141 morphological characters of ultrastructural and embryological phenotypes. This clade, that is, a group consisting of a common ancestor and all its descendants, was formally named by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes.
Remipedia is a class of blind crustaceans, closely related to hexapods, found in coastal aquifers which contain saline groundwater, with populations identified in almost every ocean basin so far explored, including in Australia, the Caribbean Sea, and the Atlantic Ocean. The first described remipede was the fossil Tesnusocaris goldichi. Since 1979, at least seventeen living species have been identified in subtropical regions around the world.
Myriapods are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial.
Atelocerata is a proposed clade of arthropods that includes Hexapoda and Myriapoda, but excludes Crustacea and Chelicerata. The name is currently used interchangeably with Tracheata. or Uniramia sensu stricto. It is an extensive division of arthropods comprising all those that breathe by tracheae, as distinguished from Crustacea, which breathe by means of gills.
Arachnomorpha is a proposed subdivision or clade of Arthropoda, comprising the group formed by the trilobites and their close relatives (Artiopoda), Megacheira and chelicerates. Under this proposed classification scheme, Arachnomorpha is considered the sister group to Mandibulata.
Pancrustacea is the clade that comprises all crustaceans, including the cladistically included hexapods. 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 is considered well accepted, with most studies recovering Hexapoda within Crustacea. The clade has also been called Tetraconata, referring to having four cone cells in the ommatidia. This name is preferred by some scientists as a means of avoiding confusion with the use of "pan-" to indicate a clade that includes a crown group and all of its stem group representatives.
Cheloniellida is a taxon of extinct Paleozoic arthropods. As of 2018, 7 monotypic genera of cheloniellids had been formally described, whose fossils are found in marine strata ranging from Ordovician to Devonian in age. Cheloniellida has a controversial phylogenetic position, with previous studies associated it as either a member or relative of various fossil and extant arthropod taxa. It was later accepted as a member of Vicissicaudata within Artiopoda.
Mandibulata, is one of two major clades of living arthropods alongside Chelicerata. It comprises the extant groups Myriapoda and Pancrustacea. The name "Mandibulata" refers to the mandibles, a modified pair of limbs used in food processing, the presence of which are characteristic of most members of the group. Members of the group are referred to as mandibulates.
The (pan)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 chelicerates, myriapods, and crustaceans, as well as 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, have been published, including several in the 2000s.
Arthropods are invertebrate animals in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin, often mineralised with calcium carbonate, a segmented body, and paired jointed appendages. In order to keep growing, they must go through stages of moulting, a process by which they shed their exoskeleton to reveal a new one. They are an extremely diverse group, with up to 10 million species.
Xenocarida is a proposed clade inside the subphylum Crustacea that comprises two classes that were discovered in the 20th century: Remipedia and Cephalocarida. Both groups are marine hermaphrodites. The clade was recovered as the sister groups to Hexapoda.
Crustaceomorpha is a proposed clade of arthropods that includes crustaceans and numerous extinct groups. Synapomorphies for the clade are that the larval antenna is a feeding or locomotory organ, and there are six endopodal podomeres in post-antennal limbs.
The subphylum Hexapoda comprises most species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura. The Collembola are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs. Most other arthropods have more than three pairs of legs. Most recent studies have recovered Hexapoda as a subgroup of Crustacea.
Tactopoda or Arthropodoidea is a proposed clade of protostome animals that includes the phyla Tardigrada and Euarthropoda, supported by various morphological observations. The cladogram below shows the relationships implied by this hypothesis.
Crustaceans belong to the subphylum Crustacea, and form a large, diverse group of arthropods including decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can be treated as a subphylum under the clade Mandibulata. It is now well accepted that the hexapods emerged deep in the Crustacean group, with the completed group referred to as Pancrustacea. The three classes Cephalocarida, Branchiopoda and Remipedia are more closely related to the hexapods than they are to any of the other crustaceans.
Gonzalo Giribet is a Spanish-American invertebrate zoologist and Alexander Agassiz Professor of zoology working on systematics and biogeography at the Museum of Comparative Zoology in Harvard University. He is a past president of the International Society for Invertebrate Morphology, of the Willi Hennig Society, and vice-president of the Sociedad Española de Malacología.
The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomorpha. Trilobites, in part due to their mineralising exoskeletons, are by far the most diverse and long lived members of the clade, with most records of other members, which lack mineralised exoskeletons, being from Cambrian deposits.
Antennulata is a proposed clade of arthropods comprising Artiopoda, Marrellomorpha and Mandibulata. As its name suggests, a proposed unifying feature of the clade is the presence of antennulae. It contrasts with the opposing Arachnomorpha hypothesis, which instead posits that artiopods and marrellomorphs are more closely related to Chelicerates.
References
- ↑ S. Richter (2002). "The Tetraconata concept: hexapod-crustacean relationships and the phylogeny of Crustacea". Organisms Diversity & Evolution . 2 (3): 217–237. doi: 10.1078/1439-6092-00048 .
- ↑ Casey W. Dunn; Andreas Hejnol; David Q. Matus; Kevin Pang; William E. Browne; Stephen A. Smith; Elaine Seaver; Greg W. Rouse; Matthias Obst; Gregory D. Edgecombe; Martin V. Sørensen; Steven H. D. Haddock; Andreas Schmidt-Rhaesa; Akiko Okusu; Reinhardt Møbjerg Kristensen; Ward C. Wheeler; Mark Q. Martindale; Gonzalo Giribet (2008). "Broad phylogenomic sampling improves resolution of the animal tree of life". Nature . 452 (7188): 745–749. doi:10.1038/nature06614. PMID 18322464. S2CID 4397099.
- ↑ Jerome C. Regier; Jeffrey W. Shultz; Andreas Zwick; April Hussey; Bernard Ball; Regina Wetzer; Joel W. Martin; Clifford W. Cunningham (2010). "Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences". Nature . 463 (7284): 1079–1083. doi:10.1038/nature08742. PMID 20147900. S2CID 4427443.
- ↑
- Omar Rota-Stabelli, Lahcen Campbell, Henner Brinkmann, Gregory D. Edgecombe, Stuart J. Longhorn, Kevin J. Peterson, Davide Pisani, Herve Philippe, and Maximilian J. Telford (2011) A congruent solution to arthropod phylogeny: phylogenomics, microRNAs and morphology support monophyletic Mandibulata. Proc R Soc B 278: 298-306.
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