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.
The family Dipluridae, known as curtain-web spiders are a group of spiders in the infraorder Mygalomorphae, that have two pairs of booklungs, and chelicerae (fangs) that move up and down in a stabbing motion. A number of genera, including that of the Sydney funnel-web spider (Atrax), used to be classified in this family but have now been moved to Hexathelidae.
The Mesothelae are a suborder of spiders that includes a single extant family, Liphistiidae, and a number of extinct families. This suborder is thought to form the sister group to all other living spiders, and to retain ancestral characters, such as a segmented abdomen with spinnerets in the middle and two pairs of book lungs. Members of Liphistiidae are medium to large spiders with eight eyes grouped on a tubercle. They are found only in China, Japan, and southeast Asia. The oldest known Mesothelae spiders are known from the Carboniferous, over 300 million years ago.
Arthrolycosidae is an extinct family of arachnids, possibly spiders. Fossils placed in the family were found in the Carboniferous to Permian, 359 to 252 million years ago. They were considered by Alexander Petrunkevitch to be "mesotheles", i.e. placed in the spider suborder Mesothelae. However, Paul A. Selden has stated they only have "the general appearance of spiders", with segmented abdomens (opisthosomae), but no definite spinnerets. At least some of the specimens placed in the family in the genus Arthrolycosa are considered to be spiders, whereas Eocteniza is "rather problematic as a spider".
The Arthromygalidae are an extinct family of arachnids, possibly spiders. Fossils placed in the family were all found in the Carboniferous, 359 to 299 million years ago. They were considered by Alexander Petrunkevitch to be "mesotheles", i.e. placed in the spider suborder Mesothelae. Petrunkevitch used the family for Carboniferous fossils he regarded as mesotheles but which lacked eyes. Paul A. Selden has shown the fossil specimens only have "the general appearance of spiders", with segmented abdomens (opisthosomae), but no definite spinnerets.
Plesiosiro is an extinct arachnid genus known exclusively from nine specimens from the Upper Carboniferous of Coseley, Staffordshire, United Kingdom. The genus is monotypic, represented only by the species Plesiosiro madeleyi described by Reginald Innes Pocock in his important 1911 monograph on British Carboniferous arachnids. It is the only known member of the order Haptopoda.
The order Trigonotarbida is a group of extinct arachnids whose fossil record extends from the late Silurian to the early Permian. These animals are known from several localities in Europe and North America, as well as a single record from Argentina. Trigonotarbids can be envisaged as spider-like arachnids, but without silk-producing spinnerets. They ranged in size from a few millimetres to a few centimetres in body length and had segmented abdomens (opisthosoma), with the dorsal exoskeleton (tergites) across the backs of the animals' abdomens, which were characteristically divided into three or five separate plates. Probably living as predators on other arthropods, some later trigonotarbid species were quite heavily armoured and protected themselves with spines and tubercles. About seventy species are currently known, with most fossils originating from the Carboniferous coal measures.
Tetrapulmonata is a non-ranked supra-ordinal clade of arachnids. It is composed of the extant orders Uropygi, Schizomida, Amblypygi and Araneae (spiders). It is the only supra-ordinal group of arachnids that is strongly supported in molecular phylogenetic studies. Two extinct orders are also placed in this clade, Haptopoda and Uraraneida. In 2016, a newly described fossil arachnid, Idmonarachne, was also included in the Tetrapulmonata; as of March 2016 it has not been assigned to an order.
Araneida is a subgroup of Tetrapulmonata. It was originally defined by Jörg Wunderlich in 2015 as a subgroup of Araneae, including all true spiders, with Wunderlich also including Uraraneida within Araneae. Araneida was redefined by Wunderlich in 2019 to include all modern spiders (Araneae), as well as Chimerarachnida, but excluding Uraraneida. Chimerarachnida and Araneae both possess spinnerets, which are absent in Uraraneida. Uraraneida and Araneida are grouped together in the clade Serikodiastida.
Archaeidae, also known as assassin spiders and pelican spiders, is a spider family with about ninety described species in five genera. It contains small spiders, ranging from 2 to 8 millimetres long, that prey exclusively on other spiders. They are unusual in that they have "necks", ranging from long and slender to short and fat. The name "pelican spider" refers to these elongated jaws and necks used to catch their prey. Living species of Archaeidae occur in South Africa, Madagascar and Australia, with the sister family Mecysmaucheniidae occurring in southern South America and New Zealand.
The evolution of spiders has been ongoing for at least 380 million years. The group's origins lie within an arachnid sub-group defined by the presence of book lungs ; the arachnids as a whole evolved from aquatic chelicerate ancestors. More than 45,000 extant species have been described, organised taxonomically in 3,958 genera and 114 families. There may be more than 120,000 species. Fossil diversity rates make up a larger proportion than extant diversity would suggest with 1,593 arachnid species described out of 1,952 recognized chelicerates. Both extant and fossil species are described annually by researchers in the field. Major developments in spider evolution include the development of spinnerets and silk secretion.
Harvestmen (Opiliones) are an order of arachnids often confused with spiders, though the two orders are not closely related. Research on harvestman phylogeny is in a state of flux. While some families are clearly monophyletic, that is share a common ancestor, others are not, and the relationships between families are often not well understood.
Attercopus is an extinct genus of arachnids, containing one species Attercopus fimbriunguis, known from flattened cuticle fossils from the Panther Mountain Formation in Upstate New York. It is placed in the extinct order Uraraneida, spider-like animals able to produce silk, but which lacked true spinnerets and retained a segmented abdomen bearing a flagellum-like tail resembling that of a whip scorpion. They are thought to be close to the origins of spiders.
Rosamygale is a genus of extinct Triassic spiders, with a single described species, Rosamygale grauvogeli. It is the oldest known member of the Mygalomorphae, one of the three main divisions of spiders, which includes well known forms such as tarantulas and Australian funnel-web spiders. It was described by Selden and Gall in 1992, from specimens found in the Middle Triassic aged Gres a Meules and Grès à Voltzia geological formations in France. It is also considered to be the oldest known member of the Avicularioidea, one of the two main divisions of Mygalomorphae.
Idmonarachne is an extinct genus of arachnids, containing one species, Idmonarachne brasieri. It is related to uraraneids and spiders.
This list of fossil arthropods described in 2012 is a list of new taxa of trilobites, fossil insects, crustaceans, arachnids and other fossil arthropods of every kind that have been described during the year 2012. The list only includes taxa at the level of genus or species.
Chimerarachne is a genus of extinct arachnids, containing a single species Chimerarachne yingi. Fossils of Chimerarachne were discovered in Burmese amber from Myanmar which dates to the mid-Cretaceous, about 100 million years ago. It is thought to be closely related to spiders, but outside any living spider clade. The earliest spider fossils are from the Carboniferous, requiring at least a 170 myr ghost lineage with no fossil record. The size of the animal is quite small, being only 2.5 millimetres (0.098 in) in body length, with the tail being about 3 millimetres (0.12 in) in length. These fossils resemble spiders in having two of their key defining features: spinnerets for spinning silk, and a modified male organ on the pedipalp for transferring sperm. At the same time they retain a whip-like tail, rather like that of a whip scorpion and uraraneids. Chimerarachne is not ancestral to spiders, being much younger than the oldest spiders which are known from the Carboniferous, but it appears to be a late survivor of an extinct group which was probably very close to the origins of spiders. It suggests that there used to be spider-like animals with tails which lived alongside true spiders for at least 200 million years.
Permarachne is an extinct genus of arachnids containing the single species Permarachne novokshonovi from the Permian (Kungurian) of Russia, found in the Koshelevka Formation near the town of Suksun in Perm Krai. It is closely related to modern spiders but unlike them, it has a long thin tail, similar to its relative Attercopus, it is known from the mostly complete holotype PIN 4909/12. It is about 1 cm in size. It initially was thought to be a spider, but is now thought to form a clade with at least its close relative Attercopus, forming the grouping Uraraneida.
Lagonomegopidae is an extinct family of spiders known from the Cretaceous period. Members of the family are distinguished by a large pair of eyes, positioned on the anterolateral flanks of the carapace, with the rest of the eyes being small. They have generally been considered members of Palpimanoidea, but this has recently been questioned. Members of the family are known from the late Early Cretaceous (Albian) to near the end of the Late Cretaceous (Campanian) of Eurasia, North America and the Middle East, which was then attached to Africa as part of Gondwana. They are generally assumed to have been free living hunters as opposed to web builders.
