Trigonotarbida

Extinct order of arachnids

The order Trigonotarbida is a group of extinct arachnids whose fossil record extends from the late Silurian to the early Permian (Pridoli to Sakmarian). 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. About seventy species are currently known, with most fossils originating from the Carboniferous coal measures.

The first trigonotarbid was described in 1837 from the coal measures of Coalbrookdale in England by the famous English geologist Dean William Buckland. He believed it to be a fossil beetle and named it Curculoides prestvicii. A much better preserved example was later discovered from Coseley near Dudley; also in the English West Midlands conurbation. Described in 1871 by Henry Woodward, he correctly identified it as an arachnid and renamed it Eophrynus prestvicii—whereby the genus name comes from [[:wikt:ἠώς (eos, meaning 'dawn'), and Phrynus, a genus of living whip spider (Amblypygi). Woodward subsequently described another trigonotarbid, Brachypyge carbonis, from the coal measures of Mons in Belgium; although this fossil is known only from its abdomen and was initially mistaken for those of a crab.

[[File:20201202 Trigonotarbus johnsoni. png|thumb|left|Reconstruction of Trigonotarbus johnsoni, the eponymous species of Trigonotarbida. This species was raised to a new, extinct, arachnid order which Karsch called Anthracomarti. The name is derived from [[:wikt:ἄνθραξ (anthrax), the Greek word for coal. A number of other fossils which would eventually be placed in Trigonotarbida were discovered around this time. Hanns Bruno Geinitz described Kreischeria wiedei from the coal measures of Zwickau in Germany, although he interpreted it as a fossil pseudoscorpion. Johann Kušta described Anthracomartus krejcii from Rakovník in the Czech Republic, and published further descriptions in a number of subsequent papers. In 1884, Samuel Hubbard Scudder described Anthracomartus trilobitus from Fayetteville, Arkansas—the first trigonotarbid from North America.

In 1892, Ferdinand Karsch suggested that the rare and rather bizarre looking ricinuleids (Ricinulei) were the last living descendants of the trigonotarbids. A similar hypothesis was reintroduced by Dunlop, who pointed out distinct similarities and possible sister group relationship between these arachnid groups. Both have opisthosomal tergites divided into median and lateral plates and both have a complicated coupling mechanism between the prosoma and the opisthosoma which 'locks' the two halves of the body together. Although cladistic analysis has tended to recover ricinuleids in their traditional position closely related to mites and ticks, further discoveries have revealed that the tip of the pedipalp ends in a small claw in both trigonotarbids and ricinuleids. If the hypothesis is true, ricinuleids, despite the lack of tetrapulmonate key characters (e. g. book lungs), may represent part of the pantetrapulmonate clade alongside trigonotarbids as well. This recovered the families Anthracomartidae, Anthracosironidae, and Eophrynidae as monophyletic. In contrast Trigonotarbidae, Aphantomartidae, Palaeocharinidae, and Kreischeriidae were not. Two clades were consistently recovered with strong support—(Palaeocharinus (Archaeomartidae + Anthracomartidae)), and Lissomartus as sister group the 'eophrynid assemblage' (Aphantomartus (Alkenia (Pseudokreischeria (Kreischeria (Eophrynus + Pleophrynus))))).

thumb|Dorsal (A) and ventral (B) morphology of a trigonotarbid. thumb|External morphology of a trigonotarbid in sagittal section. Trigonotarbids superficially resemble spiders, but can be easily recognised by having tergites on the dorsal side of the opisthosoma divided into median and lateral plates. This character is shared with ricinuleids (Ricinulei) (see also Ricinulei#Relationships). As in other arachnids, the body is divided into a prosoma (or cephalothorax) and opisthosoma (or abdomen). Body length ranges from a couple of millimetres up to about .

The prosoma is covered by the carapace and always bears a pair of median eyes.—and perhaps also Anthracosironidae—there is an additional pair of lateral eye tubercles which, at least in palaeocharinids, appear to have borne a series of individual lenses. In this sense palaeocharinids seem to be in the process of reducing a compound eye. Anterior margin of the carapace protrude into a projection referred to as clypeus. There is no evidence in well preserved fossils for the opening of a venom gland, thus trigonotarbids were probably not venomous. The chelicerae may have been slightly retractable into the prosoma. Well preserved palaeocharinids show evidence for a small, slit like mouth with an upper lip (a labrum or rostrum) and a lower lip (or labium). The pedipalps have the typical arachnid structure with a coxa, trochanter, femur, patella, tibia and tarsus. They are pediform, i. e. they look like small legs and were not highly modified. There is no evidence for a special sperm transfer device as in the modified palpal organ of male spiders. In at least the palaeocharinids and anthracomartids the tip of the pedipalp is modified into a small chela (claw) formed from the tarsal claw (or apotele) and a projection from the tarsus. As mentioned above, a very similar arrangement is seen at the end of the pedipalp in Ricinulei. presumably to help catch prey. The legs end in three claws, two large ones and a smaller median claw. Tergite of the first segment partially covered by the posterior margin of preceding carapace, forming a complicated coupling mechanism known as 'locking ridge'. A pair of ventral sacs located between the posterior operculum and following sternite had been observed in some species. Subsequent work by the researchers behind the initial publication