The order Trigonotarbida is a group of extinct arachnids whose fossil record extends from the late Silurian to the early Permian (Pridoli to Sakmarian).

Woodward subsequently described another trigonotarbid, Brachypyge carbonis, from the coal measures of Mons in Belgium;[7] although this fossil is known only from its abdomen and was initially mistaken for those of a crab.

In 1882, the German zoologist Ferdinand Karsch described a number of fossil arachnids from the coal measures of Neurode in Silesia (now Poland), including one he named Anthracomartus voelkelianus in honour of Herr Völkel, the foreman of the mine where it was discovered.

[15] †Trigonotarbita Ricinulei Araneae †Haptopoda Amblypygi Uropygi (Thelyphonida) Schizomida Early studies tended to confuse trigonotarbids with other living or extinct groups of arachnids; particularly harvestmen (Opiliones).

They questioned whether it was appropriate to define a group of animals on a variable character state and carried out the first cladistic analysis of fossil and living arachnids.

[20] In a 2007 study of arachnid relationships, the Shear et al. hypothesis was largely supported and a group Pantetrapulmonata was proposed which comprises Trigonotarbida + Tetrapulmonata.

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.

[17][27] 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.

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))))).

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.

[1] In the probably basal families Palaeocharinidae, Anthracomartidae[30]—and perhaps also Anthracosironidae—there is an additional pair of lateral eye tubercles which, at least in palaeocharinids,[31] appear to have borne a series of individual lenses.

[34] Inside the mouth there is some sort of filtering system formed from hairs or platelets which strongly suggests that trigonotarbids (like spiders and many other arachnids) could eat only preorally digested, liquified prey.

[17][27] The walking legs again follow the typical arachnid plan with a coxa, trochanter, femur, patella, tibia, metatarsus and tarsus.

Just like other lung-bearing arachnids (scorpion and tetrapulmonate), the book lungs of trigonotarbids formed by layers of trabecula-bearing lamellae, which is a feature adapted to a terrestrial, air-breathing lifestyle.

[40] Subsequent work by the researchers behind the initial publication[35] used simulation approaches to assess the efficiency of a range of gaits using an updated trigonotarbid model.