Trigonotarbid

The order Trigonotarbida is a group of extinct arachnids whose fossil record extends from the late Silurian to the early Permian (c. 419 to 290 million years ago). 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, with the 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. In July 2014 scientists used computer graphics to re-create a possible walking gait for the animal.

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 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 the back end of a crab.
A new arachnid order
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. This species
was raised to a new, extinct, arachnid order which Karsch called
Anthracomarti. The name is derived from 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.

Early studies tended to confuse trigonotarbids with other living or extinct groups of arachnids; particularly harvestmen (Opiliones).
 Petrunkevitch's division of the trigonotarbids into two, unrelated,
orders was noted above. In detail, he divided the arachnids into
suborders based on the width of the division between the two parts of
the body (the prosoma and opisthosoma). Anthracomartida and another extinct order, Haptopoda,
 were grouped into a subclass Stethostomata defined by a broad division
of the body and downward-hanging mouthparts. Trigonotarbida was placed
in its own subclass Soluta and defined as having a division of the body
which was variable in width. Petrunkevitch's scheme was largely followed
 in subsequent studies of fossil arachnids.
Pantetrapulmonata
In the 1980s, Bill Shear and colleagues carried out an important study on well preserved Mid Devonian trigonotarbids from Gilboa, New York.
 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. They showed that
trigonotarbids are closely related to a group of arachnids which have
gone under various names (Caulogastra, Arachnidea, etc.), but for which
the name Tetrapulmonata has become most widespread. Members of the Tetrapulmonata include spiders, whip spiders, whip scorpions and Schizomida and, together with trigonotarbids, share characters like two pairs of book lungs
 and similar mouthparts with fangs operating rather like a pocket knife.
 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. This has since been
corroborated in more recent cladistic analyses.

Trigonotarbids superficially resemble spiders, but can be easily recognised by having tergites on the dorsal side of the abdomen 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 5 cm (2.0 in).
Prosoma
The
 prosoma is covered by the carapace and always bears a pair of median
eyes. In the probably basal families Palaeocharinidae, Anthracomartidae—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.
The
 chelicerae are of the "pocket-knife" type consisting of a basal segment
 and a sharp, curving fang. The chelicerae are described as
paleognathic: the fangs are held parallel to one another, like those of mesothele and mygalomorph spiders, but the chelicerae hang downwards like those of araneomorph
 spiders. 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). 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.