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. 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.
Historical background
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 (, 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.
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 (), 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.
Relationships
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 (Araneae), whip spiders (Amblypygi), whip scorpions (Uropygi) and shorttailed whipscorpion (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.