Trigonotarbida

Taxobox | fossil_range = late Silurian–early Permian
name = Trigonotarbida
regnum = Animalia
phylum = Arthropoda
subphylum = Chelicerata
classis = Arachnida
ordo = Trigonotarbida †
ordo_authority = Petrunkevitch, 1949
subdivision_ranks = Families
subdivision =
Anthracomartidae
Anthracosironidae
Trigonotarbidaincertae
Palaeocharinidae
Eophrynidae
Kreischeriidae
Trigonotarbidae
Lissomartidae
TrigonomartidaeThe Order Trigonotarbida is an extinct group of arachnids whose fossil record extends from the Silurian to the Lower Permian and are known from several localities in Europe and North America. They superficially resemble spiders, to which they were clearly related.

Both have eight legs and a pair of pedipalps. They also have two main body parts (cephalothorax and abdomen). It was once thought that trigonotarbids lacked the silk-producing spinnerets that have apparently been crucial to the spider's evolutionary success, though in recent years at least one (unpublished) fossil find seems to show distinct microtubercles on its hind legs, akin to those used by spiders to direct and manipulate their silk.

They also have strongly segmented abdomen and hard, chitinous exoskeleton and don't exhibit the constriction that spiders have between the cephalothorax and the opisthosoma.

These early arachnids seem to have been adapted to stalking prey on the ground. They have been found within the very structure of ground-dwelling plants, where they possibly hid to await their prey.

They are characterized by opisthosomal tergites divided into median and lateral plates [cite journal |quotes=no |author=Dunlop, J. A. |year=1995 |title=Redescription of the Pennsylvanian trigonotarbid arachnid "Lissomartus" Petrunkevitch 1949 from Mazon Creek, Illinois |journal=The Journal of Arachnology |volume=23 |pages=118–124] . This character is shared with Ricinulei [cite journal |quotes=no |author=Dunlop, J. A. |year=1996 |title=Evidence for a sister group relationship between Ricinulei and Trigonotarbida |journal=Bulletin of the British Arachnological Society |volume=10 |pages=193–204] , supporting the sister group relationship between these two taxa recently reported by cladistic analysis [cite journal |quotes=no |author=Giribet, G., G. D. Edgecombe, W. C. Wheeler & C. Babbitt |year=2002 |title=Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data |journal=Cladistics |volume=18 |pages=5–70] .

There is also a proposal that the Trigonotarbida are members of the Tetrapulmonata [cite journal |quotes=no |author=Shear, W. A., P. A. Selden, W. D. I. Rolfe, P. M. Bonamo & J. D. Grierson |year=1987 |title=New terrestrial arachnids from the Devonian of Gilboa, New York (Arachnida, Trigonotarbida) |journal=American Museum Novitates |volume=2901 |pages=1–74 |url=http://digitallibrary.amnh.org/dspace/handle/2246/5166] .

References