Currently the oldest known land arthropods, Trigonotarbids are extinct spider-like animals that belong to the extinct order Trigonotarbida in the class Arachnida. Despite the resemblance, they were not true spiders. The trigonotarbids lack silk glands on their opisthosoma and cheliceral poison glands, and most likely represented independent offshoots of the Arachnida.

They are characterized by opisthosomal tergites divided into median and lateral plates (Dunlop, 1995). This character is shared with Ricinulei (Dunlop, 1996), supporting the sister group relationship between these two taxa recently reported by cladistic analysis (Giribet et al., 2002). There is also a proposal that the Trigonotarbida are members of the Tetrapulmonata (Shear et al., 1987).

References

• DUNLOP, J.A. (1995). Redescription of the Pennsylvanian trigonotarbid arachnid Lissomartus Petrunkevitch 1949 from Mazon Creek, Illinois. The Journal of Arachnology, 23:118-124.

• DUNLOP, J.A. (1996). Evidence for a sister group relationship between Ricinulei and Trigonotarbida. Bulletin of the British Arachnological Society, 10:193–204.

• GIRIBET, G., EDGECOMBE, G. D., WHEELER, W. C. & BABBITT, C. (2002). Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data. Cladistics, 18:5–70.

• SHEAR, W. A., P. A. SELDEN, W. D. I. ROLFE, P. M. BONAMO & J. D. GRIERSON. (1987). New terrestrial arachnids from the Devonian of Gilboa, New York (Arachnida, Trigonotarbida). American Museum Novitates, New York, 2901: 1 -74.

External links

• Chelicerata

A pdf of Shear et al. 1987 article may be found at the AMNH biblio resource: ^*