Palaeozoic arachnids and their significance for arachnid phylogeny
Jason A. Dunlop Department of Earth Sciences, University of Manchester, Manchester, M13 9PL, UK
Chelicerata originated from among the Arachnomorpha, though their relationships to other arachnomorph taxa remain uncertain. Chelicerata can be defined on the presence of a median eye tubercleand a postabdomen and comprise (Lemoneites (Xiphosura (Chasmataspida (Eurypterida + Arachnida)))). Arachnid origins are uncertain. Arachnida are currently recognized as monophyletic, though Scorpiones and Eurypterida may represent sister taxa and a number of arachnid characters, e.g. book-lungs, are convergent terrestrial adaptations. Some of the Gilboa trigonotarbids may, in fact, be early representatives of extant orders. Trigonotarbids show evidence of a vestigial third cheliceral segment, suggesting two-segmented chelicerae evolved through the loss of the most basal segment, and a new cheliceral orientation, palaeognathy, of paraxial, downwards-hanging chelicerae. Of the extinct orders, Trigonotarbida may be sister group of Ricinulei, Phalangiotarbida resemble Opilioacari and Haptopoda resemble Uropygi. A possible sister group of Schizomida has been identified among fossil Uropygi.
Proceedings of the 16th European Colloquium of Arachnology, 65-82
Trouble is since publishing this phylogeny model most of my subsequent work has been about trying to disprove it. Such is life. This paper reviews out a lot of my previous theories. By the way I pefer Arachnata to Arachnomorpha now.