Evidence for a sister group relationship between Ricinulei and Trigonotarbida
Jason A. Dunlop
Departmet of Earth Sciences, University of Manchester, Manchester, M13 9PL
A review of previous placements of the arachnid order Ricinulei with particular reference to the fossil order Trigonotarbida, and comparison of the morphology of these two orders, has resulted in a revised scheme of opisthosomal segmentation for Ricinulei. Cladistic analysis suggests that Ricinulei are a sister group to Trigonotarbida, these two orders being the sister group of the extant tetrapulmonate arachnids (Araneae, Amblypygi, Uropygi and Schizomida). Suggested synapomorphies of Ricinulei and Trigonotarbida are: longitudinally divided tergites, fusion of tergites 2 and 3 into a diplotergite, a dorsal locking ridge and ventral recesses in the opisthosoma to accomodate the leg 4 coxae. Doubts still remain over the position of the Acari and Opiliones in this scheme.
Bulletin of the British Arachnological Society, 10, 193-204 (1996)
Author's comments:
This was one of my favourite pieces of comparative anatomy between a living and fossil group. I've heard that the cladogram may be wrong, but to be honest I didn't want to include the bloody thing anyway (it really is quite shit), the whole point of the paper was to demonstrate characters that could be used to support Ricnulei + Trigonotarbida and maybe put them in a bigger analysis of all the arachnids later.
So far 
people were interested in trigonotarbid / ricinuleid relationships