Trigonotarbus johnsoni Pocock, 1911, revealed by X-ray computed tomography, with a cladistic analysis of the extinct trigonotarbid arachnids

Trigonotarbids are a group of Palaeozoic arachnids, best known from Coal Measures sites in Europe and North America. Here, we review previous work on their higher systematics and redescribe Trigonotarbus johnsoni Pocock, 1911, from the Coseley Lagerstatte, UK, on the basis of both hand specimens and X-ray microtomography reconstruction. This is a historically important species – being the eponymous taxon for the entire order – but lacks recent or comprehensive description. We also present the first cladistic analysis of Trigonotarbida, based on a novel matrix of 49 characters, 20 trigonotarbids, and four outgroups. Two families, Archaeomartidae and Lissomartidae, were only represented by a single exemplar. At least two exemplars were scored to test monophyly for other families. Anthracomartidae, Anthracosironidae, and Eophrynidae were all recovered as monophyletic, but Palaeocharinidae, Trigonotarbidae, Aphantomartidae, and Kreischeriidae were not. In a wider context, our data support a (Trigonotarbida + Ricinulei) clade; albeit based on a limited set of outgroups. Relationships within Trigonotarbida were not so well resolved, although two previously anticipated clades were consistently recovered in the agreement subtrees with strong support: (1) (Palaeocharinus (Archaeomartidae + Anthracomartidae)) and (2) Lissomartus as sister group to a so-called ‘eophrynid assemblage’ (Aphantomartus (Alkenia (Pseudokreischeria (Kreischeria (Eophrynus + Pleophrynus))))). © 2014 The Linnean Society of London