Author(s): Russell J. Garwood (1), Jason Dunlop (2)
Arachnids and their relatives (Chelicerata) form a major branch of the arthropods, with around 112,000 living species (Zhang, 2011). They also have an extensive palaeontological record, including more than 2,200 fossil species at the time of writing (Dunlop, Penney & Jekel, 2014). Chelicerates can be found back into the Cambrian (Waloszek & Dunlop, 2002; Dunlop, Anderson & Braddy, 2004), although their record through deep time is patchy and tends to be concentrated into windows of exceptional preservation such as the late Carboniferous Coal Measures and various Cretaceous and Cenozoic ambers. Currently, sixteen arachnid orders can be recognised. Twelve have living representatives: scorpions (Scorpiones), harvestmen (Opiliones), pseudoscorpions (Pseudoscorpiones), camel spiders (Solifugae), palpigrades (Palipgradi), mites (Acariformes and Parasitiformes), ricinuleids (Ricinulei), spiders (Araneae), whip spiders (Amblypygi), whip scorpions (Thelyphonida) and schizomids (Schizomida). Four arachnid orders are extinct: trigonotarbids (Trigonotarbida), phalangiotarbids (Phalangiotarbida), haptopodids (Haptopoda) and the spider-like uraraneids (Uraraneida). To this can be added two marine groups with living representatives, the sea spiders (Pycnogonida) and horseshoe crabs (Xiphosura), as well as two extinct groups which were likely to have been primarily aquatic, the sea scorpions (Eurypterida) and the rare chasmataspidids (Chasmataspidida).
Resolving relationships between the arachnid and/or chelicerate lineages remains a challenge. Important cladistic studies include the comprehensive morphological analyses of Weygoldt & Paulus (1979), Shultz (1990) and Shultz (2007), as well as numerous applications of molecular data-sometimes with morphology combined (e.g., Wheeler & Hayashi, 1998; Giribet et al., 2002; Pepato, da Rocha & Dunlop, 2010; Rehm et al., 2011). Few of these include fossil terminals (but see Giribet et al., 2002; Shultz, 2007)-despite the fact that extinct species provide a valuable source of data (Edgecombe, 2010). Several arthropod-wide analyses-both molecular and morphological-also include chelicerates (Regier et al., 2010; Legg, Sutton & Edgecombe, 2013; Rota-Stabelli, Daley & Pisani, 2013). Yet, as noted in a recent review (Dunlop, Borner & Burmester, 2014), there is still no single accepted phylogeny for arachnids and their relatives, and there are evident discrepancies between trees derived from morphological and molecular data. Dunlop, Borner & Burmester (2014) thus recognised a minimum consensus tree, i.e., supported by various methodologies, of the form (Pycnogonida (Xiphosura (Scorpiones (Araneae (Amblypygi (Thelyphonida + Schizomida)))))). This rather extensively pruned phylogeny still excludes diverse and important groups like mites, harvestmen and pseudoscorpions, and does not place any of the fossil taxa. Xiphosura was recently interpreted as paraphyletic (Lamsdell, 2013), at least with respect to Palaeozoic 'synziphosurines' which may include lineages eventually evolving into both crown-group horseshoe crabs and, separately, into arachnids.
Fossils have sometimes proved controversial in phylogenetic reconstruction, and for arachnids some authors simply excluded them completely (e.g., Wheeler & Hayashi, 1998). Extinct taxa offer direct evidence of early-and possibly quite different-body plans, but often have large amounts of missing data when compared to living taxa. Furthermore, scoring morphological character states in fossils involves a degree of interpretation, and objective inferences have to be made. Despite the challenges inherent in using fossils in such analyses, recent studies...