ARACHNIDA LAMARCK, 1801

Lamsdell, James C., 2013, Revised systematics of Palaeozoic ‘ horseshoe crabs’ and the myth of monophyletic Xiphosura, Zoological Journal of the Linnean Society 167 (1), pp. 1-27 : 21

ARACHNIDA LAMARCK, 1801

Diagnosis: Sclerophorata with reduced head shield lacking cardiac lobe; anteroventrally directed mouth; proventricular crop lost; adult instars without appendages of the first opisthosomal segment.

Remarks: A monophyletic Arachnida is one of the most strongly supported clades within the analysis, even without characters traditionally considered to unite the group that have been suggested to represent convergences towards a terrestrial mode of life ( Dunlop & Webster, 1999). Dunlop et al. (2012) recently reviewed the cephalic tagmosis in solifuges, acariformes, schizomids, and palpigrades, which have the prosoma separated anteriorly into a propeltidium consisting of the first four appendage pairs, that they suggested may be homologous to the ‘euarthropod head’ (see Chen, 2009). Dunlop et al. suggest that a divided prosomal region consisting of a propeltidium and two separate segments bearing walking limbs may be plesiomorphic for arachnids. This would, however, necessitate a separate acquisition of the prosoma in each of the aquatic euchelicerate groups. The solifuge Galeodes armeniacus Birula, 1929 was included in the current analysis with its head composition coded polymorphically as both 1 + 3 and 1 + 5, representing the fact that it possesses a propeltidium as a subdivision of the prosoma. The results, however, show that the plesiomorphic condition for arachnids is a fully consolidated prosoma, and the division into propeltidium is a derived state (or possibly a reversal). Alternatively coding G. armeniacus as only possessing a 1 + 3 head did not change the tree topology, or the arachnid ground plan, in any way.