Fossil evidence, terrestrialization and arachnid phylogeny

Geological and morphological evidence suggests that the earliest scorpions were at least partially aquatic and that terrestrialization occurred within the scorpion clade. Scorpions and one or more other arachnid lineages are therefore likely to have come onto land independently. The phylogenetic pos ition of scorpions remains controversial and we question Dromopoda, in whic h scorpions are placed derived within Arachnida, as this is not supported b y scorpions' lateral eye rhabdomes, embryology and sperm morphology. We pro pose a synapomorphy for scorpions + eurypterids, a postabdomen of five segm ents as part of an opisthosoma of 13 segments. Scorpions and tetrapulmonate s must have evolved their book lungs convergently while fossil evidence ind icates that a stomotheca, synapomorphic for Dromopoda, is probably converge nt too. 'Arachnid' characters such as Malpighian tubules, the absence of a carapace pleural margin, and an anteriorly directed mouth may also be conve rgent, although their status as synapomorphies fan be defended using parsim ony. Convergence is difficult to prove unequivocally, but when there are st rong grounds for suspecting it, such characters are questionable evidence f or arachnid monophyly.