LEG EVOLUTION The legs of terrestrial arthropods are characterised by a differentiation in podomere length and an oval/compressed cross section in order to support the animals weight on land. We should be careful in using this feature as a sole indication of habitat though, as in very large aquatic eurypterids such as Tarsopterella this can also be seen - because they are so big they needed greater support!. It is generally thought that terrestrialisation of the scorpion lineage took place somewhere in the Palaeoscorpiones. If this is true then you might expect the aquatic protoscorpions to demonstrate the primitive condition. Sure enough, the four genera belonging to the Protoscorpiones (Allopalaeophonus, Palaeoscorpius, Palaeophonus and Dolichophonus) all have podomeres of roughly equal length (albeit decreasing distally) and tubular cross sections. Although Stockwell (In a very important, widely read but unpublished PhD thesis!) claimed the proscorpioids also showed this feature, Jeram (1997) codes these fossils as possessing the more typical, modern condition.
PEDIPALP EVOLUTION Chelate (pincer-like) pedipalps are found in ALL scorpions, but if scorpions evolved from mixopterid eurypterids (which also have chelate pedipalps) this is unsuprising. Scorpion pedipalps come in 2 main forms, GRACILE (with fingers longer than the palm) and ROBUST (with fingers shorter than the palm). The different pedipalp morphs are related to the mode of feeding in scorpions (Gracile morphs being found in much more toxic varieties and Robust form being used to crush prey in less toxic scorpions). It is of little importance in the systematics of fossil scorpions, but in recent scorpions it can be quite important as the Buthidae are all Gracile scorpions.
Photo of the extant, gracile scorpion Lychas suctilus taken from the 'Scorpion Files'.
Image of the Robust clawed Tarsoporosus yustizi taken from the 'Scorpion Files'.