When discussing the macroevolution of scorpions, little of interest can be discussed about the tail! However, in lower level classification, keels, carinatation and other ornamentation can be quite important. One trend of note though is that in more toxic species (ie. buthids) scorpions have more gracile pedipalps and much thicker tails. Also, from Palaeopisthacanthus onwards the pre-anal segment (the anus is located just before the sting) is longer than the segment before it and this isn't seen in earlier scorpions.
All scorpions possess a stinger. This can be split into the bulbous, venom gland- containing vesicle and the hypodermic needle which delivers the sting, called the aculeus. The venom that scorpions release is genereally neurotoxic, and is of variable potency, but the Buthidae are among the worst offenders. As a general rule however, those scorpions with fat tails and gracile claws (those where the fingers are longer than the palm) are likely to be the most toxic, whilst those with robust claws and thin tails are more likely to use their claws in killing their prey. That said, scorpions do possess 2 types of venom, one to merely stun their prey and another, stronger form, designed to kill.
Given a eurypterid ancestory, it is thought that the scorpion telson evolved from the telson of eurypterids. In most eurypterids, this was involved in propulsion or balance during swimming, however, in the Mixopteroidea it was very simialr in shape to that of scorpions and is believed to have been envenomated and held over-head like scorpions do. Little of use can be discussed regarding the evolution of the telson and venom, as venom does not preserve and there has been little change in the shape of the telson. That said, featurs such as the ratio of aculeus: vesicle, the presence of setae, Latero-basal aculear serations, granulation and carination are all of use in the lower (Family/Subfamily) systematics of modern scorpions. Some Palaeozoic oddities are of note, such as Palaeobuthus, which developed an incredibly long aculeus in the Carboniferous and it should also be made clear that the sub-aculear teeth (convergently evolved in a number of scorpion familes) are not known in any Palaeozoic or Mesozoic forms, possible only arising in the Tertiary.