Restudy of the Silurian Pentland Hills eurypterid fauna helps to resolve the poorly understood early phase of eurypterid evolution. ‘Drepanopterus’ bembycoides and ‘D’. lobatus are not stylonurines, but identified as synonyms of ‘Nanahughmilleria’ conica, representing a distinct new genus with an intriguing set of characters; an epistoma, appendages II-IV with poorly-expressed paired spines, and a pediform appendage VI with a modified podomere 7a (as in Eurypterina; swimming forms). It is assigned to the superfamily Moselopteroidea, along with Moselopterus and Vinetopterus, that share an oval metastoma with anterior notch, anterior ‘ears’ on the coxa of appendage VI, a pediform appendage VI with a podomere 7a, and serrate posterior margins of the postabdominal segments. This new genus is the most primitive Eurypterina known, and shows that both basal Eurypterina and Stylonurina possess a three-segmented genital operculum and a pediform appendage VI. Eurypterina are distinguished by the presence of a podomere 7a and the lack of transverse sutures on the prosoma. The new genus shows some adaptation towards a sediment-grubbing mode of life which suggests that the ‘swimming paddle’ of Eurypterina initially developed for digging in sediment much as in modern Limulus. Furthermore, some ‘derived’ characters, such as genital spatulae and epistomal sutures are now found in the most basal Eurypterina, which comprises Moselopteroidea, Eurypteroidea and an unnamed clade consisting of Onychopterella and Tylopterella. Megalograptus, previously considered to be a basal taxon, resolves as a member of the Mixopteroidea, suggesting a rapid diversification event early in eurypterid evolution, probably during the late Ordovician.