A redescription of the Carboniferous arachnid Plesiosiro madeleyi Pocock, 1911 (Arachnida : Haptopoda)

Plesiosiro madeleyi Pocock, 1911, the monotypic representative of the fossi l order Haptopoda Pocock, 1911 from the British Middle Coal Measures (Upper Carboniferous, Westphalian B) of Coseley, Staffordshire, is redescribed. A specimen from Sparth Bottoms, Lancashire is not a Plesiosiro and is referr ed to Arthropoda incertae sedis. Plesiosiro was probably a predator, having tooth-like projections on the femora for prey capture, and shows adaptatio ns of the limbs for crawling in narrow spaces. The uncertain nature of the chelicerae, eyes and respiratory organs makes resolving the phylogenetic po sition of Plesiosiro difficult, though Haptopoda deserves its status as a s eparate order. Plesiosiro resembles both cythophthalmid and trogulid opilio nids. Its broad prosoma-opisthosoma junction could be interpreted as a syna pomorphy for Plesiosiro + Opiliones, but this character is not unique to th ese orders. Alternatively, Plesiosiro has a large, ventral opisthosomal scl erite. This is interpreted here as a genital operculum, a synapomorphy for Tetrapulmonata, and Plesiosiro is tentatively referred to this clade. Based on the synapomorphies of a divided sternum and subdivided telotarsi a rela tionship of the form: (Plesiosiro (Amblypygi (Thelyphonida + Schizomida))) is proposed.