This is the supplemental data for the paper X-ray microtomography of the Late Carboniferous whip scorpions (Arachnida, Thelyphonida) Geralinura britannica and Proschizomus petrunkevitchi. It comprises the following files: Garwood_Dunlop_Character_list.pdf - The character list for the phylogenetic analysis Garwood_Dunlop_chelicerate_dates_2020_Proschizomus_dates.csv - A CSV file of the dates for the terminals in the phylogeny, used to render the results in R using the STRAP package Garwood_Dunlop_chelicerate_matrix_2020_Proschizomus.nex - The phylogenetic matrix in MrBayes nex format Garwood_Dunlop_chelicerate_matrix_2020_Proschizomus.tnt - The phylogenetic matrix in TNT format Garwood_Dunlop_SI_Figure_01.png - A supplementary figure Garwood_Dunlop_SI_Figure_02.png - A supplementary figure Garwood_Dunlop_SI.mkv - A movie showing renders of the two taxa redescribed in the paper Garwood_geoscale_script.r - The R script used to render the phylogenies Geralinura britannica In31265 VAXML.zip - A model of Geralinura britannica in the VAXML interchange format Proschizomus petrunkevitchi I7912 VAXML.zip - A model of Proschizomus petrunkevitchi in the VAXML interchange format Whip scorpions (Thelyphonida) and schizomids (Schizomida) are closely related arachnid orders, whose low species diversity compared to other arachnid groups is reflected in a limited fossil record. Here we investigate two key fossil whip scorpions from the British Middle Coal Measures of Coseley, Staffordshire (Late Carboniferous, ca. 315 Ma), UK using X-ray microtomography, and incorporate the taxa into an updated arachnid phylogeny. Geralinura brittanica Pocock, 1911 is an unequivocal whip scorpion, whose pedipalps are closer to the modern condition than previously assumed. The trochanter bears a dorsal flange with prodorsal teeth, and the whole pedipalp is rendered subchelate by the presence of at least a patellar apophysis. These results suggest a high degree of stasis in the whip scorpion body plan since the Late Carboniferous, and the genus Geralinula Scudder, 1884 is within the crown-group; i.e. the extant family Thelyphonidae. Proschizomus petrunkevitchi Dunlop & Horrocks, 1996 lacks median eyes, a character shared with Schizomida, but unlike schizomids the pedipalp also has patellar and tibial apophyses; albeit with the appendage articulating at an angle of ~45° rather than articulating horizontally as in living whip scorpions, or vertically as in schizomids. Our phylogeny refutes a previous hypothesis that P. petrunkevitchi is a stem-schizomid, and places it among the whip scorpions instead. However, the species may have branched early from the whip scorpion lineage, and thus its morphology might reflect a plesiomorphic arrangement in which the pedipalps were transitioning from a more leg-like to a fully raptorial and claw-like set of prey-catching appendages