Abstract:  X‐ray microtomography (XMT) was applied to three species of the extinct arachnid order Trigonotarbida, hosted in siderite nodules from the late Carboniferous British Middle Coal Measures. All three, Cryptomartus hindi (Pocock, 1911), Cleptomartus plautus (Petrunkevitch, 1949) and Maiocercus celticus (Pocock, 1902), belong to the family Anthracomartidae. As well as providing interactive three‐dimensional visualisations of their likely appearance in life, XMT study has resolved new morphological detail, yielding key data about the likely stance and habitus of these early arachnids. Similarities in the form of the carapace, eyes and coxosternal region between anthracomartids and the Devonian genus PalaeocharinusHirst, 1923 are highlighted. Analogies in limb disposition are drawn between anthracomartids and modern crab spiders (Araneae: Thomisidae), which exhibit sit‐and‐wait style predatory behaviour. Anthracomartids are relatively common fossils at many Coal Measures localities. Although they appear to exhibit limited morphological variation, numerous genera and species have been proposed in the literature, often distinguished from each other on rather trivial characters. From our reconstruction of well‐preserved examples, we have resolved a number of common features likely to be present in a typical anthracomartid. Comparisons to this model suggest that the characters on which many anthracomartid genera are distinguished may be artefacts of preservation. We hence treat PromygaleFrič, 1901, BrachylycosaFrič, 1904, PerneriaFrič, 1904, CoryphomartusPetrunkevitch, 1945, CryptomartusPetrunkevitch, 1945, PleomartusPetrunkevitch, 1945, CleptomartusPetrunkevitch, 1949 and OomartusPetrunkevitch, 1953 as junior synonyms of the type genus of the family, AnthracomartusKarsch, 1882. Only the genera BrachypygeWoodward, 1878 and MaiocercusPocock, 1911 are additionally retained. Within Anthracomartus, Cleptomartus planusPetrunkevitch, 1949, Cryptomartus meyeriGuthörl, 1964, Cleptomartus hangardiGuthörl, 1965 and Cryptomartus rebskeiBrauckmann, 1984 are treated as junior synonyms of A. hindiPocock, 1911 (all syn. nov.). Cleptomartus plautusPetrunkevitch, 1949 and Anthracomartus denuitiPruvost, 1922 are treated as junior synonyms of A. priestiPocock, 1911 (both syn. nov.).