We report a third archosaur from the Late Triassic cavity-fill sediments of Pant-y-ffynnon Quarry in South Wales, in addition to the basal crocodylomorph Terrestrisuchus and the basal sauropodomorph dinosaur Pantydraco. The new taxon, Aenigmaspina pantyffynnonensis, previously referred to informally as ‘Edgar,’ was recovered in 1952 and is preserved as a tight cluster of vertebrae, ribs, and a scapula in a single block, as well as numerous associated, but now isolated, elements, including skull bones, vertebrae, further putative elements of the forelimb, and a partial pelvis. We present a digital model based on computed tomography (CT) scans of the main associated blocks, which shows exquisite detail of an associated series of 12 cervical and anterior dorsal vertebrae, with ribs and a scapula. The well-developed spine tables and eight matching osteoderms confirm that this is a crurotarsan archosaur, but it lacks apomorphies of all major crurotarsan clades. Phylogenetic analysis gives equivocal results, showing weak relationships with Proterochampsidae and Ornithosuchidae; the new taxon might lie close to Erpetosuchidae, based on the ‘Y'-shaped spine tables.