A revised diagnosis and expanded description are presented for the amphiumid salamander Proamphiuma cretacea based on the original topotypic collection of vertebrae and on new atlantes, trunk vertebrae, and caudal vertebrae from the holotype locality. P. cretacea is the type and only species in the genus and is reliably known only from the Bug Creek Anthills (late Maastrichtian or early Paleocene in age), in the Hell Creek Formation, Montana, USA. A referred dentary originally believed to be from the same locality is shown to have come from the upper part (late Paleocene in age) of the Ravenscrag Formation of Saskatchewan, Canada, and is re-interpreted as belonging to an indeterminate amphiumid genus and species. Contrary to a recent claim, P. cretacea is a diagnosable amphiumid taxon and stands as the geologically oldest member of the family. Proamphiuma differs from the type and only other unequivocal amphiumid genus, Amphiuma (four species: late Paleocene–Recent, USA), in primitively retaining weaker vertebral crests and processes. Proamphiuma differs further from the three extant Amphiuma species (conditions unknown for the late Paleocene A. jepseni) in three character states that are potentially autapomorphic within the family—indistinct postzygapophyseal process on atlas, vertebrarterial canal closed posteriorly in trunk vertebrae, and spinal foramen present in anterior caudals—plus vertebral plesiomorphies and one vertebral character state of uncertain polarity. The following relationships are hypothesized within the Amphiumidae: Proamphiuma (A. jepseni (A. pholeter (A. means A. tridactylum))). This phylogeny and fossil occurrences imply that the characteristic atlanto–cranial joint and elongate trunk and the unique vertebromuscular complex of amphiumids were established by the Cretaceous–Tertiary boundary, while elongation of the snout and associated cranial modifications seen in living amphiumids appeared after the late Paleocene. In terms of its morphology and provenance, Proamphiuma remains a good structural ancestor for, if not directly ancestral to, Amphiuma.