The genus Sphenothallus was erected by Hall (1847, p. 261), who originally considered it a land plant. Sphenothallus was later classified as a marine invertebrate. Moore and Harrington (1956, p. F65) regarded Sphenothallus as a hydrozoan or scyphozoan. Van Iten et al. (1992, p. 139) supported Moore and Harrington's idea, and argued that it displays a close relationship to conularids (also see Li, 2000, p. 91). However, Mason and Yochelson (1985, p. 93–94) suggested that Sphenothallus is an annelid or “worm” (also see Fauchald et al., 1986, p. 64; Feldmann et al., 1986, p. 344–345; Choi, 1990, p. 403; Bartels et al., 1998, p. 114–117). The exact phylogenetic affinity of Sphenothallus is still debated.