Sphenothallus Hall, 1847, one of the most widely distributed and longest ranging genera in the fossil record, has been documented from all systems of the Paleozoic Erathem except the Permian (Table 1), although it has been stated (e.g., Choi, 1990; Bolton, 1994; Fatka et al., 2012) that the genus also occurs in that system. At present the first appearance of this epibenthic, polypoid medusozoan cnidarian lies in Cambrian Stage 3, while the previously known youngest occurrences are in the Pennsylvanian System. Sphenothallus has been found in numerous formations on all continents except Australia and Antarctica. It occurs in a variety of marine facies ranging from shallow nearshore to deep offshore and has even been found in strata of coastal lacustrine origin, probably as an allochthonous element (Lerner and Lucas, 2011). Many of the rock units known to contain Sphenothallus also contain conulariids (Table 1), an extinct group of marine scyphozoans that may have been closely related to Sphenothallus (Van Iten et al., 1992, 1996). Van Iten et al. (1992) interpreted Sphenothallus as a medusozoan cnidarian of uncertain class-level affinities, but later Dzik et al. (2017) documented internal peridermal structures that may be homologous to similar features in the periderm of coronate scyphozoans (see for example illustrations in Van Iten, 1992, and Van Iten et al., 1996).
