|Specimen in Vienna|
It was a 25-30 cm or 10-12 inches long "cladodont-toothed stethacanthid shark"  The first material known from the genus were the prominent fin spines that curved anteriorly over the head of the animal. When first described in 1883 from the St. Louis Limestone, these remains were given the name Physonemus falcatus. However, in 1985, fossils of a new type of condrichthyan from Montana were described that displayed a high degree of sexual dimorphism. The same spines that were previously named P. falcatus were found on one of the morphs, identified as the male due to the presence of valvae.