Zenostephanus is true ammonite genus from the lower part of the Upper Jurassic of eastern England and Scotland belonging to the ammonitid family, Perisphinctidae.revised name for Xenostephanus
Zenostephanus is revised from Xenostephanus, a preoccupied name for a new genus of toxodont notoungulate from Argentina. The type is Aulacostephanus ranbyensis
References
Perisphinctidae in the Treatise on Invertebrate Paleontology, Part L, Mollusca 4.