Article: The Cretaceous ammonite Eopachydiscus and the origin of the Pachydiscidae

Eopachydiscus Wright, 1955 is confirmed as the earliest, Upper Albian representative of the Pachydiscidae, derived in all probability from a bullate member of the Beaudanticeratinae and giving rise to the Cenomanian-Coniacian Lewesiceras Spath, 1939. Nuclei of Eopachydiscus are ribbed, tuberculate, and constricted, showing even at this stage and date typical pachydiscid ornament; the evolution of Lewesiceras simply involved retention of these features into middle growth. The suggestion that the Pachydiscidae arose from Arrhaphoceras Whitehouse, 1927 of the Hoplitinae is discounted on morphological, stratigraphical, and biogeographical grounds.