Zemistephanus is an extinct ammonite genus from the middle Jurassic of western North America and Western Australia, named by McLearn 1927. Zemistephanus is included in the stephanoceratoid family Otoitidae, which are characterized by shells that begin as barrel-shaped cadicones  but which become planulate later in life.
Zemistephanus is an extinct ammonite genus from the middle Jurassic of western North America and Western Australia, named in 1927 by McLearn. This genus is part of the stephanoceratoid family Otoitidae, characterized by shells that start as barrel-shaped cadicones but transform into planulate forms later in life.