Dietze V. (2010) Über Ammonites Humphriesianus umbilicus Quenstedt, 1886 an seiner Typus-Lokalität (östliche Schwäbische Alb, Südwestdeutschland) // Zitteliana. Bd. A50. S.55-69.
pdf: http://rogov.zwz.ru/Dietze,2010.pdf
The ammonite Stephanoceras umbilicum is the index species of the umbilicum horizon (Humphriesianum Subzone, Humphriesianum Zone, Lower Bajocian, Middle Jurassic), which is widely used in Middle Europe. The Stephanoceras fauna from the type horizon of S. umbilicum at its type locality in Essingen near Aalen is described. The fauna consists of the morphospecies S. umbilicum, S. mutabile and S. scalare. The section at Essingen is correlated with other sections in southern and northern Germany, France and southern England. Lectotypes are designated for S. umbilicum (Quenstedt, 1886) and S. mutabile (Quenstedt, 1886). The term Umbilicum (Sub) Zone, however, should not be used. Thanks to Volker Dietze for this pdf
