Abstract

The Early Tithonian ammonite genus Semiformiceras represents a lineage that lasted almost 3 myr. Previously, a gradual, anagenetic evolutionary pattern has been proposed on the basis of both biostratigraphic data and the interpretation of morphological changes in successive species, subspecies or morphotypes. The existence of two almost homeomorphic species, S. birkenmajeri and S. gemmellaroi, at two separate stratigraphic intervals in the early Tithonian contrasts with the transformation of characters suggested by the stratigraphic distribution of others. New descriptions of 18 specimens of the poorly known species Semiformiceras gemmellaroi (Zittel) allow a more accurate analysis of its characters. Cladistic analysis of a taxon-character matrix is based on nine Semiformiceras phena (species, chronosubspecies and morphotypes) and two outgroup taxa (Taramelliceras and Streblites), using 14 morphological characters. The phyletic relationships inferred from these do not fit the anagenetic pattern and question the adequacy of the fossil record in reconstructing phyletic lineages.