Palaeobiogeography, biostratigraphy and palaeoecology of Platyceramus undulatoplicatus zone

Abstract

Platyceramus undulatoplicatus zone is defined by the first occurrence of Platyceramus undulatoplicatus (Roemer, 1852) and range to its last occurrence (Lamolda & Hancock, 1996). P. undulatoplicatus belongs to family Inoceramidae and shows two main important features: it has a widespread palaebiogeographic distribution and it is restricted to the lowermost Santonian. Moreover, this species is very easy to be recognised by it is large size (up to 2.0 metres high), very flat shell, and prominent and divergent ribs dominating on concentric rugae. Taxonomically, this species has been assigned to genus Cladoceramus Seitz or to Platyceramus Seitz it depending on the criteria of authors. But in agreemenf with Matsumoto et al. (1982) and López (1992) we consider that similarities in shape, convexity, and concentric ornamentation between Cladoceramus and Platyceramus, as well as the presence of transitional forms between them, are enough to accept Cladoceramus as a junior synonimous of Platyceramus.