Eoradiolites liratus (Bivalvia, Radiolitidae) from the Upper Cenomanian Galala Formation at Saint Paul, Eastern Desert (Egypt)

In Egypt, Cretaceous rudistid bivalves were first recorded in the latter half of the nineteenth century (e.g., von Zittel, 1883); detailed descriptions can be found in Douvillé, 1910, Douvillé, 1913, Douvillé, 1915. Subsequently, these bivalves have only received scant attention; among more recent studies, reference may be made to Parnes, 1987, Steuber and Bachmann, 2002 and El-Sabbagh and El-Hedeny (2003).

In the Eastern Desert, rudists occur exclusively in Cenomanian and Campanian strata (Douvillé, 1913, Metwally and Abd El-Azeam, 1997). There are no records from deposits of Turonian, Coniacian and Santonian age in this region. In contrast to Campanian species from both the Eastern and Western deserts (Gebel Ataqa and Farafrah Oasis, respectively), which were discussed in detail in a monograph by Douvillé (1913), our knowledge of Cenomanian taxa is still poor.

Parautochthonous and monospecific assemblages of Eoradiolites liratus are common in the Upper Cenomanian at Saint Paul (Fig. 1), yet are not dominant, occurring occasionally in bouquets or loose associations. Generally, Eoradiolites is considered the most primitive genus of the Radiolitidae (Gallo Maresca, 1994). It should be noted, however, that despite the fact that several species of Eoradiolites have recently been revised (Gallo Maresca, 1994, Masse and Gallo Maresca, 1997, Masse et al., 1998a, Masse et al., 1998b, Steuber, 1999, Skelton and Masse, 2000, Steuber and Bachmann, 2002), the taxonomy of the group is still confusing. Type material of a number of species comprises transverse sections of right valves, and diagnoses are predominantly concerned with minor differences in the outline of radial bands (Steuber, 1999).

The aim of the present paper is to describe the morphological variation within a collection of late Cenomanian Eoradiolites liratus from Saint Paul (Fig. 1) and to use these data to document a case of polymorphism. In addition, the stratigraphic ranges and palaeogeographic distribution of this species are discussed, and the palaeoecology and post-mortem changes are evaluated.