Shell adaptation and ecological variability• in the pelecypod species Sphenia anatina (Basterot) from the Korytnica basin (Middle Miocene; Holy Cross Mountains, Central Poland)

Abstract

The pelecypod species Sphenia anatina (Basterot) from the shore facies of the Korytnica basin (Middle Miocene; Holy Cross Mountains, Central Poland) was adopted to live in more or less empty borings produced by diverse pelecypods (Gastrochaena, Aspidopholas, Jouannetia, Lithophaga). In result of the tendency to fill the space of the boring tightly to protect the body against the rocking and shaking prevailing all over the biotope, the shell of particular individuals acquired the shape of the occupied boring. In consequence, the four groups of the shell shape may be distinguished, all of them almost strictly counterparting the shape of the primary occupants. These groups, the ecotypes (gastrochaenicola, aspidophoiicoia, jouanneticoia, lithophagicola, respectively), although of no taxonomic importance, may be well indicated in the hitherto described specimens from the Neogene deposits of Europe. The revision of taxonomy of all those specimens shows that presumably only one species of the genus Sphenia Turton lived in the Neogene of Europe, viz. Sphenia anatina (Basterot). and all the shape-deviated and variously called specimens are the ecological variants of this very species. Finally, this extinct species is compared, in the terms of its shell characteristics. taxonomy, and ecological requirements, with the present-day species Sphenia binghami Turton which, if taxonomically really separate, has developed directly from its Neogene ancestor, Sphenia anatina (Basterot).