Paläobiologie und Systematik der neuen Überfamilie Hippocardioidea Pojeta & Runnegar, 1976 (Rostroconchia, Mollusca)

Rostroconchia POJETA, RUNNEGAR, MORRIS & NEWELL, 1972 is the name of an extinct class of molluscs whose members occurred from Cambrian to Permian time. The class is divided into three orders and has a specific significance as its members are the ancestors of the bivalves and / or the scaphopods, depending on the phylogenetical concept applied, and therefore has a key position in the phylogeny of the molluscs. This thesis concerns taxa of the so-called “modern rostroconchs” which belong to the order Conocardiida NEUMAYR, 1891 and which are characterized by a specific distinguishing feature, the hood. Its members occurred from the Ordovician to the Permian. By analysis of the order’s (Conocardiida) history, a number of nomenclatural problems were solved which, a. o., correct the spelling of the order’s name and the definition of the type species of some of the families. In the course of the revision of the family Hippocardiidae POJETA & RUNNEGAR, 1976, originally consisting of six genera, a new superfamily Hippocardioidea POJETA & RUNNEGAR, 1976 – was erected, including three families, ten new subfamilies as well as 29 genera, 16 of which are new. This revision is based upon examination of the collection material of 44 international museums; further information on the material studied is compiled in a database (attached on cd). The palaeobiological studies of morphology, ontogeny, shell microstructure and mode of life led to new insights into the growth of larval and adult shells and the microstructural construction of the shell. Furthermore, it is shown that hippocardioid rostroconchs, in addition to detritus feeding, were dependent on the symbiosis with photoautotrophic zooxanthellae in order to meet the basic nutritional demands. This flexibility in the mode of feeding made it possible for this group to occupy other biotopes than the shallow marine shelf regions to which they were limited before and to compete successfully with the biomechanically more advanced and efficient bivalves. With respect to phylogenetic aspects, a new discussion on the present theories on molluscan phylogeny as well as on the fossil material has been carried out. Results from this work to support the concept of PEEL (1991a, b) rather than that of RUNNEGAR & POJETA (1974) on which their monograph of the class is based (POJETA & RUNNEGAR 1976). Preliminary palaeobiogeographical analyses show the extinction of the last members of the superfamily in the middle Permian as being linked with the reduction of their habitats as a consequence from the formation of Pangean Continent.