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The structure of the shell of Patella crenata connected with suggestions to the classification and evolution of the Archaeogastropoda

by Klaus Bandel & Werner Geldmacher, Hamburg

BANDEL, K. (1996): The structure of the shell of Patella crenata connected with suggestions to the classification and evolution of the Archaeogastropoda. - Freiberger Forschungshefte, C 464, Paläontologie, Stratigraphie, Fazies. - Heft 3. - S. 1-71. - Freiberg 1996.

Abstract

Aragonitic crossed lamellar structure is found only in molluscan shells and not among other invertebrates. The docoglossan archaeogastropod Patella in addition has a unique crossed lamellar structure the Docoglossa from other Archaeogastropoda. Both types of crossed lamellar structure in the same shell of Patella crenata are described in detail. In the diagenetically altered fossil shells both structural types are difficult to distinguish from each other. The shell structure of modern and fossil archaeogastropods is discussed and connected with the morphology of the shell. An inner nacreous layer is commonly found connected to an outer complex crossed lamellar layer that resembles the crossed composed of calcite. It distinguishes, but is not identical with it. After diagenesis this outer layer can resemble a true crossed lamellar layer. A coherent place in the taxonomic system is presented for the Archaeogastropoda which is based mainly on features of the ontogeny of the shell and can also be detected on a fossil. Archaeogastropod evolution began latest during the Ordovician. From that time onward their development occurred indepedently from the members of the other still existing subclasses Neritimorpha, Caenogastropoda and Heterostropha. Due to the absence of a larval shell in the archaeogastropod shells provide less information than in the other three subclasses of the Gastropoda, and tracing of lineages within the subclass is difficult. Here shell structure data are helpful. Nacre and calcitic crossed lamellar structure are only found in some archaeogastropods among the Gastropoda. The classification available from the text books is not satifactory especially in regard to the numerous fossils. Thus morphogroups are introduced and characterized which are thought to be more helpfull in tracing the course of evolution than traditional taxa which often obscure relations more than reveal them.