Taxonomy and palaeoecology of the Early Jurassic (Pliensbachian) bivalves from Buttenheim, Franconia (Southern Germany)

摘要

The dark clays of the Lower Jurassic (Upper Pliensbachian) Amaltheenton Formation in Buttenheim, Franconia yield a diverse and well-preserved benthic and nektonic macrofauna, dominated by bivalves, gastropods, and ammonites. In the present study the bivalve fauna is described based on new collections of ca. 7000 specimens. A comprehensive taxonomic study of the Early Jurassic bivalves from Franconia has not been conducted for over eighty years. The earliest taxonomic study of the Pliensbachian bivalves from Franconia was by VON MONSTER and GOLDFUSS in the 19th century, the last one was performed by KUHN in the 1930s. Type material of these studies is housed in the Bayerische Staarssammlung fur Palaontologie und Geologie and is revised herein. The bivalve fauna from Buttenheim comprises 57 species belonging to 40 genera. It is the most diverse macrobenthic faunal group in Buttenheim, and the assemblage is one of the most diverse Lower Jurassic bivalve faunas in Europe. Paracuspidaria and Eothyasira are established as new genera; two new species (Rollieria franconica sp. nov., Nicaniella schoberti sp. nov.) and two replacement names (Rollieri a goldfussi nom. nov. for Nucula subovalis GOLDFUSS, 1837 and Limopsis (Limopsis) quenstedti nom. nov. for Nucula aurita QUENSTEDT, 1856) are proposed. Composition, crophic character and life habit of the bivalves show adaptations to a generally low-energy soft bottom environment, characterized by intermittent dysoxic conditions on the sea floor. Occasionally, this quiet scare was interrupted by brief high-energy events. Most specimens are only small to moderate in size, which is pardy species-specific, partly it reflects palaeoenvironmental conditions. Infaunal bivalves (e.g., Ryderia) represent the most diverse and abundant bivalve group, followed by semi-infaunal (e.g., Myoconcha) and epifaunal (e.g., Pseudopecten) bivalves. The most abundant species is the plicamlid Harpax spinosus, which initially lived cemented to a hard substrate (mostly other shells) and later in life became a free recliner. Although not abundant, five species belonging to five genera (Solemya,Nucinella, Lu ciniola Sphaeriola, Eothyasira) probably had a chemosymbiotic mode of life. Paracuspidaria goldfussiana and Paracuspidaria walteri belong to the oldest members of the microcarnivorous family Cuspidariidae. Eothyasira antiqua is the oldest representative of the family Thyasiridae.