Decapod crustaceans from the Middle Jurassic Opalinus Clay of northern Switzerland, with comments on crustacean taphonomy

Abstract

Four species of decapod crustaceans from the Middle Jurassic Opalinus Clay (Aalenian) of Northern Switzerland are described. Of these, Mecochirus cf. eckerti is the most common one, while Eryma cf. bedelta, Glyphea sp. and Aeger sp. were present as individuals, or only a few specimens. The preservation of these crustaceans ranges from moderate to excellent, reflecting the favourable taphonomic conditions of the depositional environment. An interesting aspect of the taphocoenosis in the Opalinus Clay is that the decapod crustaceans are by far outnumbered by small peracarid crustaceans (isopods and tanaids). This is interpreted as reflecting the original differences in abundance. Yet this distribution is not frequently encountered in sedimentary sequences where decapods (although rare) are far more common than isopods and tanaids. In rare instances, this reflects the original predominance of decapods, more often it is a consequence of the differential taphonomic behaviour of these two groups. A new model relating the ecology to the taphonomic behaviour of decapod and peracarid crustaceans is proposed. According to this model, decapods dominate in settings that were deposited under extremely dysoxic (peracarids wiped out by seasonal anoxia) as well as under fully oxic conditions (peracarids destroyed by taphonomic processes). Only in muddy dysoxic depositional environments are peracarid crustaceans frequently preserved. In these settings with equal preservation potential of decapods and peracarids, the original composition of the crustacean fauna would show a predominance of peracarid crustaceans. Examples from some well known fossiliferous settings are provided to illustrate the use of the new model.