| 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. |
