Faunal turnovers and trilobite morphologies in the upper Cambrian Leptoplastus Zone at Andrarum, southern Sweden

The Furongian (upper Cambrian) Leptoplastus Zone marks a time of critical changes in the evolution of olenid trilobites. This zone, unexposed at Andrarum in Skåne, southern Sweden, has been re-excavated and the sequence of faunas and sediments logged in detail. The faunal succession accords with that previously described from borehole cores by Westergård, and the subzones of L. paucisegmentatus, L. raphidophorus, L. crassicornis, L. ovatus, L. angustatus, and L. stenotus have been recognized. In the first two subzones the olenid assemblages are monospecific. At the base of the L. crassicornis Subzone more than one species is present and morphotypes with long genal spines appear for the first time. Faunal turnover is rapid, but the incoming of new species is invariably linked to an abrupt change in sedimentation, or follows an unfossiliferous interval; species either arose or migrated in after a time of environmental perturbation. Particular faunal associations are often confined to discrete sedimentary packages though some species may range through a succession of sedimentary changes. Leptoplastus crassicornis has very long genal spines, adapted for resting on the sea floor; it may have competed with the coeval, and very similar, L. angustatus. Subsequently, L. angustatus is accompanied by the stout-bodied, short-spined L. ovatus, which presumably occupied a different niche within the same environment. Leptoplastus stenotus is convergent on the much earlier L. paucisegmentatus, and likewise is found as a monospecific assemblage, presumably being adapted to a similar niche.