MOULTING AND COPULATION IN A GREGARIOUS SITUATION: PALAEOECOLOGY OF THE EARLY CAMBRIAN TRILOBITE BALCORACANIA FROM SOUTH AUSTRALIA

Monospecific assemblages of the trilobite Balcoracania dailyi occur in Lower Cambrian strata (Pararaia janeae Zone; late Botoman) within the Adelaide Geosyncline in South Australia. Biostratinomic data from single bedding plane assemblages within the Warragee and Coads Hill Members of the Billy Creek Formation and White Point Conglomerate reveal a range of taphonomic signatures from census to within-habitat, time-averaged assemblages. These assemblages are interpreted as having inhabited very shallow marine environments, typically shallow subtidal to intertidal zones. Size-frequency distributions, coupled with taphonomic data, show that the Warragee Member census assemblage represents a living population in a physically stressful intertidal environment, while the original population structure of the Coads Hill Member and White Point Conglomerate assemblages has been lost due to varying degrees of taphonomic overprinting. Integration of taphonomic, stratigraphic and sedimentological data supports the interpretation of B. dailyi as representing an opportunistic species. A preserved body cluster from the Warragee Member assemblage is considered to characterise a congregation formed for the purpose of synchronous reproduction and ecdysis, representing one of the oldest examples of gregarious behaviour in the arthropod fossil record. Furthermore, by analogy with modern horseshoe crabs, the high number of larvae and juveniles (i.e., protaspides and early meraspides) within the same assemblage are believed to be constituents of a nursery within the intertidal zone, with adults migrating onto the intertidal flats to copulate and spawn. Preserved moult ensembles from the Coads Hill Member and White Point Conglomerate have enabled the description of exuviation techniques for B. dailyi.