The Oldest Bivalved Arthropod Assemblage from the Early Cambrian of South Australia
Timothy P. Topper1†, Christian B. Skovsted2, Glenn A. Brock1 and John R. Paterson3
1 Department of Biological Sciences, Macquarie University, New South Wales, Australia
2 Department of Earth Sciences, Uppsala University, Sweden
3 Division of Earth Sciences, School of Environmental & Rural Science, University of New England, Armidale, New South Wales, Australia
Bradoriids and phosphatocopids are small bivalved arthropods that were an important component of Cambrian faunal assemblages before disappearing in the middle Ordovician. Historically considered to be ancestral ostracods, appendage morphology of exceptionally preserved specimens from China and Sweden suggests that both groups are only distantly related to extant and fossil Ostracoda. Bradoriids are currently thought to be a potential sister group to the Crustacea and phosphatocopids a sister group to the crown group Eucrustacea. Bradoriids and phosphatocopids had a worldwide distribution during the Cambrian and have been recorded from all the major Cambrian lagerstätten. They have been sporadically documented from Cambrian successions in central and northern Australia equivalent to Cambrian Series 2 (Stage 4) and Series 3 (Stage 5 to Guzhangian). The diverse faunas from older parts of the traditional lower Cambrian in South Australia (Series 2, Stage 3) remain to be fully described.
Bradoriids occur coevally with the first trilobites on several continents, but appear slightly earlier than the first recorded trilobite (Abadiella) in southern China. The oldest bradoriids previously documented from South Australia were svealutids from the Lower Cambrian (Abadiella huoi Zone, Atdabanian equivalent). These occur in association with species belonging to the Hipponicharionidae and Monsateriidae. The oldest phosphatocopid in Australia (Indianidae gen. et sp. indet.) has also been documented from this trilobite zone in South Australia.
Recent systematic investigations by the authors have revealed well preserved, diverse (total of 8 taxa) and prolific bradoriid and phosphatocopid assemblages (including new taxa) in acid resistant residues from the lower Cambrian Ajax Limestone in the northern Flinders Ranges. In the Ajax Limestone, the first bradoriids occur some 20 m below the FAD of the zonal trilobite A. huoi. This pre-trilobitic occurrence suggests an early Series 2, Stage 3 age for the assemblage and represents the oldest bivalved arthropod assemblage hitherto known from the lower Cambrian succession of South Australia. The recognition of distinct bradoriid assemblages associated with the A. huoi (Atdabanian), Pararaia tatei, P. bunyerooensis and P. janeae (all Botomian) trilobite biozones in South Australia indicates great potential for future regional, and possibly intercontinental, biostratigraphic correlation.