New data on 'Oikozetetes' (Mollusca: Halkieriidae) from the lower Cambrian of South Australia

Abstract

A new species of 'Oikozetetes, O. mounti' n. sp. is described from the upper Mernmerna Formation (equivalent to Cambrian Series 2) in the eastern Flinders Ranges, South Australia. Description of both shell morphotypes (morph A and B) from the same stratigraphic horizon favors the interpretation of a two-shelled scleritome for 'Oikozetetes'. Identification of at least two types of halkieriid sclerite (palmates and siculates) in stratigraphic association with the 'Halkieria'-like shells of 'Oikozetetes' suggests that these elements are derived from the same scleritome. This provides evidence against previous suggestions that the Gondwanan species was aspiculate; the scleritome arrangement is interpreted to be similar to 'Halkieria evangelista' Conway Morris and Peel, 1995 and supports placement in the family Halkieriidae Poulsen, 1967. Comparison of modes of accretionary growth in 'Oikozetete's shell morphotypes to 'Halkieria' shells and terminal plates in modern polyplacophorans, supports a scleritome model that places shell morphs A and B in posterior and anterior locations, respectively, along the axis of the body.