Isoxys and Tuzoia with Soft-Parts from the Emu Bay Shale: Comparison with Material from Other Cambrian Lagerstätten
Diego C. García-Bellido1, John R. Paterson2, Gregory D. Edgecombe3, James G. Gehling4, James B. Jago5 and Michael S. Y. Lee4
1 Departamento de Paleontología, Instituto de Geología Económica (CSIC-UCM), Madrid, Spain
2 Division of Earth Sciences, School of Environmental and Rural Science, University of New England, Armidale, New South Wales, Australia
3 Department of Palaeontology, Natural History Museum, London, UK
4 South Australian Museum, North Terrace, Adelaide, Australia
5 School of Natural and Built Environments, University of South Australia, Mawson Lakes, Australia
6 School of Earth and Environmental Sciences, University of Adelaide, Australia
Abundant material from Buck Quarry, a new outcrop of the lower Cambrian (Series 2) Emu Bay Shale lagerstätte (Kangaroo Island, South Australia), contains preserved soft-bodied features previously unknown from the original Big Gully locality. This has allowed the revision of Isoxys communis and Tuzoia australis, two bivalved arthropod taxa described in 1979 by Glaessner. The collections have also produced fossils that represent two new species, one of each of Isoxys and Tuzoia. Among the soft parts preserved in these taxa are stalked eyes, digestive structures and cephalic and trunk appendages.
Comparison with species of Isoxys with soft-parts from the Burgess Shale, the lower Cambrian Maotianshan Shale (China) and the Sirius Passet assemblage (Greenland) show that the genus exhibits a considerable range of morphologies of cephalic appendages, ranging from long and spiny to short and spineless. However, the nature of the postcephalic appendages seems more conservative, with a small endopod and a well developed exopod fringed with setae. Other non-mineralized structures, such as stalked eyes and digestive structures are very similar across the genus.
Tuzoia is also widespread (Canada, eastern and western USA, south China, Australia, Czech Republic), but preservation of soft parts is extremely rare: out of more than four hundred specimens studied by Vannier and colleagues, only three specimens of T. burgessensis and Tuzoia sp. from the Burgess Shale have preserved soft parts. In contrast, soft part preservation in Tuzoia from the Emu Bay Shale is more common, being present in 5% of specimens. Both Australian species preserve soft parts in the form of eyes, while the appendages and other anatomy of these taxa remain elusive. The Australian species fall within the lower and upper size range of the genus, with the larger species from the Emu Bay Shale only surpassed in size by Tuzoia sp. from Bohemia; both exceed 13 cm in maximum sagittal length.