Lower Cambrian helcionelloid macromolluscs from South Australia |
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| Institution: | 1. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China;2. State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;3. Tianjin Institute of Geology and Mineral Resources, China Geological Survey, Tianjin 300170, China;4. School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China;1. Department of Biological Evolution, Faculty of Biology, Moscow State University named after M.V. Lomonosov, Moscow GSP-1, 119991, Russia;2. Paleontological Institute, Russian Academy of Sciences, ul. Profsoyuznaya 123, Moscow 117997, Russia;3. School of GeoSciences, University of Edinburgh, King''s Buildings, West Mains Road, Edinburgh EH9 3JW, UK;1. Department of Earth and Environmental Sciences, Korea University, Seoul 136-713, Republic of Korea;2. Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China;3. School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Republic of Korea |
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| Abstract: | Early Cambrian univalve molluscs are predominantly represented by microscopic forms (body length of 1–3 mm), preserved mainly as phosphatised internal moulds with limited definable features. Macromolluscs (≥ 5 mm) are generally rare, occur in low abundance and are poorly preserved, often lacking apical features and ornament which hinders taxonomic assessment. New and previously undescribed material from lower Cambrian Hawker Group carbonates of the Flinders Ranges in South Australia includes four new taxa, Minastirithella silivreni gen. et sp. nov., Galeacalvus coronarius gen. et sp. nov., Helcionella histosia sp. nov., and Ilsanella enallaxa sp. nov. Three-dimensional silicified shell material preserved with intact apices offers new insight into protoconch morphology, growth habit (isometric vs. allometric) and developmental mode. This material supports previous suggestions that some micromollusc taxa may in fact be early ontogenetic stages (juveniles) of larger macroscopic taxa; such that the millimetric size range of helcionelloids conforms to the dimensions of earliest apical portion in some macromolluscs documented herein. However, taphonomic limitations associated with phosphatisation show that the morphology (especially height vs. width) of the apex can greatly influence the probability of steinkern formation and preservation potential for both micro- and macro-scale helcionelloids. Artificial inflation in the appearance of millimetric helcionelloids with an optimal morphology for phosphatisation is thus directly linked to taphonomic biases in the fossil record. |
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