Ontogeny and dimorphism of Isoxys auritus (Arthropoda) from the Early Cambrian Chengjiang biota,South China |
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| Institution: | 1. State Key Laboratory of Continental Dynamics, Early Life Institute and Department of Geology, Northwest University, Xian 710069, PR China;2. Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, Uppsala SE-752 36, Sweden;3. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, PR China;4. Coastal Ecosystems Research Station of the Yangtze River Estuary, Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, Fudan University, Shanghai 200433, PR China;1. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China;2. Institute of Evo/Developmental Biology, Nanjing University, Nanjing 210093, China;1. Palaeoscience Research Centre, School of Environmental and Rural Science, University of New England, Armidale, New South Wales, 2351, Australia;2. Palaeontology Department, Australian Museum Research Institute, Sydney, New South Wales, 2010, Australia;3. Department of Biological Sciences, Macquarie University, Sydney, New South Wales, 2109, Australia |
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| Abstract: | The morphology of Isoxys auritus Jiang, 1982 is reinterpreted in the light of abundant new specimens from the Early Cambrian Chengjiang biota, South China. I. auritus was a bivalved arthropod, its shield armed with two cardinal spines sub-equal in length. Two morphotypes (shield with and without ornamentation) which are of several original differences were interpreted as sexual dimorphs. 81 specimens examined here, which range between 4.8 mm and 47.0 mm, represent a successive developmental sequence. The earliest stages were characterized by short cardinal spines, large spherical eyes, a pair of elongated antennulae, and seven pairs of post-antennular appendages. The slim antennula is uniramous, consisting of nine articles, each armed with short spines. It differs from that of great appendage and lacks any grasping function. During the ontogeny, the body length increases, accompanied by addition of trunk somites and appearance of primary reticulated ornaments, and both cardinal spines become evident. In the fully grown adult, there are up to 11 pairs of post-antennular appendages, equipped with the stout endopod composed of 6 or 7 podomeres lacking endites, and a terminal claw; the paddle shaped exopod is fringed with long setae along its posterior margin. Secondary reticulation of the shield has developed inside each primary one; the cardinal spines more elongated; and the ratio of length to height of shield increases. However, the size of eye and antennula relative to the body length significantly decreases. Accordingly, intraspecific variation, including sexual dimorphs and developmental change, is evident in I. auritus. Recognizing such differences is important for detecting possible synonymies in the genus. Furthermore, the ontogenetic changes of I. auritus described here, particularly the postembryonic segment addition and possible allometric growth may also improve our understanding of development of Cambrian arthropods in Burgess-shale type preservation, especially those possessing such a large shield. |
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