1. State Key Laboratory of the Continental Dynamics; Shaanxi Key Laboratory of Early Life & Environments; Department of Geology; Northwest University; Xi'an 710069 China
2. Department of Palaeobiology; Swedish Museum of Natural History; Box 50007 SE-104 05 Stockholm Sweden
3. State Key Laboratory of Palaeobiology & Stratigraphy; Nanjing Institute of Geology & Palaeontology; Chinese Academy of Sciences; Nanjing 210008 China