1 Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK; Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA. Electronic address: jo314@cam.ac.uk.
2 Key Laboratory for Palaeobiology, Yunnan University, Kunming 650091, China.