1. Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg 2050, South Africa e-mail: michael.day@nhm.ac.uk
2. Evolutionary Studies Institute and School for Geosciences, University of the Witwatersrand, Johannesburg 2050, South Africa e-mail: bruce.rubidge@wits.ac.za