1. Department of Biology and Biochemistry, The University of Bath The Avenue, Claverton Down, Bath BA2 7AY, UK; E-mail: m.a.wills@bath.ac.uk
2. Department of Palaeontology, The Natural History Museum Cromwell Road, South Kensington, London SW7 5BD, UK; E-mail: p.barrett@nhm.ac.uk
3. Department of Earth Sciences, Birkbeck College, University of London Malet Street, London WC1E 7HX, UK