1. Aldridge, R. J., Briggs, D. E. G., Smith, M. P. Clarkson, E. N. K. Clark, N. D. L. Phil. Trans. R. Soc. Lond. 338, 405–421 (1993).
2. Sansom, I. J. et al. Science 256, 1308–1311 (1992).
3. Briggs, D. E. G. Science 256, 1285–1286 (1992).
4. Forey, P. & Janvier, P. Nature 361, 129–134 (1993).
5. Dzik, J. in Problematic Fossil Taxa (eds Hoffman, A & Nitecki, M. H.) 240–254 (Oxford University Press, New York, 1986).