Sansom, I. J., M. P. Smith, H. A. Armstrong and M. M. Smith, 1992. Presence of the earliest vertebrate hard tissues in conodonts. Science 256: 1308-1311.
Shu, D.-G., S. Conway Morris and X.-L. Zhang, 1996. A Pikaia-like chordate from the Lower Cambrian of China. Nature 384: 157-158.