Smith, M. Paul; Sansom, Ivan J.; Cochrane, Karen D. (2001). “The Cambrian origin of vertebrates”. In Ahlberg, Per Erik. Major Events in Early Vertebrate Evolution: Palaeontology, Phylogeny, Genetics and Development. London; New York: Taylor & Francis. pp. 67–84. ISBN0-415-23370-4. LCCN00-62919. OCLC51667292
Smith, M. Paul; Sansom, Ivan J.; Cochrane, Karen D. (2001). “The Cambrian origin of vertebrates”. In Ahlberg, Per Erik. Major Events in Early Vertebrate Evolution: Palaeontology, Phylogeny, Genetics and Development. London; New York: Taylor & Francis. pp. 67–84. ISBN0-415-23370-4. LCCN00-62919. OCLC51667292