Loydell, D.K., Štorch, P., Melchin, M.J. Taxonomy, evolution and biostratigraphical importance of the Llandovery graptolite Spirograptus. Palaeontology. 1993, 36(4), 909-926. ISSN 0031-0239. E-ISSN 1475-4983. Available: https://www.palass.org/publications/palaeontology-journal/archive/36/4/article_pp909-926