Goldman, D., Leslie, S. A., Nõlvak, J., Young, S. 2007.
The Black Knob Ridge section, southeastern Oklahoma, USA: The global stratotype-section and point (GSSP) for the base of Katian Stage of the Upper Ordovician series.Acta Palaeontologica Sinica46,
Suppl,144-154.
Nõlvak, J., Bauert, G. 2006.
Distribution of Ordovician chitinozoans.Estonian Geological Sections Bulletin7,
9-11. Kerguta (565) drill core, pp. 9-11.