Rothwell, G. W.; Scheckler, S. E.; Gillespie, W. H. (1989). “Elkinsia gen. nov., a Late Devonian gymnosperm with cupulate ovules”. Botanical Gazette. 150 (2): 170–189. doi:10.1086/337763.
"The oldest fossils reveal evolution of non-vascular plants by the middle to late Ordovician Period (~450-440 m.y.a.) on the basis of fossil spores" Transition of plants to land