<ref name="EngelRen2008">{{cite journal |last1=Pigg |first1=K.B. |coauthors = Manchester S.R. & Wehr W.C. |year=2003 |title=''Corylus'', ''Carpinus'', and ''Palaeocarpinus'' (Betulaceae) from the Middle Eocene Klondike Mountain and Allenby Formations of Northwestern North America |journal= International Journal of Plant Sciences |volume=164 |issue= 5 |pages=807-822 |url=}}</ref>