Leaves of Cornus (Cornaceae) from the Paleocene of North America and Asia Confirmed by Trichome Characters

Abstract

The identification of Cornus foliage in the fossil record previously has relied primarily on similarities in venation, particularly the eucamptodromous secondary veins and widely spaced, transversely oriented tertiary veins. These features, while consistent with Cornus, are not by themselves diagnostic for the genus. Double‐armed, acicular trichomes mineralized with calcium carbonate are an additional characteristic feature found in all extant species of the genus. The presence of such trichomes provides a means of confirming leaf impression fossils attributed to Cornus. Reexamination of previously described Cornus leaves from the Paleocene of the Rocky Mountains and Great Plains region leads us to reject Cornus nebrascensis Schimper (=Cornus newberryi Hollick) from the genus. Another species, Cornus hyperborea Heer, is provisionally accepted as Cornus based on its venation, although we were unable to confirm the trichomes in the type material from Greenland or in the assigned specimens from North Dakota. We also review other Eurasian Paleocene Cornus‐like leaf remains, including Cornus platyphylla Saporta and Cornophyllum hebridicum (Johnson) Boulter and Kvaček. We recognize two new Paleocene species, whose identity as Cornus is confirmed by the presence of characteristic trichomes and venation: Cornus swingii sp. n., from the Paleocene of Wyoming, Montana, and North Dakota, and Cornus krassilovii sp. n., from the Paleocene Tsagayan flora of Russia. These occurrences, along with fruit records, indicate that the extant genus was well established in the Northern Hemisphere early in the Tertiary.