Cretaceous origin of dogwoods: an anatomically preservedCornus(Cornaceae) fruit from the Campanian of Vancouver Island

Abstract

BackgroundCornaceae consists of 58 species, all within the genusCornus. The Cenozoic record ofCornusis extensive and well documented. Molecular divergence-time studies suggest that crown-groupCornusmay have originated by the Late Cretaceous. However, there has been no formal report ofCornusfrom Cretaceous deposits. Here, we characterize a permineralized fossil fruit assignable toCornussubg.Cornusfrom the Upper Cretaceous (Campanian) Shelter Point locality of Vancouver Island, British Columbia, Canada.MethodsSerial sections of the specimen were made using the cellulose acetate peel technique. Peels were mounted onto microscope slides and studied by light microscopy.ResultsThe fossil fruit consists of a tri-locular woody endocarp with dorsal germination valves. The locules are sub-triangular to ellipsoidal in transverse section and are separated by thin septa. Endocarp tissue consists of elongated and isodiametric sclereids and secretory cavities. Internal vascular tissue was not observed, but is interpreted to have been located along the outer periphery of the septa for some length, common in many cornalean taxa. There is one seed in each locule, one of which was found to have endosperm and a dicotyledonous embryo.DiscussionWoody endocarps with germination valves, without central vascular bundles, and with one seed per locule are characteristic of several families within the order Cornales. The interpreted vascular pattern and presence of secretory cavities indicates that the fossil fruit is assignable toCornussubg.Cornus. Comparative analysis suggests that the fossil is most similar toCornus piggae, a species described from the Paleocene of North Dakota. This fossil is the first evidence of crown-group Cornaceae from the Cretaceous and sheds light on both the plesiomorphic fruit characters and the timing of the initial diversification of the family and basal asterid lineage, Cornales.