Leaves and fruits of Aesculus (Sapindales) from the Paleocene of North America

Previous reports of Aesculus from the paleontological record have been based mainly on isolated leaflet impressions. Recently discovered complete palmately compound leaves and associated trivalved fruits from the Fort Union Formation of North Dakota and Wyoming confirm that the genus was common in the Paleocene of North America. Leaves of Aesculus hickeyi sp. nov. bear three to five obovate, sessile leaflets with finely serrate margins. Previously the isolated fossil leaflets were commonly misidentified as Carya antiquorum Newberry. Whereas the type specimens of Carya antiquorum Newberry are leaflets with well-impressed percurrent tertiary venation and coarsely serrate margins that can be matched to pinnately compound leaves of the Juglandaceae, the leaflets of A. hickeyi have weakly impressed tertiary venation and finely serrate margins. Fruits associated with A. hickeyi are globular, trivalved, loculicidal capsules. The spiny ornamentation on the fruit valves matches that of extant Aesculus glabra (eastern North America) and Aesculus hippocastanum (eastern Europe); all other extant species have smooth or verrucate ornamentation. Aesculus hickeyi differs from extant Asian species by its sessile, somewhat asymmetrical leaflets. The fruits and seeds of A. hickeyi are smaller than those observed in extant species, and the three locules of the fruits are usually equally well developed in contrast to those of extant species, in which one locule usually dominates. These fossils indicate that the extant genus was already established by the late Paleocene in North America, complementing foliar remains from the Paleogene of Spitzbergen and Kamchatka and predating the Oligocene occurrences from mainland Europe. New data concerning the affinity of Carya antiquorum, including a recently recovered specimen showing the leaves to be pinnately compound, unlike the palmately compound leaves of Aesculus, are also presented.