Original paper

On the history of the butternuts (Juglans section Cardiocaryon, Juglandaceae)

van der Ham, Raymond W. J. M.

Abstract

The largely northern hemisphere genus Juglans (Walnut) consists of three sections, of which Cardiocaryon (butternuts) includes three extant species: one in northern America and two in eastern Asia. Fossil nuts are known from many localities, especially in Europe and Japan. A total of 15 fossil species have been described in the past, several of which were merged later. It is argued in the present paper that except for J. lacunosa all these species should be combined into a single widespread species (J. bergomensis), because clear-cut differences are absent. The North American Oligocene J. lacunosa is accepted as an early butternut. J. bergomensis ranges from the middle Miocene to the early Pleistocene (c. MIS 30), though one less supported record leaves the possibility for an earlier first appearance date (early Miocene). Accepting J. lacunosa as an early representative of section Cardiocaryonq would imply that a similar, American species might have been among the ancestors of J. bergomensis. Westward migration in the Miocene from northern America via the Bering land bridge to Asia and further to Europe is more likely than migration via a North Atlantic land bridge or through long-distance dispersal. This scenario is corroborated by most cladistic analyses of extant Cardiocaryon.

Keywords

Walnuts; fossils; Neogene; Pleistocene; northern hemisphere; palaeobiogeography; butternnut