| The Juglandaceae (walnut family) has an excellent fossil record in the Northern Hemisphere which includes foliage, wood, inflorescences, pollen and fruits. A review of this record, focusing on, but not limited to, fruits, reveals patterns of evolution, distribution and extinction in the history of the family.; Palynological evidence suggests origin and rapid diversification of the Juglandaceae during the late Cretaceous and Paleocene. Well-documented juglandaceous megafossils first occur in the Paleocene of North America. By the late Paleocene and early Eocene, modern tribes and some modern genera (e.g. Cyclocarya and Platycarya) are recognizable on the basis of fruits. The family reached its greatest diversity by the middle Eocene. In contrast to the eight genera recognized today, fossil fruit data indicate the presence of at least ten distinct genera. Early Tertiary assemblages included both modern genera (e.g. Platycarya, cyclocarya, Pterocarya, and Juglans) and extinct genera (e.g. Paraengelhardtia, Paleooreomunnea, and Polyptera gen. nov.). As today, most of the early Tertiary genera had winged nutlets adapted for wind dispersal. The evolution of large wingless nuts coincides well with diversification of rodents. Although basic morphological patterns characteristic of modern genera were evolved early in the Tertiary, post-Eocene evolution is also evident, particularly involving shape and size of fruits.; There were two major periods of extinction in the history of the family. During the first episode, late Eocene, at least four genera became extinct and the ranges of others were substantially reduced. The surviving genera were those which had achieved a broad European-American distribution by the mid-Eocene. The second episode occurred during the Pleistocene, and also resulted in widescale reductions in geographic ranges, but not in generic extinction. These periods of extinction, both apparently climate-related, account for the relatively limited modern diversity and distribution of the walnut family. |
