Investigations of angiosperms from the Eocene of southeastern North America: samaras of Fraxinus wilcoxiana Berry

Abstract

Fruits of Fraxinus L. from Eucene sediments of North America constitute the oldest unequivocal macrofossil evidence of the genus, of the subfamily Oleoideae, and of the family Oleaceae. We briefly review North American Paleogene records of Fraxinus, and report the results of a study of 30 samaras of Fraxinus wilcoxiana Berry (including the holotype) from two localities in the Middle Eocene Claiborne Formation of western Tennessee, USA. The diagnosis and description of this species are emended to include new information on its persistent calyx, fruit and seed cuticles, and variation in samara dimensions and morphology. The fossil samaras lack characters used in the systematic grouping of infrageneric taxa, and therefore cannot be assigned to a modern section or subsection of the genus. However, samara cuticle bears striae and two distinct classes of peltate trichomes useful in confirming the oleaceous affinities of the fossils. Fraxinus wilcoxiana samaras are essentially modern in terms of their morphology, seed size, single basal pendulous seed, and cuticle ornamentation. The presence of Fraxinus in Paleogene sediments attests to an early divergence of the subfamilies Oleoideae (basal chromosome number 23) and Jasminoideae (basal chromosome number 11, 12 or 14). Fraxinus wilcoxiana samaras also demonstrate that anemochory (wind dispersal of disseminules) is an ancient dispersal strategy among Oleaceae.