Some aspects of the pollination biology of middle eocene angiosperms

Abstract

Angiosperms are characterized by their sophisticated adaptations for wind and insect pollination. Modern relations between the angiosperms and anthophilous insects and the fossil records of both groups suggest that co-evolution was important in the radiation and perhaps even the origin of the angiosperms. An integral part of understanding the evolution of the angiosperms is understanding the evolution of their pollination mechanisms. Enough well-preserved flowers and inflorescences have now been assembled from the Claiborne Formation to allow a consideration of the types of floral morphology and of probable pollination systems that had evolved by the Middle Eocene. Flowers and inflorescences with floral and pollen morphology well adapted for wind pollination are common in the Clairborne Formation. Among these are representatives of two important extant families of the “Amentiferae”. These had diversified by the Middle Eocene and were as well adapted for wind dispersal of pollen as modern representatives of the families. Furthermore, it appears that significant evolution has taken place within the two families since the Middle Eocene. Flowers and inflorescences with morphology suggestive of pollination by all four orders of anthophilous insects are present in sediments of the Claiborne Formation. Several of the trends commonly postulated to have taken place with increasing specialization for insect pollination have progressed to a significant degree by the Middle Eocene, but evidence for certain features thought to be characteristic of more sophisticated insect pollination mechanisms is missing from the fossil flower record of that time. In addition, the fossil flower record of the Middle Eocene suggests that there has been significant evolution within certain extant families since the Middle Eocene and that certain families of angiosperms have originated since the Middle Eocene.