Palynological Evidence on the History of pantropical Plants

In a recent paper by Germeraad, Hopping and Muller (1968) a stratigraphical zonation for tropical Tertiary sediments is developed, based on the distribution in time and space of fossil pollen grains and spores. Data were derived from large scale samples of coastal oil-well samples in E. South America (Columbia, Venezuela, Trinidad), W. Africa (Nigeria), and Malesia (NW. Borneo). This study was made for the prosaic purpose of providing the necessary geological information for oil exploration and it is remarkable that such academic subjects as the migrations and extinctions which have taken place in the ancient plant world thus suddenly become of interest for oil-companies. It will be clear that such a large scale effort has yielded many data of interest to botanists and these are summarized in a paragraph on ”Botanical results”. Here those fossil pollen and spore species of which the botanical affinities are well established, are discussed with special emphasis on phytogeographical aspects. In this review a few selected examples will be briefly cited. The first group concerns still living pantropical genera: Alchornea (Euphorbiaceae) has at present a pantropical distribution and this was the case already for the major part of the Tertiary. The typical pollen grains of this genus first appear in the Eocene. Rhizophoraceae are represented in the fossil record by the Rhizophora pollen type, which also may include pollen of Bruguiera and Ceriops. This group also appears first in the Eocene, but in W. Africa it has not been found before the Miocene, which suggests later immigration in this area. Acanthaceous pollen is found only from the Miocene upwards, which raises the question whether the present day pantropical distribution of these genera may be of relatively recent origin.