Piptochaetium (Gramineae) and Associated Taxa: Evidence for the Tertiary Migration of Plants Between North and South America

Abstract

Disjunct distributions are frequent among higher plant taxa and the paucity of detailed fossil records for such plants, especially herbs, has rendered explanation of dispersal difficult. The late Tertiary fossil record of central North America provides data on the history of a number of herbaceous taxa which are now disjunctly distributed in amphitropical America and between North America and Eurasia. The North American Tertiary data provide evidence supporting the concurrence of evolution of herbaceous taxa and grazing mammals. Extensive migrations of grazing mammals occurred after the Central American land bridge emerged in middle Pliocene time. It is concluded, on the basis of available modern and fossil data, that these plants may reasonably have crossed the land bridge with animals with which they had evolved a degree of interdependence. The applicability of the hypothesis of long distance dispersal to the case of amphitropical American disjunctions is reviewed and certain deficiencies of the hypothesis are noted. In view of the data presented in this paper, it is concluded that the observed pattern of amphitropical American plant taxa disjunctions is the result of the combined processes of migration and long distance dispersal.