Definitions of, evolutionary trends within, and classification of early brevaxonate pollen

Abstract

The vast majority of brevaxonate pollen morphotypes were produced by plants which have long been extinct. As a result, satisfactory answers to a number of questions pertaining to their evolution, morphology, and classification have yet to be proposed. Among the more vexing problems associated with the Brevaxones, and which are considered in this article, are: (1) the time of their first appearance; (2) a clear and concise definition of the early (i.e., middle and Late Cretaceous) Brevaxones groups; and (3) the evolutionary trends within the early Brevaxones, as revealed by TEM and SEM studies. In addition, a new definition is proposed for the Normapolles group with respect to the Postnormapolles, and a three-fold morphologic subdivision of the Normapolles is presented.