THE PROGYMNOSPERMOPSIDA: BUILDING A CONCEPT

Summary Fifteen years have elapsed since the establishment of the Progymnospermopsida, a class of plants showing both pteridophytic reproduction and gymnospermous anatomy. Subsequent work has yielded many taxa ascribed to the group but only a few show the dual characteristics upon which the class was founded: Archaeopteris, Protopitys, Tetraxylopteris, Aneurophyton and Rellimia. Our concept of this extinct group must be based on these few genera. All five are similar in the branching pattern of their fructifications, dichotomous then pinnate. This is obvious in Tetraxylopteris, Rellimia and Protopitys , less so in Archaeopteris and Aneurophyton. The other taxa merely show similarities to one or another of the above, but not the combination of characters diagnostic of the group. They should be treated as form genera assigned only tentatively to the progymnosperms. Such isolated pieces of anatomy, vegetative axes and sporangial fragments add little to the concept. If incorrectly assigned, due to parallelisms in other plant groups, the evolutionary, ecological and stratigraphic value of the Progymnospermopsida would be obscured.