Speculation on Pre-Cretaceous Angiosperms has been made by several authors, and many authors have hypothesized the possible mechanisms of the evolutionary change. Many of the purported pre-angiosperm ancestors have "angiosperm" leaf characters (net-like venation pattern) which has arisen independently in several clades.

The apparent burst of monocot and dicot angiosperms into the fossil record without "transitional" forms has led workers to a quandary and a search for pre-Cretaceous angiosperms. There is little doubt that angiosperms display the most advanced seed plant state, and that the features that characterize them originated from a gymnosperm ancestor.

The Gnetales, a small relict group of gymnosperms, may hold the key to angiospermy. Recent investigations have shown that one of the primary characters of angiospermy - double fertilization, anastomosing leaf venation, reduction of the male gametophyte, tetrasporic megagametophyte with free nuclei serving as eggs, and a feeder in the embryo - occurs sporadically in Ephedra, and a welwitschian-type reproductive structure (Archaestrobilus) found in the Late Triassic of Texas has been interpreted to have been a precursor to angiosperms. There is growing agreement that these plants may be the closest living relative to flowering plants.

To date, our best evidence for angiosperms is still in the Cretaceous but Doyle and Hickey (1976) have demonstrated in the Potomac Group that unequivocal angiosperms existed in Barremian (100 my) strata. Both pollen (monocolpate) and leaves (laminate, entire) of angiospermous origin have been recovered. These assemblages are poorly diversified and morphologically possess "disorganized" patterns. Their arrival into depositional sites has spawned postulation that the group originated in a variety of settings:

Recent discoveries of a herbaceous angiosperm fossil record in the Early Cretaceous has led to two competing hypotheses - that early angiosperms were paleoherbs, rather than trees or shrubs.

Albian 113-97.5 MY Flowers with four gynoecial valves and an erect ovule Reticulate tricolpate pollen
Aptian 119-113 MY Expanded laminae with reticulate venation pattern; Tricolpates & Monoporates
Barremian 124-119 MY Zone I (Doyle & Hickey, 1977) leaves are predominantly entire, some marginally toothed leaves, and only 1 with lobate leaves. Afropollis, Brenneripollis and tricolpate pollen
Hauterivian 131-124 MY Flowers (Friis, Pedersen & Crane 1994) weakly Monosulcate
Valanginian 137-131 MY Tectate-columellae structure (Brenner & Bickoff, 1992)
Berriasian 144-137 MY

©Copyright 1997 by Robert A. Gastaldo. All rights reserved. No part of or transmitted in any form or by any means, electronic or mechanical, including information storage and retrieval system, without permission from the author.