Several authors have evaluated the diversification of flowering plants from their initial appearance in the fossil record. These patterns have been based either on megascopic or palynological remains. Lidgard and Crane evaluated absolute species richness of leaf data (the number of taxa reported in the literature) for angiosperms, conifers, pteridophytes (ferns, lycopophytes, sphenophytes), cycadophytes (cycads, cycadeoids, and pteridosperms with simple pinnate leaves), ginkgoes (Ginkgo and Czekanowskiales), and other seed plants. Beginning about 90 MY ago (mid-Albian), there is a widespread dominance in megafloral assemblages of the Northern Hemisphere. As can seen from their figures (below), there appears to be competitive displacement of cycadophytes and pteridophytes.

Lidgard & Crane Generic
DiversityLidgard & Crane Species Diversity

Between 115-90 MY, there was within floral diversification of the angiosperms, with essentially no response or reaction within the coniferophytes. The rise to dominance with increased diversity seen in megafloral assemblages is not paralleled coevally with the palynological record. Based on palynological data, angiosperm dominance is achieved in the Turonian. Diversification occurred at a rapid rate, following a kinetic growth model (S-shaped growth curve).

Cycadophytes begin a decline in diversity before angiosperms become dominant, and the opening of this ecological niche may have provided the opportunity for angiosperms to diversify.

Patterns of Diversification

Cretaceous Vegetational types

Floral provinces of the Cretaceous are primarily based on palynological suites, and during the Barremian-Cenomanian Brenner has recognized 4 floral provinces:

  1. Northern Laurasian Province - temperate, humid, now above 60oN; Bisaccate pollen of Pinaceae, NO Classopollis (Cheirolepsis) or Ephedra-type pollen; monosulcate pollen of the cycadophytes is rare. Fern pollen is abundant, but at a reduced diversity than in lower latitudes.
  2. Southern Laurasian - warm, temperate to subtropical. High diversity in the middle latitudes with abundant and diverse ferns; coniferous pollen of Cheirolepidaceae (Classopollis) dominating the spectrum (60-80%).
  3. Northern Gondwana - tropical to semi-arid with bizarre forms of pollen; low fern spore diversity and a paucity of Schizaceous ferns. NO Bisaccate Pollen of Pinaceae, araucarian and podocarp pollen; Cycadophyte and Ephedraceae pollen.
  4. Southern Gondwana - warm temperate to subtropical climate. Podocarp bisaccate pollen abundant, pteridophyte diversity.
Predicted mean annual rainfall for the Cenomanian (from Parrish et al., 1982). Wet=Purple; Humid=Green; Semi-arid=Orange; Arid=Yellow.

In the Late Cretaceous of North America, three vegetational types have been recognized:

Vegetational Response at the K/T Boundary

Takhtajan has subidvided the floras of the Late Cretaceous into three paleobiogeographic zones within the Northern Hemisphere:

Widespread disruption occurred in parts of the northern hemisphere ecosystem. Palynological and megafloral evidence indicates that there was a mass-kill event followed by a successional pattern of recovery. Ecological disruption is evidenced by the relative abundance of characteristic pollen and spores, with a change from angiosperm-dominated assemblages just below the K/T boundary to fern-dominated assemblages above. Above the K/T boundary, angiosperm pollen markedly declines in abundance, with the relative abundance of fern spores rising to between 70-100%. This has been termed the "fern spike." Studies of dispersed cuticles corroborate the palynological data set.

Northern vegetation shows less disruption at the K/T boundary than do floras from Eastern North America and the southern Western Interior. This may be due to the development of more efficient dormancy mechanisms in these polar forests. Plant groups in which dormancy mechanisms are not well developed appear not to have survived the K/T boundary event successfully.

Two features of the climate/vegetational environment were significantly altered:

©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.