Northern Hemisphere Triassic

Early Triassic vegetation is not well represented in the stratigraphic record, with a higher number (and better preservation) of assemblages known from the Late Triassic. Triassic vegetation of Laurasia, in general, is characterized by:

Ginkgoaleans appear to have been geographically important, as they are preserved in abundance (and with high diversity) in northerly paleolatitudes. The pycnoxylic woody trees (Ginkgos, Coniferales) were medium-large in stature and probably formed a diffuse canopy. Ginkgos are thought to have occupied a position in the understory. There is little evidence to indicate that coniferous gymnosperms evolved the wide variety of growth habits experienced by extant angiosperms (herbs, vines, floating aquatics). It appears that rhizomatous ferns colonized open, dry, and/or nutrient poor sites, possibly forming savannah or "grassland" communities.

Monotypic stands of lycopids and sphenopsids have been identified. Lycopsids (e.g., Pleuromeia) are known to occur with freshwater-to-marine invertebrates which indicates a tolerance to salinity. Sphenopsids, similar to their niche today, were probably primary colonizers of open or disturbed moist soils.

The Late Triassic Chinle Formation (Petrified Forest National Park) preserves a low-diversity, vegetational mosaic in which:

Southern Hemisphere Triassic

Early Triassic floras are better represented in the southern hemisphere probably in response to a landscape sculpted by deglaciation. Preserved communities include:

Sometime in the Late Triassic, this assemblage was replaced by a more universal conifer-cycadeoid flora, similar to that of the northern hemisphere.

Jurassic Vegetation

The Late Triassic communities continued into the Jurassic, composed of a mixture of various woody gymnosperms and herbaceous ferns.

Coniferous halophytes (mangroves/mangals) have been identified based upon autochthonous assemblages in the Purbeck Beds of the Late Jurassic. These mangroves are moderately tall, sparsely branched, and some examples had multiple, thick trunks arising from a single rootstock. These conifers belong to the extinct Cheirolepidiaceae, and there is some evidence to indicate that these plants were deciduous. Cheirolepidiaceous conifers were not restricted, though, to a mangrove habit. They are known from a wide variety of depositional settings.

Our understanding of paleoclimates is mainly from analyses of permineralized woods that existed on floodplains. In the Late Jurassic petrified forest of Suihent, Mongolia (paleolatitude 46o N), wood-ring analysis has demonstrated that these trees were subjected to seasonal growth patterns. Narrow latewood has been documented in these woods with low mean and annual sensitivity values. Growth was apparently steady through the growing season, with an abrupt cessation that has been attributed to limiting water supply consistent with a monsoonal climate.

Coastal marshes have been interpreted to have been dominated by monospecific thickets of large sphenopsids (Equisetities), small ferns, and a cycadeoid (Ptilophyllum).

Southern hemisphere floras appear very similar to northern hemisphere floras with the equatorial groups (Cycadeoids, Cheirolepid conifers, and some fern groups) expanding their geographic range. Ginkgoalean trees are not common in the southern hemisphere; rather, podocarpaceous conifers are important in the southern region (a feature that continues until the Recent).

Early Cretaceous (pre-angiosperm)

The Kootenai flora of Montana exemplifies pre-angiosperm communities, which are very similar to those of the Late Cretaceous in general composition. This swamp forest consisted of:

Better-drained sites are believed to have been colonized by ferns, caytonialean pteridosperms, ginkgos, and other conifer genera, whereas stream side vegetation appears to have been podocarpaceous conifers and cycadaleans. Non-swamp floodplains were occupied by a heterogenous flora that included conifers, cycadaleans, and ferns.

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