A VERY BRIEF OVERVIEW OF THE CENOZOIC

Northern Hemisphere

The consistency of angiospermous floras in the Arctic of the northern hemisphere has been suggested to be heterochronous by Hickey and others. That is, the vegetation in the high arctic paleolatitudes is characterized by a palynoflora that is of Early Paleocene (of the mid-latitudes) in aspect. The megaflora has reported to follows this trend; that is, the leaf assemblages of these Arctic floras appear to be at least 1/2 stage/epoch ahead (18 million year phase shift) of the lower latitudinal floras. By the beginning of the Eocene, the floras are synchronous. This observation has led Hickey and others to suggest that certain taxa (i.e., Metasequoia occidentalis & "Carya" antiquorum) probably evolved at high latitudes and migrated southward during climatic deterioration, while diversity gradients in the remainder of the Cretaceous show higher diversity at low-latitudes.

In the late Cretaceous and Early Tertiary, the northern hemisphere continents were vegetated by a flora that was nearly cosmopolitan. There has been considerable question as to the relationship of the eastern Asia and both western and eastern US floras because they both have similar systematic affinities.

• In the late Cretaceous, 1/2 - 2/3 of the floras in these areas are similar or identical
• In the Early Tertiary (Late Eocene) 1/4 - 1/2 of the floras are the same.

Hsu has suggested that there was a direct migration between these portions of the northern continents with the development of a modern flora after the Eocene. The similarities between these two extant floras has been interpreted to represent a remnant distribution based on temperature centers.

Latitudinal organization of early Paleocene vegetation was different from that of the Late Cretaceous. Assemblages north of 50^oN paleolatitude represent broad-leaved deciduous forests (low proportion of entire-margined taxa), which is about 10-15^o south of their Late Cretaceous limit. Assemblages south of 50^oN paleolatitude exhibit characteristics of broad-leaved evergreen forest; those from 46^oN and south (Raton Basin and Gulf Coast) have a high proportion of entire-margined taxa reflecting megathermal conditions.

		Wolfe's suggested vegetational map of the latest Paleocene-early Eocene with continental positions adapted from Smith et al. (1981). Tropical rainforest=Green; Paratropical rainforest=Yellow; Broadleaved evergreen forest=Black; Mixed northern hardwood forest=Transparent

The trend into the Eocene reflects a return to dominance of broad-leaved evergreen plants in the northern Rocky Mountains, followed by significant drying in the Western Interior and eastern North America. The dry-season deciduous vegetation continued into the early Oligocene.

		Wolfe's suggested vegetational map of the late middle Eocene with continental positions adapted from Smith et al. (1981). Tropical rainforest=Green; Tropical/Paratropical semideciduous rainforest=Orange; Paratropical rainforest=Yellow; Broadleaved evergreen forest=Black; Mixed northern hardwood forest=Transparent; Mixed coniferous forest=Blue

Western United States

Wolfe has stated that the Late Cretaceous-Early Tertiary generally had a higher temperature and equitability through the Paleocene, with a deterioration in climate near the terminal Eocene event . The Paleocene vegetation includes magnolias, Ficus (figs), and other typically subhumid to subtropical plants. Several of these taxa are now restricted to Central America.

• In the Mid-High latitudes, the entire leaf margins, large, coriaceous leaves with attenuated apices suggest that the region underwent high annual temperatures.

The Puget Group and equivalent floras are characteristic of broad-leaved evergreen rain forests 20-30 degrees poleward of its present distribution. In the Mid-Eocene, the vegetation was paratropical rainforest with the presence of palms, mangroves, and other tropical families. The vegetation that was established at high latitudes, particularly floras known from Alaska, Greenland and Spitzbergen, are considered to be representatives of temperate to cold temperate climates. These include angiosperms - Juglans (walnuts), Ulmus (elm), Quercus

Floras from Oligocene strata depict a change in climate, with the beginning of a cooling trend that is accompanied by temperate-character forests. This continues into the early Miocene, although a slight warming trend is apparent in the mid-Miocene of the middle latitudes. This is in spite of the fact that Alaska began to experience glaciation. Mid-latitude floras reflect changing rainfall conditions, with floristic representatives presently found in the southeastern US (e.g., Magnolia, Liquidambar, and Ulmus).

By the Pliocene, Asian and southeastern US taxa are no longer found in the region. The present-day flora is essentially established in this area. The vegetation records a gradual climate shift to cooler and drier conditions, and culminated in the Pleistocene with the inception of continental-scale glaciation.

Before the end of the Tertiary, the forests in the western US become dominated by conifers (Wolfe, 1979). A similar pattern is seen in Europe where, in the Pliocene, Sequoia is dominant in some regions. Taxodium-dominated lowland swamps, with taxa similar and identical to those found in the Holocene flora of the southeastern US, existed in northern Europe (Denmark, German, Netherlands). This mixed conifer-deciduous forest extended into the Quaternary. Wolfe emphasizes that the loss of conifers in the Quaternary of Europe is an important vegetational and, hence, climate change.

Southeastern United States

The southeastern US has undergone a similar vegetational change in the Early Tertiary, although it differs slightly.

• The coastal plain vegetation in the Early Paleocene is representative of tropical rain forest vegetation with a deterioration in climate near the late Paleocene resulting in paratropical rainforest.
• By the Eocene, the southeastern US vegetation is characteristic of dry, tropical & warm climate floras in response to the cooling trend of the Eocene.
• During the Oligocene-Neogene, temperate broadleaf floras replace the broad-leaved evergreen floras. The terminal Eocene event apparently lowered overall diversity, and within the Oligocene a re-diversification and more modern appearing flora appeared.

During periods of maximum glaciation in the Pleistocene, temperate hardwood forests appear here and in northern Mexico. Reconstructions of global vegetation patterns in the Pleistocene are available in the Global Atlas for Paleovegetation.

Southern Hemisphere

Cenophytic floras in this hemisphere are varied in diversity.

• During the Eocene, the tropical South American and Africa were approximately 600 km apart with numerous volcanic islands interspersed. The Early Tertiary floras appear to be similar, but after the end of the Eocene and distances between South America and Africa 1400 km apart, principal evolutionary divergences began.
• Africa became accessible to dispersal from Asia for approximately 18 m.y., but with climatic deterioration in Africa and a drying accompanied by mass extinction, the flora of Africa dramatically changed.
• During the Miocene-Recent, the altitude of Africa has increased 8000 ft and accompanied aridity has impoverished the tropical flora as compared to South America.

©Copyright 1997-1998 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.