The Amazon river basin was formed in the Paleozoic period(two thousand million to five thousand million years ago).And the forest was mainly formed after Cenozoic(65 million years ago), when angiosperm started to thrive. Now, half of present-day rain forest occur in South and Middle America. The following is the brief explanation of the history of amazon rainforest.

Table 1                                                       Table 2

Table 1is from http://www.stonecompany.com/geotime/images/eralace.GIF
Table 2 is from http://www.stonecompany.com/geotime/images/cenozoic.GIF

The history of rainforest overall (from Robert J. Morley Origin and Evolution of Tropical Rain Forests 1999)

Cenomanian(96-92 Ma)

The earliest moist megathrmal forests were developed.
It may not have been identical to our classic impressions of present-day tropical rain forests(multistratal communities), but more likely they were single-stratum open forests.

Paleocene (66-54 Ma)

In Early Paleocene(66-60 Ma), closed, multistratal, tropical aspect rain forests became widespread in each of the three moist megathermal belts( Boreotropical, Palmae, and Southern Megathermal, from north to south).
The diversification of mammals, such as dinasaurs, which developed fruit and seed eating, and helped seed dispersal of large, closed-canopy trees.

In the end of the Paleocene(60-54 Ma), the the floras of the Boreotropical and Southern Megathermal Provinces expanded poleward, mainly because of the climate change.

Eocene (54-36 Ma)

Tropical aspect and paratropical rain forests and widespread mangroves became established over much of Europe and North America.
the warm periods continued into the Late Eocene(39-36 Ma) in North America, whereas in Europe there was a tendency toward cooler climates after the Mi
In the Middle and Late Eocene, equatorial floras were characterised by the appearance of many elements of modern aspect in each of the three tropical bolook. (the isolation of South America and Africa, which led to the different floras evolved).

The terminal Eocene event (36 Ma)

The cooling event occurred.
megathermal elements of the Boreotropical Province virtually disappeared from northrn mid-latitudes.

Middel Miocene(16-10 Ma)

Climatic amelioration.
Renewd global warming during the latest Early and earliest Middle Miocene resulted once.

The history of rainforest in south america (from Robert J. Morley Origin and Evolution of Tropical Rain Forests 1999)

Epoch Northern Southern
Plio-Pleist (5 - 2 Ma) Northern neotropical Southern neotropical Nothofagus
Miocene (25 - 5 Ma) Northern neotropical Southern neotropical Nothofagus
Oligocene (45 - 25 Ma) Neotropical Mixed (former)Mixed / (latter)Nothofagus)
Eocene (57 - 45 Ma) Neotropical Mixed Mixed
Paleocene (65 - 57 Ma) Palmae  Mixed Mixed
Net tropical climates enabled the development of diverse vegetation.
Palmae ---- Diverse Palmar, megathermal taxa;
Mixed --- megathermal elements, southern hemisphere taxa

Characterized Plants --- palms (considerable diversity)

Key Species:  manglove palm (nypa, mauritia, longapertites, etc.), Megathermal taxa (Anacolosa, ctenolophon, bombacaceae, annoaceae, proxapertites, etc.)  --- in Palmae
                        pollen of casuarina, beauprea, nothofugns, gunnera, restionaceae etc.; conifers of podocarpus, microcachrys, etc. ; macrofossil of cunoniaceae, peumus, etc. --- in Mixed
The cooling event all over the world ---> less effect on South America. (this is considered to be one of the reasons of th high genetic diversity of Palmae in SouthAmerica.)

early --- well represented by palm pollen (mauritia, etc.)
middle --- pollen of annonaceae, gramineae, cruclin, coryocar, mirabilis, etc.
latter ---- mangroves with rhizophora in North South Amrica

subhumid wooded savannas became widespread in S South America
Grass-dominated vegitation  in North America

Mincerie and so on

2.7 Ma --- Sudden Cooling
2.2 - 1.0 Ma --- cold period

However, the desiccation associated with glacial maxima were not pronounced in South America as in Africa. ---> which enabled the survival of very diverse rain forest communities through the most unfavorable,periods of the Pleistocene)

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