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Origins and Evolution of Tropical Rain Forests
Jaboury Ghazoul
Chapter 6:
From the beginning: origins
and transformation
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The Growth and Spread of Trees
Gilboa: the earliest tree
Archaeopteris: the earliest branched tree
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The First Trees
Devonian (416‐359 Ma)
Two growth forms:
Single‐stemmed ‘Gilboa’, perhaps the world’s oldest tree
Archaeopteris, a multi‐
stemmed pro‐gymnosperm
Archaeopteris
Gilboa
The First Forests
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Pro‐gymnosperms: the first forest trees
Archaeopteris, extinct relative of seed plants: the first large and widespread tree
Appeared in the Middle Devonian (375 Ma) and became extinct in the Early Carboniferous (345 Ma)
The Carboniferous Coal Forests (360‐300 Ma)
Carboniferous coal deposits due to:
• Appearance of lignin‐rich woody trees
• Lower sea levels – created extensive lowland swamps and forests • Few animals and bacteria could digest the new lignin
Lepidodendron
50 m
Sigillaria
40 m
Accumulating woody debris eventually formed coal deposits 4
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Lepidodendron
Scale trees of the Late Carboniferous
Cone
The Rise of Land Plants and Animals
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Carboniferous Arthropods
High oxygen levels (up to 35%) resulted in frequent fires and giant arthropods
Encounters with Arthropleura
in the Carboniferous forests
The Permian (299 – 251 Ma)
Extensive rain forests of the Carboniferous disappeared during the hot and dry Permian
Smaller areas of wet tropical vegetation persisted in coastal areas (in what is now China) and comprised ferns, pteridosperms (seed ferns), and the first Ginkgoales
Gingko trees diverse and widespread in Permian tropical forests
Leaf of a fern and Sphenopteris (a seed fern)
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Eduard Suess
(1831 –1914)
Proposed, in 1885, the existence
of Gondwana, based on Glossopteris fossil distribution
Terra Nova Expedition, 1911‐1912
Edward Wilson
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Permian Temperate Forests
The fossil distribution of the Glossopteridales
provided evidence for continental drift theory Glossopteris
woody, seed‐bearing plant reaching 30 m
Distribution of Glossopteris across southern Pangea in the Permian
Fossil leaves of Glossopteris
Triassic and Jurassic (250–145 Ma)
Rain forests on coasts, with Araucariaceae trees reaching 60 m, understorey of ferns, cycads and giant horsetails. Large herbivorous dinosaurs were common. Late Jurassic (152 Ma) – initial break‐up of Gondwana
Petrified cone of Araucaria sp. From the Jurassic (210 Ma)
Dicroidium, a tree‐like seed fern
Petrified Araucarioxylon
trunk from Arizona
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Cretaceous: the first flowering plants
Isolation of rain forests on separate continental fragments lays the foundation for distinctive modern biotas.
Late Cretaceous (80‐65 Ma) – extensive wet forests of angiosperms (flowering plants), and first modern rain forests
Leefructus mirus, 125 Ma
Angiosperm origins and diversification
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Insect Diversification
110‐100 Ma Gondwana was breaking‐up and warm wet climates were extensive
Tropical angiosperm families diverged
Start of Angiosperm radiation
Ants, bees and other pollinating insects diversified alongside angiosperms …
… but evidence for coevolution is limited
Birds, mammals, lizards, freshwater fish, and other groups also underwent radiations
Insect familial diversity from Triassic to present
Angiosperm diversity in the Cenozoic
Fossil pollen reveals long‐term diversity changes in South America – a gradual rise then fall in plant diversity from 65 to 20 million years ago
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The Cenozoic (65 Ma ‐ present)
Extensive forests largely disappeared at the end Cretaceous extinction (65 Ma)
Rain forests replaced by ferns, but recovered within 1.4 million years
Early Tertiary climate favoured rain forests
Angiosperms, as well as mammals and birds, diversified once more
After 35 Ma the world began to cool and forests retreated The last 10‐15 million years has seen C4‐
grasslands emerge as a major tropical biome
The evolution of grazing species along with fire restricted the distribution of forests
Regional differences: Neotropics
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Forests Through Glacial Cycles
Under glacial conditions wet tropical climates cooled and dried, montane vegetation descended and sea levels dropped
Cooling lowered altitudinal vegetation bands by as much as 1500 m and temperate species (e.g. Quercus, Alnus and gymnosperms) migrated into the tropics
Regional differences: Madagascar
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Regional differences: Southeast Asia
The Dipterocarpaceae: Gondwana origin, India rafting, and Asian radiation
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Regional differences: New Guinea
Coastal extension and deep sea division
Land bridges during lower sea levels permitted faunal exchange across Southeast Asia
Deep water channels limited exchange across the Wallace Line
17,000 yrs BP
9,500 yrs BP
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Regional differences: Australasia
• Australasia separated from Gondwana at about the same time as South America
• Evolved its own distinctive flora
• Rainforest flora comprises ancient conifer groups (Agathis) that dominated Gondwanan forests
Agathis microstachya (Araucariaceae)
• Many angiosperm families also occur elsewhere because of the ancient Gondwana link
Bowenia spectabilis (Stangeriaceae)
Take home messages
• Tropical forest with complex structures have arisen repeatedly over geological time.
• Tropical rain forests acted as a cradle of angiosperm and animal diversification.
• Tectonic and biogeographical processes have interacted to shape tropical rain forest distributions and compositions
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