Geoscience Reference
In-Depth Information
mid-oceanic ridge that created the Atlantic Ocean. In the early Cretaceous Period
(around 120 mya), Madagascar and India separated from the Gondwanan land-
mass and began to move east and slightly north. Africa remained isolated for a long
period during the late Cretaceous and early Tertiary Periods, developing a large
endemic group of mammals. By about 50 mya, the continents began to resemble
today's configuration. Each of the Tropical Rainforest Biome regions of the world
carries a bit of that geologic history. Fossils of primitive flowering plants and early
fauna, along with evolutionary relationships discovered between current species,
provide evidence of their connected past. Even today, the similarities between
these forests reveal these earliest connections.
At the time of Pangaea, the land was dominated by gymnosperms (conifers,
ginkgos, and cycads) and ferns, which were adapted to a drier, more continental
climate. Gymnosperms were the first seed plants; they have their ovules and seeds
exposed on the plant surface; that is, they have naked seeds. This is in contrast to
the flowering plants, angiosperms, whose ovules are contained within the flower
and their seeds within a fruit. Even before the Pangaean supercontinent was cre-
ated, primitive gymnosperms were present and evolving on the earlier southern
landmass as it moved toward the Equator. Soon the evolution of flowering plants
began. Although questions about the origin and evolution of flowering plants
remain, current evidence points to a great explosion of angiosperms during the Cre-
taceous Period (around 144-65 mya), between 45
S latitude, although
some may have been present prior to that time. Recently, fossils of primitive angio-
sperms found in China were dated to the mid-Jurassic Period (175 mya). The abun-
dance of moisture, sunlight, and heat helped support this great evolution and
diversification of angiosperms, and these flowering plants began to dominant the
central part of the world. At this time, Africa and South America remained close to
each other and were able to share species.
The tropical areas of Gondwana experienced abundant sunlight, rain, and con-
stant warm temperatures for millions of years. During the period when gymno-
sperms and ferns were dominant, dinosaurs roamed the land. Early mammals were
just beginning to emerge. With the extinction of the dinosaurs in the Jurassic and
Cretaceous periods (200-65 mya), habitat and niches became available for an ex-
plosive diversification of mammals, which rapidly began to evolve and diversify.
Marsupial mammals arose between 250-100 mya. Evolutionary changes in repro-
duction and regeneration in flowering plants occurred simultaneously with mam-
mal evolution as plants provided food for the mammals and the mammals aided
the pollination, seed dispersal, and germination of plants. At this time South
America, Antarctica, Australia, and New Guinea were separating from Africa.
They began to move south from equatorial Gondwana, carrying some of the more
primitive mammals, as well as primitive vascular plants and gymnosperms, with
them. The plate containing Australia and New Guinea broke away from South
America and began to move in a northeastern direction. This island block
remained isolated for millions of years, and ancient gymnosperms and marsupials
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