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Deep Time |
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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era |
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Cenozoic Era: (248 mya-present)
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Paleocene | Eocene | Oligocene | Miocene | Pliocene | Pleistocene | Holocene
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Eocene Epoch (54-33 mya)
Early in the Eocene, the global climate remains warm. As the continents
move ever closer to their present-day positions, this plate activity alters ocean and air
circulation patterns. By the end of the Eocene, temperatures cool considerably and a drying
period commences. In subtropical latitudes, open woodlands covered with ferns and shrubby
plants replace forests.
Land mammals, which move out of the forests and into the open spaces,
become noticeably larger. Rhinoceroses, three-toed horses, and early relatives of pigs,
camels, and hippopotamuses first appear. As the temperature cools, some groups go extinct,
especially those in higher latitudes. Primates, which had thrived in the more tropical
conditions, manage to survive the extinction. With the evolution of whales from wolf-like
land animals to sea-going creatures, mammals now occupy land, air, and sea.
Animal migrations continue, especially between continents in the northern
hemisphere. Asian animals invade Europe, replacing some native species (like European tapirs)
with non-native ones (like Asian rhinos).
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50 mya: Whales
Features apparent in early whale fossils -- specialized teeth,
nostrils near the tip the nose, hind limbs, and a pelvis attached to the skeleton --
offer irrefutable evidence that whales evolve from land mammals. While they shed
many mammalian characteristics, like hair, whales still bear live young, breathe air,
and have large and complex brains as they adapt to aquatic life.
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Read more
Return to the sea (50 mya)
Gradually, over 5-10 million years, whale ancestors abandon their
terrestrial ties in favor of a sea-going existence. Hoofed, wolf-like, carnivorous
mammals called mesonychids hunt for food along the seashore. They probably spend more
and more time offshore because a more abundant food supply exists in the water. Those
better suited for aquatic environments thrive and reproduce, and eventually full-time
marine creatures evolve.
Fossil finds in Pakistan's Valley of the Whales, now a desert but
once home to inland seas, have allowed scientists to trace this transformation stage
by stage, as generations of species subtly change in appearance and habit. These
intermediate species are known in evolutionary terms as "transitional forms." They
help define the relationship between living things and their ancestors, thereby
improving our understanding of evolution and its many paths.
By comparing the ear regions of ancestral mesonychids, early
Eocene whales, and whales known from later in the period, for example, we can
understand the structural changes that enabled the later whales to hear underwater
and to regulate pressure when diving.
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40 mya: Continents near present-day positions
Following North America and Greenland's split from Europe and
Australia's separation from Antarctica, continents drift near their present-day
positions.
40 mya: Drying and cooling trend begins
Plate movements change ocean and atmospheric circulation
patterns, triggering drier, more seasonal weather and the return of ice to the poles.
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Late Eocene extinction
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Date:
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33 mya
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Intensity:
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3
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Affected:
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As many as 50-90 percent of
species in certain groups of marine invertebrates, mammals, and plants go extinct
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Hypotheses:
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Global cooling
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Summary:
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Two extinction pulses likely
occur -- the first at 37 mya, and the second at about 33 mya -- affecting both
marine and land faunas. In marine communities, tiny single-celled
organisms with shells called foraminifera suffer gradual but severe losses,
as do gastropods, bivalves, and echinoids (sea urchins).
On land, plants, rodents, primitive primates, and ungulates (hoofed mammals)
are affected. While impact craters have been identified from this time period,
evidence in rock layers suggests that meteors do not trigger the extinction.
Instead, global cooling is a more likely cause. This hypothesis
is supported on land by the appearance of open woodlands where densely forested
habitats previously existed, and the extinction of many animal groups that
lived in warm, higher latitudes.
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-> Go to the Oligocene Epoch
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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era |
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