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BMEN 515 Lecture Notes - Cenozoic, Pleistocene, Herbaceous Plant

Biomedical Engineering
Course Code
BMEN 515
William Huddleston

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The modern biota evolved during the Cenozoic era
By early Cenozoic ear (65 mya), the positions of the continents resembled those of today.
Cenozoic was characterized by extensive radiation of mammals.
Flowering plants diversified extensively and came to dominate world forests, except in cool regions.
Mutations of two genes in one group of plants allowed them to use atmospheric nitrogen directly by forming symbioses
w/ a few species of nitrogen-fixing bacteria; dramatically increased the amount of nitrogen available for terrestrial plant
o Australia began its northward drift.
o Early in this era, it was hot and humid; the ranges of many plants shifted latitudinally.
o In the middle of this era, Earth’s climate became considerably drier and cooler.
o Many lineages of flowering plants evolved herbaceous forms; grasslands spread over much of Earth.
o Snakes and lizards underwent extensive radiations (birds and mammals too).
o Three waves of mammals dispersed from Asia to North America across the land bridge.
o Rodents, marsupials, primates, and hoofed mammals appeared in N. America for the first time.
o Is subdivided into two epochs, the Pleistocene and the Holocene.
o The Pleistocene was a time of drastic cooling and climate fluctuations; 4 major and 20 minor “ice ages” cause
the spreading of massive glaciers across the continentsranges of the animals and plants shifted toward the
o Relatively few species became extinct during these climate fluctuations.
o Pleistocene was the time of hominoid evolution and radiation, resulting in Homo sapiens.
o Many large bird and mammal species became extinct in Australia and in the Americas when H.sapiens arrived
on those continents (40 000 to 15 000 years ago).
Three major faunas have dominated life on Earth
Fossil records reveal three great evolutionary radiations.
o The Cambrian explosion (first)
o The second, 60 million years later, resulted in the Paleozoic fauna.
o The great Permian extinction 300 million years later was followed by the third greater radiation Triassic
explosion leading to modern fauna.
o During Cambrian explosion, organisms ancestral to most of the present day animal groups appeared.
o The Paleozoic and Triassic explosions resulted in considerable diversification of the existing major groups of
Why Do Evolutionary Rates Differ Among Groups of Organisms?
“Living fossils” exist today
Species whose morphology has changed little over millions of years are known as “living fossils.” (e.g. horseshoe crabs).
Evolutionary changes have been gradual in most groups
The most striking feature of life’s evolution is that rates of change are, on average, very slow.
A likely reason is that climates have usually changed slowly.
Rates of evolutionary change are sometimes rapid
If the physical or biological environment changes rapidly, some lineages may also change rapidly.
Rates of extinction have also varied greatly
More than 99 percent of the species that have ever lived are extinct.
At least 5 major extinction events that severely reduced the planet’s biota were often followed by high rates of
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