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CENOZOIC LIFE. The CENOZOIC = “The Age of Mammals".

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The CENOZOIC = “The Age of Mammals".

The original mammals evolved from reptiles. These small, insect eating, shrew-like creatures, had the advantage of being endothermic - they produced their own heat internally by consuming great amounts of food. This gave them the advantage of being active at night, when reptiles couldn't compete because of the cold. This enabled the mammals to become dominant at the end of the Mesozoic, when the climates got colder and many of the larger reptiles died out.

Harry Williams, Historical Geology

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Another mammalian feature related to heat was the development of body hair, to help conserve the heat generated internally.

Unlike reptiles, mammals also NURTURE their young using milk. Again, this helped to keep the young warm and ensured rapid growth in the vulnerable early stages of life.

So, mammals had good survival skills.

Harry Williams, Historical Geology

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Animals that are transitional between the reptiles and true mammals are still around today - the duck-billed platypus and the anteater are endothernmic, have hair, nurture their young with milk, but they both lay eggs (MONOTREMES).

Harry Williams, Historical Geology

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The problem of keeping the young warm resulted in other adaptations, for example the MARSUPIALS (kangeroos, wombats, wallabies, opossums) - which nurture their young in a pouch; and the PLACENTAL MAMMALS - which carry the young inside the body for a longer period of development, feeding them nutrients from a placenta.

Harry Williams, Historical Geology

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Apart from evolutionary changes designed to keep mammals warm; other changes also occurred which were adaptations to the changing environment. Perhaps the most important of these changes was the appearance of GRASSES and vast PRAIRIES in the Miocene. This led to the evolution of grazing mammals - the UNGULATES.

Harry Williams, Historical Geology

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Ungulates are characterized by continuously-growing cheek teeth for chewing grass and multi-chambered stomachs for digesting tough grass. Due to the lack of cover on the open prairies, these mammals also developed speed to escape predators - long legs for running and running on toes, which became HOOVES. The ungulates are classified according to the number of toes; PERISSODACTYLS = odd-toed (if middle toe carried weight-> single hoof). Examples: zebra, horse, rhino, tapir.

Harry Williams, Historical Geology

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ARTIODACTYLS teeth for chewing grass and multi-chambered stomachs for digesting tough grass. Due to the lack of cover on the open prairies, these mammals also developed speed to escape predators - long legs for running and running on toes, which became = even-toed (if middle 2 toes carried weight -> cloven hoof). Examples: bison, pig, deer, hippo.

Harry Williams, Historical Geology

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Other important orders included the carnivorous predators that preyed on the grazing mammals - ORDER CARNIVORA. Examples: Smilodon (extinct), dogs, raccoons, bears.

Harry Williams, Historical Geology

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and the mammals with trunks, forerunners of the modern elephant -ORDER PROBOSCIDEA. Examples: mastodons (extinct), mammoths (extinct), elephants.

Harry Williams, Historical Geology

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Particularly important, of course, was the evolution of the order PRIMATES, distinguished by stereoscopic vision and grasping hands - both of which probably evolved to facilitate a life in the trees. The primates evolved into the Late Cenozoic ancestors of Homo sapiens, who appeared in the Pleistocene (more next class).


Harry Williams, Historical Geology

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MIDDLE EOCENE SCENE order (primitive ungulates, mainly browsers, still lots of forest)

Harry Williams, Historical Geology

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Browsing teeth for crushing leaves, fruit, nuts, branches etc.. (mastodon)

Grazing teeth for grinding grass (mammoth)

Harry Williams, Historical Geology

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LATE EOCENE SCENE etc.. (mastodon)(more open grassy land – more long-legged grazers)

Harry Williams, Historical Geology

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EARLY MIOCENE SCENE etc.. (mastodon)(cooler drier climate; more advanced ungulates)

Harry Williams, Historical Geology

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EARLY PLIOCENE SCENE etc.. (mastodon)(prairies – woods restricted to bottom lands near rivers)

Harry Williams, Historical Geology

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LATE PLEISTOCENE SCENE etc.. (mastodon)(predominately grazers)

Harry Williams, Historical Geology