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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


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


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


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


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


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


ARTIODACTYLS = even-toed (if middle 2 toes carried weight -> cloven hoof). Examples: bison, pig, deer, hippo.

Harry Williams, Historical Geology


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


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

Harry Williams, Historical Geology


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


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

Harry Williams, Historical Geology


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

Grazing teeth for grinding grass (mammoth)

Harry Williams, Historical Geology


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

Harry Williams, Historical Geology


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

Harry Williams, Historical Geology


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

Harry Williams, Historical Geology


LATE PLEISTOCENE SCENE (predominately grazers)

Harry Williams, Historical Geology