Cenozoic Life

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Cenozoic Life
www.geo.ucalgary.ca/macrae/timescale/time_scale.g
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Cenozoic Life History

• The Cenozoic was the time during which
• Earths present-day fauna and flora evolved
• trends established millions of years earlier
  continued
• Fewer skull and jaw bones during the transition
• from fish to amphibians
• and then to reptiles
• and finally to mammals

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Good Fossil Records

• Cenozoic rocks are especially common
• in western North America
• also found along the Gulf and Atlantic coasts
• horses, rhinoceroses, rodents, rabbits, and
  camels have very good fossil records

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Changing Climatic Patterns

• Changing climatic patterns
• accompanied by shifting plant distributions
  characterize the Tertiary
• During the Paleocene and Eocene
• mean annual temperatures were high
• abundant precipitation fell
• tropical to semitropical forests covered much of
  North America

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Leaf Structure

• Leaf structure can give information about
• Paleoprecipitation
• Paleotemperature
• Precipitation drip tips
• Reduces fungal infections
• Reduces parasitic plant infections
• Temperature degree of leaf serration
• Correlation between big smooth leaves (entire
  margin) and warm climates
• Not clear why

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Plant leaves as Climatic Indicators

• Climatic trends for four areas in North America

• based on the percentages of plant species with
  entire margin leaves

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Major Climatic Change

• A major climatic change took place at the end of
  the Eocene

• when mean annual temperatures dropped 7 degrees C
  in 3 million years

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Climatic Change

• Since the Oligocene
• mean annual temperatures have varied somewhat
  worldwide
• overall have not changed much in the middle
  latitudes except during the Pleistocene

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Decrease in Precipitation

• A general decrease in precipitation
• over the last 25 million years
• in the midcontinent region of North America
• As the climate became drier vast forests of the
  Oligocene
• gave way first to savanna conditions
• grasslands with scattered trees
• and finally to steppe environments
• short-grass prairie of the desert margin

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Mammal Diversification

• With the demise of dinosaurs and their relatives
• mammals adaptively radiated
• remarkable diversification continued throughout
  the Cenozoic Era
• The Age of Mammals had begun

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Marsupial Mammals

• Marsupial mammals give birth to live young
• born in a very immature, almost embryonic
  condition
• undergo further development in the mother's pouch
• Marsupials probably migrated to Australia,
• the only area in which they are common today,
• via Antarctica before Pangaea fragmented
  completely

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South American Marsupials

• Quite widespread in South America until a few
  millions of years ago
• Most South American marsupials died out
• when a land connection was established between
  the Americas
• and placental mammals migrated south
• Now the only marsupials
• outside Australia and some nearby islands are
  species of opossums

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Placenta

• Like marsupials, placental mammals give birth to
  live young,
• but their reproductive method
• differs in important details
• In placentals, the amnion of the amniote

• has fused with the walls of the uterus
• forming a placenta

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Marsupial Placenta Less Efficient

• Nutrients and oxygen flow
• from mother to embryo through the placenta
• permitting the young to develop much more fully
  before birth
• marsupials also have a placenta
• but it is less efficient
• explaining why their newborn are not as fully
  developed

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Success of Placental Mammals

• A measure of the success of placental mammals
• is partially related to their method of
  reproduction
• More than 90 of all mammals
• fossil and extinct, are placentals

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Fossil Record of Horses

• With the possible exception of camels,
• no group of mammals has a better fossil record
• horse fossils are so common,
• especially in North America
• where most of their evolution took place
• that their overall history and evolutionary
  trends are quite well known

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Horse Evolution

• Some evolutionary trends in horses
• an increase in size
• lengthening of the limbs
• reduction in the number of toes
• development of high-crowned teeth with complex
  chewing surfaces

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Trends in Horses

• Size increase
• Legs and feet become longer for running
• Lateral toes reduced to vestiges
• Straightening and stiffening of the back
• Adaptations for grinding abrasive grasses
• Larger, more complex brain

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Horse Evolution Branched

• Horse evolution proceeded along two distinct
  branches
• One led to three-toed browsing horses
• all now extinct
• The other led to three-toed grazing horses
• and finally to one-toed grazers
• The appearance of grazing horses
• with high-crowned chewing teeth
• coincided with the evolution and spread of
  grasses during the Miocene

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Low- and High-Crowned Teeth

• Once grasses had evolved
• many hoofed mammals became grazers
• developed high-crowned, abrasion-resistant teeth

• Low-crowned teeth
• typical of many mammals with varied diets

• High-crowned, cement-covered chewing teeth
• are adapted for grazing

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Mammals of the Ice Age

• The most remarkable aspect
• of the Pleistocene mammalian fauna
• is that so many very large species existed
• Mastodons, mammoths, giant bison,
• huge ground sloths, immense camels, beavers 2 m
  tall
• present in North America

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Cooler ConditionsLarger Sizes

• Many smaller mammal species also existed
• but obvious trend among Pleistocene mammals was
  large body size
• Perhaps this was an adaptation
• to the cooler conditions
• Large animals have less surface area
• compared to their volume
• thus retain heat more effectively than do smaller
  animals
• (but what about big dinosaurs?)

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Frozen Mammals

• Some of the world's best-known fossils
• come from Pleistocene deposits
• frozen mammals found in Siberia and Alaska,
• such as mammoths, bison, and a few others
• These extraordinary fossils,
• although very rare,
• provide much more information than most fossils do

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Frozen Baby Mammoth

• Frozen baby mammoth found
• in Siberia in 1971

• 1.15 m long and 1.0 m tall
• had a hairy coat
• Recovered from permafrost

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Pleistocene Extinctions

• Extinctions have occurred continually
• at times of mass extinctions, Earth's biotic
  diversity sharply declined
• as at the ends of the Paleozoic and Mesozoic eras
• In marked contrast,
• the Pleistocene extinctions were rather modest
• did have a profound effect on genera of large
  terrestrial mammals

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Extinctions

• (1) What caused these extinctions?
• (2) Why did these extinctions eliminate mostly
  large mammals?
• (3) Why were extinctions more severe in Australia
  and the Americas?
• No completely satisfactory explanation exists
• but two competing hypotheses are currently being
  debated

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Extinction Hypotheses

• Rapid climatic changes at the end of the
  Pleistocene
• Prehistoric overkill
• holds that human hunters were responsible

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Climate and Vegetation Changes

• Rapid changes in climate and vegetation
• occurred over much of Earth's surface during the
  Late Pleistocene
• as glaciers began retreating
• In North America and northern Eurasia
• conifer and broadleaf forests replaced
  open-steppe tundras
• warmer and wetter conditions prevailed

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Climate and Vegetation Changes

• The southwestern U.S. region changed
• from a moist area with numerous lakes
• where saber-toothed cats, giant ground sloths,
  and mammoths roamed
• to a semiarid environment unable to support a
  diverse large mammalian fauna

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Why Didn't Large Mammals Migrate?

• Rapid changes in climate and vegetation
• can certainly affect animal populations
• but the climate hypothesis presents several
  problems
• First, why didn't the large mammals migrate to
  more suitable habitats as the climate and
  vegetation changed?
• many other animal species did

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Mammal Migration in Europe

• For example, reindeer and the Arctic fox
• lived in southern France during the last
  glaciation
• migrated to the Arctic when the climate became
  warmer

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Argument Against the Climatic Hypothesis

• The second argument against the climatic
  hypothesis
• is the lack of correlation between extinctions
  and the earlier glacial advances and retreats
  throughout the Pleistocene Epoch
• Previous changes in climate
• were not marked by episodes of mass extinctions

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Arrival of Humans

• Proponents of the prehistoric overkill hypothesis
  
• argue that the mass extinctions in North and
  South America and Australia coincided closely
  with the arrival of humans
• Perhaps hunters had a tremendous impact
• on the faunas of North and South America
• about 11,000 years ago because the animals had no
  previous experience with humans

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Arrival of Humans

• The same thing happened much earlier in
  Australia soon after people arrived about 40,000
  years ago
• No large-scale extinctions in Africa and most of
  Europe
• because animals in those regions had long been
  familiar with humans

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Extinctions on Oceanic Islands

• How could a few hunters decimate so many
  species of large mammals?
• Humans have caused major extinctions on oceanic
  islands
• in a period of about 600 years after arriving in
  New Zealand, humans exterminated several species
  of the large, flightless birds called moas

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Hunters Concentrate on Small Animals

• A problem is that present-day hunters concentrate
  on smaller, abundant, and less dangerous animals
• remains of horses, reindeer, and other small
  animals are found in many prehistoric sites in
  Europe
• whereas mammoth and woolly rhinoceros remains are
  scarce

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Other Arguments

• Few human artifacts are found among the remains
  of extinct animals in North and South America
• and there is usually little evidence that the
  animals were hunted
• Countering this argument
• is the assertion that the impact on the
  previously unhunted fauna
• was so swift as to leave little evidence

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Multiple Reasons

• The reason for the extinctions
• of large Pleistocene mammals is still unresolved
• It may turn out that the extinctions
• resulted from a combination of different
  circumstances
• Populations that were already under stress from
  climatic changes
• were perhaps more vulnerable to hunting
• especially if smaller females and young animals
  were the preferred targets

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How do we know we had ice ages?

• Geologic evidence
• Moraines
• Poorly sorted sediments
• Scratched rocks

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Moraines

• Most important glacial deposits
• chaotic mixtures of poorly sorted sediment
  deposited directly by glacial ice
• An end moraine is deposited
• when a glaciers terminus remains stationary for
  some time

Mt. Cook, 1999
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Recessional Moraine

• If the glaciers terminus
• should recede and then stabilize once again
• another end moraine forms
• known as a recessional moraine

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Glacial Features

• Features seen in areas once covered by glaciers
• glacial polish
• the sheen
• striations
• scratches?

Devils Postpile National Monument, California
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Glacial Sediment

• Glaciers typically deposit poorly sorted
  nonstratified sediment

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How do we know how cold it got?

• Isotopes of oxygen!
• Oxygen
• All isotopes have 8 protons
• Most common isotope has 8 neutrons
• Extremely rare 9 neutrons
• Rare but detectable 10 neutrons
• (why are 10 and 8 more common than 9?)

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Oxygen Isotope Ratio
18O
16O
16O
http//www.ngdc.noaa.gov/paleo/ctl/about.html
18O
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Oxygen Isotope Ratio
16O
18O
16O
http//www.ngdc.noaa.gov/paleo/ctl/about.html
18O
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Oxygen Isotope Ratio
16O
18O
16O
http//www.ngdc.noaa.gov/paleo/ctl/about.html
18O
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Onset of the Ice Age
Cenozoic Glaciations
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Why the Icehouse?

• Long-term climate drivers
• Plate tectonics
• Opening/closing of seaways
• Ocean currents are our heat and AC
• Uplift and erosion of mountains
• Weathering reduces atmospheric CO2
• Life catastrophic evolution of new capabilities
• O2
• Astronomical drivers
• Other bodies (moon, sun) pull on the Earth,
  changing its distance to the sun

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Milutin Milankovic