www.geo.ucalgary.ca/~macrae/timescale/time_scale.gif

Cenozoic Life

• The Cenozoic was the time during which– Earth’s 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

Cenozoic Life History

• 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

Good Fossil Records

• 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

Changing Climatic Patterns

• 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

Leaf Structure

• Climatic trends for four areas in North America

Plant leaves as Climatic Indicators

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

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

Major Climatic Change

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

• Since the Oligocene– mean annual temperatures have varied somewhat worldwide

– overall have not changed much in the middle latitudes except during the Pleistocene

Climatic Change

• 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

Decrease in Precipitation

• With the demise of dinosaurs and their relatives– mammals adaptively radiated

– remarkable diversification continued throughout the Cenozoic Era

• The Age of Mammals had begun

Mammal Diversification

• 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

Marsupial Mammals

• 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

South American Marsupials

• Like marsupials, placental mammals give birth to live young, – but their reproductive method– differs in important details

• In placentals, the amnion of the amniote

Placenta

– has fused with the walls of the uterus

– forming a placenta

• 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

Marsupial Placenta Less Efficient

• 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

Success of Placental Mammals

• 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

Fossil Record of Horses

• 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

Horse Evolution

• 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

Trends in Horses

• 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

Horse Evolution Branched

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

Low- and High-Crowned Teeth

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

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

• 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

Mammals of the Ice Age

• 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?)

Cooler Conditions—Larger Sizes

• 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

Frozen Mammals

• Frozen baby mammoth found – in Siberia in 1971

Frozen Baby Mammoth

– 1.15 m long and 1.0 m tall

– had a hairy coat

• Recovered from permafrost

• 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

Pleistocene 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

Extinctions

• Rapid climatic changes at the end of the Pleistocene

• Prehistoric overkill – holds that human hunters were responsible

Extinction Hypotheses

• 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

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

Climate and Vegetation Changes

• 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

Why Didn't Large Mammals Migrate?

• For example, reindeer and the Arctic fox – lived in southern France during the last glaciation

– migrated to the Arctic when the climate became warmer

Mammal Migration in Europe

• 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

Argument Against the Climatic Hypothesis

• 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

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

Arrival of Humans

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

Extinctions on Oceanic Islands

• 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

Hunters Concentrate on Small Animals

• 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

Other Arguments

• 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

Multiple Reasons

How do we know we had ice ages?

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

• Most important glacial deposits – chaotic mixtures of poorly sorted sediment deposited directly by glacial ice

– An end moraine is deposited– when a glacier’s terminus remains stationary for some time

Moraines

Mt. Cook, 1999

• If the glacier’s terminus – should recede and then stabilize once again

– another end moraine forms

– known as a recessional moraine

Recessional Moraine

• Features seen in areas once covered by glaciers • glacial polish

– the sheen

• striations– scratches?

Glacial Features

Devil’s Postpile National Monument, California

• Glaciers typically deposit poorly sorted nonstratified sediment

Glacial Sediment

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?)

Oxygen Isotope Ratio

http://www.ngdc.noa

a.gov/paleo/ctl/about.html

18O

18O16O

16O

Oxygen Isotope Ratio

http://www.ngdc.noa

a.gov/paleo/ctl/about.html

18O

18O

16O

16O

Oxygen Isotope Ratio

http://www.ngdc.noa

a.gov/paleo/ctl/about.html

18O

18O

16O

16O

QUATERNARY

60 Ma

Today

~2 Ma - Northern Hemisphere

10,000

CENOZOIC ERA

~45 Ma - East Antarctic

~30 Ma - West Antarctic

Cenozoic Glaciatio

ns

Onset of the Ice Age

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

Milutin Milankovic