Life of the Cenozoic

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Chapter 16

• Life of the Cenozoic

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The Age of Mammals

• The Cenozoic Era is sometimes called the Age of
  Mammals.
• Mammals came to dominate the Earth, much as
  reptiles had done during the Mesozoic.
• A spectacular adaptive radiation of mammals near
  the beginning of the Cenozoic resulted in the
  appearance of mammals as diverse as bats and
  whales, descending from shrew-like mammalian
  ancestors in as little as 12 m.y.

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Appearance of Homo sapiens

• The appearance and evolution of primates led to
  the ancestors of humans by the Neogene.
• Homo sapiens appeared in the Pleistocene Epoch.

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• We know more about the life of the Cenozoic Era
  than we know about life of any other time.
• The fossils are better preserved and have had
  less time to be destroyed
• They are stratigraphically uppermost, and thus
  more accessible for study.
• In addition, Cenozoic fossils more closely
  resemble life today.

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Causes of Biologic Changes

• Biologic changes in the Cenozoic can be tied to
  changes in the environment and geographic change.
  
• Changes in climate to cooler and drier
  conditions, led to the expansion of the
  grasslands, which influenced the evolution of
  herbivorous mammals.
• Continental breakup as a result of plate
  tectonics, stimulated biological diversity. This
  resulted in distinct faunal radiations on
  separate landmasses, and in isolated marine
  basins.

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Diversity of Life in the Cenozoic
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Cenozoic Plant Life

• The flowering plants or angiosperms diversified
  throughout the Cenozoic to become the dominant
  vascular land plant on Earth.
• Ferns, cycads, conifers, and other plants
  declined relative to the angiosperms

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Grasslands Expand and Mammals Respond

• Grasses, a flowering plant commonly eaten by
  grazing mammals, became widespread during the
  Miocene.
• The expansion of the grasslands across the plains
  of North America and other continents was related
  to cooling and drying of the global climate.
• Mammals evolved in conjunction with the spread of
  the grasslands.

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• Modern grasses include
• Maize (corn)
• Wheat
• Rice
• Oats
• Rye
• Barley
• Grain amaranth
• Bamboo
• Sugar cane

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Teeth Adapt to Grasses

• Many grasses contain siliceous secretions, and
  because they grow close to the ground, grasses
  are often coated with fine particles of soil. As
  a result, grasses are abrasive to the teeth of
  grazing mammals.
• To compensate for the tooth abrasion resulting
  from chewing grasses, the major groups of
  herbivorous mammals evolved high-crowned cheek
  teeth that continue to grow at the roots during
  part of the animals' lives.

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Teeth Adapt to Grasses

• The resistant enamel of the
  chewing teeth became infolded.
• As the teeth wore down, a complex pattern of
  enamel ridges became apparent on the grinding
  surface of the teeth.
• The incisors (front teeth) gradually aligned into
  a curved arc, which served for biting the
  grasses.
• The length of the face in front of the eyes
  increased in the grazing mammals to provide space
  for these teeth (for example, in the horses).

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Limbs Adapt to Grasslands

• The limbs of grazing mammals changed to become
  better adapted to life on the grasslands.
• Grasslands provide few places to hide from
  predators, so grazing herbivores developed
  modifications to run more quickly.
• The bones of the limbs and feet were lengthened,
  strengthened, and modified by natural selection
  to permit rapid fore-and-aft motion, and to
  prevent rotation.
• The ankle was elevated, and the grazing mammals
  ran on their toes like sprinters.

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Limbs Adapt to Grasslands

• Many grazing mammals gradually developed hoofs as
  an adaptation to protect the bones of the toes as
  they ran across the hard prairie sod.
• Mammals with hoofs are called ungulates.
• These grazing mammals also lost some of their
  side toes.

Evolution of the lower foreleg in horses
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Cenozoic Life in the Seas
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Marine Phytoplankton

• Entire families of phytoplankton became extinct
  at the end of the Mesozoic. Only a few species in
  each major group survived into the Cenozoic.
• Surviving species of phytoplankton diversified
  rapidly in the Paleogene due to decreased
  competition.
• Cenozoic phytoplankton include
• Diatoms
• Dinoflagellates
• Coccolithophorids

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Marine Zooplankton

• Zooplankton diversified in the Cenozoic, and
  became abundant in the seas. Cenozoic zooplankton
  include
• Benthonic foraminifera
• Planktonic foraminifera
• Radiolarians

Planktonic foraminifera
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Significance of Foraminifera

• Large benthic foraminifera resembling coins in
  size and shape, called nummulitic foraminifera,
  lived in the Tethys seaway and other areas. Their
  remains accumulated to form thick beds of
  nummulitic limestone, which were used to build
  the Great Pyramids and Sphinx in Egypt.
• Forams are useful in correlating rocks of
  Cenozoic age, particularly in oil fields around
  the world.
• Benthonic forams can be used as water depth
  indicators.

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Dominant Cenozoic Invertebrates

• Arthropods
• Crustaceans
• Insects (on land)
• Echinoderms
• Starfish
• Echinoids

• Sponges
• Scleractinian corals
• Bryozoans
• Brachiopods
• Molluscs
• Bivalves
• Gastropods
• Cephalopods

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Corals

• Corals are found both as solitary types
  (primarily in the Paleogene), and as colonial,
  reef-building types.
• Atolls are ring-like coral reefs that grow in
  shallow tropical waters around a volcano which
  subsides beneath the water.

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Molluscs

• Cenozoic molluscs are dominated by
• Bivalves (clams)
• Gastropods (snails)

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Molluscs

• Cephalopods are also present, but not as
  widespread and abundant as previously.
• Cenozoic cephalopods include
  the Nautilus, and other forms without
  a shell (or with a
  reduced shell)
• Squid
• Octopus
• Cuttlefish

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Echinoderms

• Echinoderms are also present in the Cenozoic,
  particularly free-moving types (as opposed to the
  attached crinoids of the Paleozoic). Echinoderms
  include the echinoids (sea urchins, sand dollars,
  sea biscuits), and the starfish.

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Arthropods - Crustaceans

• Modern crustaceans (such as crabs, shrimp,
  lobsters, barnacles) became well established in
  the seas during the Cenozoic.

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Arthropods - Insects

• One of the world's best locations for
  fossil insects is the Oligocene
  Florissant Formation, Florissant
  Fossil Beds National Monument, Colorado.
• Insect fossils are preserved in fine volcanic ash
  which has been compressed to form shale.
• The ash settled into an ancient lake, burying
  insects and plants.

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Cenozoic Vertebrates

• Cenozoic vertebrates include
• Fishes
• Amphibians
• Reptiles
• Birds
• Mammals

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Fishes

• The Eocene Green River Formation in Wyoming
  contains abundant well-preserved
  fossil fish deposited in a freshwater environment

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Sharks

• Sharks were common in the Cenozoic. Sharks
  have skeletons of cartilage rather than of
  bone, and the skeletons are rarely preserved.
• Shark teeth are well preserved in Cenozoic
  sedimentary rocks.

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Amphibians

• Cenozoic amphibians resembled modern forms. All
  are relatively small with smooth skin (unlike the
  large Paleozoic amphibians).
• Cenozoic amphibians include
• Frogs
• Toads
• Salamanders

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Reptiles

• Cenozoic reptiles include the following
• Turtles
• Crocodilians
• Lizards
• Snakes
• The tuatara, the only surviving rhynchocephalian,
  which resembles a large lizard, and is found on
  islands near New Zealand.

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Turtles

• The turtle lineage dates back to the Late
  Permian. Turtles have no teeth. Their jaws are
  covered by a beak that is used to slice through
  plants or animal flesh.

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Crocodilians

• Crocodilians appeared in the Triassic.
• Modern crocodilians include
• Alligator (broad snout)
• Crocodile (narrow snout)
• Gavial (very narrow snout).

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Lizards and Snakes

• The lizards and snakes belong to an order of
  reptiles called the squamates.
• Lizards are the ancestors of snakes.
• Snakes are modified from lizards by the loss of
  limbs, the change of the skull to become more
  flexible to engulf prey, and the addition of more
  vertebrae and ribs.
• Some primitive snakes retain vestigal rear limb
  and pelvic bones, attesting to their tetrapod
  ancestry.

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Snakes

• Snakes began to diversify during the Miocene.
• Poisonous snakes evolved with specialized teeth
  for injecting venom into their prey.
• The diversification of snakes may be linked to
  the diversification of mammals, which serve as
  their prey.
• Fossil snakes are found in rocks as old as Early
  Cretaceous.

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Birds

• Characteristics of birds include
• Lightweight skeleton with thin and hollow bones
• More neck vertebrae than most other animals (13
  to 25).
• Jaws form a toothless horny beak
• Keeled breastbone or sternum for attachment of
  the large flight muscles leading from the breast
  to the wing

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Birds

• Fused collarbone (wishbone)
• Pelvic girdle and vertebrae are fused together to
  provide rigidity during flight
• Fusion of bones of the "hand" to help support the
  wing
• Four chambered heart
• Constant body temperature

36
Birds

• Bird fossils are rarely preserved, so the
  Cenozoic fossil record of birds is poor.
• Birds have undergone extraordinary adaptive
  radiation to produce
• Songbirds
• Forest birds (owls)
• Seagoing birds
• Wading birds
• Flightless aquatic birds (penguins)
• Flightless land birds (ostrich, emu)

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Birds

• The fossil record is better for large flightless
  land birds than for small birds because they have
  more robust skeletons.

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Birds

• Diatryma, a large flightless bird from the Eocene
  of North America, was about 2 m tall and weighted
  about 300 pounds.
• It had massive legs, clawed feet, and a huge
  beak, suggesting that it was a predator.
• Others interpret it as a scavenger or browsing
  herbivore.

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Mammals

• Mammals have the following characteristics
• Warm-blooded
• Hair or fur (insulating body cover)
• Mammary glands
• Differentiated teeth (incisors, canines, molars)

Note the differentiated teeth in the modern
coyote skull, Canis latrans.
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Mammals

• Single bone on either side of jaw. (Reptiles and
  birds have several jaw bones)
• Ear bone-structure is derived from bones of the
  ancestral reptilian jaw
• Seven neck vertebrae in most mammals, except for
  manatee and sloth (low metabolic rates)
• Large braincase compared to other vertebrates
• Secondary palate separating mouth cavity from
  nasal passages, allowing simultaneous breathing
  and feeding (needed for infants to nurse)

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Origin of Mammals

• Mammals originated from an advanced group of
  synapsids called therapsids (mammal-like
  reptiles) that lived in the Permian and Triassic.
  
• Mammals appeared in the Late Triassic.
• After the extinction of the dinosaurs, mammals
  expanded into habitats vacated by the dinosaurs,
  plus additional ones.

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Early Mammals

• The first mammals were small.
• Insulation by hair aided survival by preventing
  heat loss.
• Mammary glands are modified sweat glands. The
  young may have been nourished by secretions from
  glands that preceded the development of true
  mammary glands.
• Tooth patterns show early mammals ate insects.
• Skulls show that smell and hearing were well
  developed, suggesting they were nocturnal.

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Types of Mammals

• Monotremes
• Marsupials
• Placentals
• Insectivores
• Edentates
• Rodents
• Rabbits
• Bats

• Meat-eaters
• Primates
• Ungulates

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Monotremes

• Primitive egg-laying mammals, such as the
  platypus (living in Australia and Tasmania), and
  two species of spiny anteater or echidna (living
  in Australia and New Guinea).
• Milk is secreted from special glands onto hairs
  on the abdomen, where the young can lick it up.

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Marsupials

• Mammals with pouches in which they keep their
  young. Many Australian forms.
• Opossum
• Kangaroos
• Wallabies
• Wombats
• Koalas
• Others

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Marsupials

• Many marsupials developed in South America, and
  resemble placental mammals found in North America
  (including a South American marsupial sabertooth
  cat).
• This is an example of convergent evolution.

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Placentals

• Placental mammals appeared during the Cretaceous
  as small insectivores.

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Insectivores

• Insect-eating mammals such as the moles. The
  descendants of this group include
• Edentates
• Bats
• Primates
• Rodents
• Carnivorous mammals
• Herbivorous mammals
• Marine mammals

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Edentates

• Toothless mammals.
• This group includes the living armadillos, tree
  sloths, and South American anteaters.
• Extinct fossil edentates include the glyptodonts
  and giant ground sloths.

Glyptodont
Armadillo
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Rodents

• The rodents probably outnumber all other mammals.
  They have adapted to many habitats.
• Includes partially aquatic mammals (beaver and
  muskrat), desert-dwelling mammals (jerboas and
  kangaroo rats), and tree-dwelling mammals
  (squirrel).
• Also includes hamsters, gerbils, guinea pigs,
  chipmunks.
• Teeth are specialized for gnawing and nibbling.
  They lack canine teeth and have two upper and
  lower pairs of continuously-growing incisors.

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Rabbits

• Rabbits are not rodents.
• Their teeth are similar to those of rodents, but
  rabbits have two upper pairs of incisors, and
  only one lower pair.
• The tail is reduced.
• The hind legs are strengthened for hopping.

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Bats

• Flying mammals, the bats evolved during the
  Cenozoic.
• Bat teeth have been discovered in Paleocene
  strata.
• The wings are developed on elongated fingerbones.

Rock made of bat bones
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Meat-eaters

• The earliest meat-eating placental mammals are
  Late Cretaceous in age.
• Creodonts - Extinct small-brained animals with
  short limbs and claws. Dominant meat-eating
  mammals in the Paleocene.
• Carnivores - Cats, hyenas, dogs, wolves,
  raccoons, bears, and weasels.Aquatic carnivores
  include the seals, sea lions, and walruses.
  Larger brains than the creodonts. They replaced
  the creodonts by the Miocene.

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Primates

• Prosimians (lemurs and tarsiers)
• Monkeys, apes, and humans.

Chimpanzee skull human skull
Male gorilla skull
Lemur
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Ungulates

• Mammals with hoofs, including horses, cattle,
  sheep, goats, deer, antelopes, camels, tapirs,
  rhinos, and other animals.
• Also includes the descendants of mammals with
  hoofs, including whales, manatees and dugongs,
  elephants and other animals with trunks
  (proboscideans), such as the extinct mastodons
  and wooly mammoths.

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Ungulates
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Odd-toed Ungulates

• Also called perissodactyls
• An odd number of toes (either 1 or 3 toes) on
  each foot. Reduction of lateral toes.
• Includes modern horses, rhinos, and tapirs, as
  well as extinct chalicotheres, brontotheres, and
  other groups.

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Evolution of the Horse

• The modern horse is an odd-toed ungulate that
  evolved from small (about 40 cm tall) Eocene
  browsing horses with 4 toes on the front feet and
  3 toes on the rear feet.
• The horse changed from a small animal with a
  short skull and low-crowned teeth to a larger
  animal with fewer toes, longer skull, larger
  brain, and complexly-ridged high-crowned teeth
  for chewing grasses.

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Evolution of the Horse

• Summary of the evolution of the horse, showing
  the relative sizes of the skeletons. Left to
  right Pliohippus (10 m.y.a.), Merychippus (25
  m.y.a.), Mesohippus (40 m.y.a.), Hyracotherium
  (55 m.y.a.).

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Other odd-toed ungulates include rhinos and tapirs

• Wooly rhino fossil

Extinct odd-toed ungulates include the large
brontotheres and chalicotheres.
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Even-toed Ungulates

• Also called artiodactyls
• An even number of toes (2 or 4 toes) on each
  foot.
• Those with two toes have cloven hoofs.
• Includes cattle, pigs, deer, hippos, goats,
  sheep, camels, llamas, giraffes, and antelope.
• This group of animals is important to humans
  because it provides meat, milk, and wool.

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Even-toed Ungulates
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Even-toed Ungulates

• Hippos are the only modern amphibious even-toed
  ungulates.

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Even-toed Ungulates

• Some even-toed ungulates are ruminants that have
  multichambered stomachs and chew their cuds, for
  digesting coarse vegetation.
• Ruminants include sheep, cattle, giraffes, and
  deer.
• Extinct even-toed ungulates include the oreodonts
  and entelodonts.

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Proboscidians

• Mammals with a proboscis or trunk, including
  elephants and the extinct mastodons and wooly
  mammoths.

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Cetaceans

• Mammals that have adapted fully to life in the
  sea, such as the whales, porpoises, and dolphins.
  
• They are descended from hoof-bearing land
  dwellers related to the hippo.

Bottle-nosed dolphin skull
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Cetaceans

• Early whales had tiny, vestigial hind legs that
  were too small to be of use in swimming and too
  small to hold the animal up on land.
• The Eocene whale, Pakicetus, is found in
  non-marine shales indicating that they lived in
  lakes, streams, and estuaries
• Later, whales made the transition to the sea.

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Whales with Legs

• Georgiacetus vogtlensis, the Georgia whale
• Eocene, 42 m.y. old.
• Oldest whale skeleton from North America.
• Note the rear legs. The hip bone is not firmly
  anchored to the rest of the skeleton, so it
  probably could not walk on dry land.
• On display at Georgia Southern University,
  Statesboro, GA.

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Whales with Legs

• Fossil whale with vestigial legs (note femur and
  pelvis). Ambulocetus from the Eocene of Pakistan.

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Cenozoic Migrations

• The southern continents (South America,
  Australia, and Antarctica) were separated from
  North America and Eurasia during most of the
  Cenozoic.
• As a result, distinctive assemblages of mammals
  developed on the southern continents, showing
  convergent evolution with northern hemisphere
  species.

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• The development of the Panamanian land bridge
  about 3 m.y. ago (during the Late Pliocene) led
  to the migration of mammals between North and
  South America.

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Panamanian Land Bridge

• Marsupials went northward
• Placentals went southward
• Eventually, the marsupials began to decline.
• All of the hoofed marsupials became extinct.
• Ground sloths and glyptodonts also became
  extinct.
• The land bridge caused many species of South
  American marsupial mammals to go extinct, because
  of migrants from the north.

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Bering Land Bridge

• The Bering land bridge existed between North
  America and Eurasia during the Pleistocene (now
  occupied by the Bering Sea).
• Camels, horses, mammoths, and a wide variety of
  other land mammals migrated across the Bering
  land bridge during the Pleistocene.
• The land bridge was also used by early humans to
  enter North America at least 14,000 years ago.

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Extinction of the Large Pleistocene Mammals

• About 17,000 years ago, during the last
  glaciation, North America supported large numbers
  of many types of large mammals
• Odd-toed ungulates
• Even-toed ungulates
• Giant beavers
• Mammoths
• Mastodons
• Elks
• Huge ground sloths

Extinct Irish elk, Megaloceros
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Extinction of the Large Pleistocene Mammals

• Most of these large land mammals began to become
  extinct around 8000 years ago.
• Why? There are two hypotheses
• Climate change associated with global warming at
  the end of the last Ice Age.
• Human hunting and predation