The Earth Through Time, 10th Edition
by Harold L. Levin
CHAPTER 16—LIFE OF THE CENOZOIC
CHAPTER OUTLINE FOR TEACHING
I.
Overview of Cenozoic: “age of mammals”
A. Continental Breakup: stimulated biological diversity and radiations
1. Separated landmasses
2. Isolated marine basins
B. Groups that Flourished
1. Marine: bivalves, gastropods, crustaceans, echinoids, bony fishes,
marine mammals
2. Terrestrial: mammals (rodents, bats, carnivores, elephants, bison,
camels, horses, rabbits)
C. Southern Continents (South America, Australia, Antarctica) Split from North
America and Eurasia
D. Key Migrations due to Land Bridges
1. Isthmus of Panama: extinction of South American marsupials
2. Bering Land Bridge: camels and horses reintroduced to North America
E. Primate Evolution
1. Neogene: human-ancestor primates
2. Pleistocene: Homo sapiens appeared
F. Mass Extinction
1. Between 50,000 and 10,000 years ago
2. Involved loss of 2/3 of large land animals
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II. Plant Evolution and Link to Mammalian Evolution
A. Pre-Cenozoic (Middle Cretaceous) “Explosion” of Plants
1.
2.
3.
4.
Angiosperms appeared in Mid-Cretaceous
Rapidly spread and dominated floras
Miocene was time of spread of grasses
Gradual evolution toward today’s flora
B. Link to Mammalian Evolution
1. Miocene grasslands led to proliferation of foraging and grazing
mammals (herds)
a. changes in teeth: high-crown cheek teeth that continually
grow
b. length-of-face increase to accommodate teeth
c. folding of enamel layers increased grinding surfaces
d. incisors aligned for nipping and chopping grass
e. four-chambered stomach developed for grass digestion
(ungulates)
2. Miocene open plains
a. speedy flight (running) modifications: lengthening of limb and
foot bones
b. redesign foot and limb to reduce rotation and allow fore-aft
motion
c. elevation of ankle for speed (toe running)
d. loss of useless side toes
e. protective hoofs developed
III. Marine Phytoplankton
A. Most Families lost during terminal-Cretaceous Extinction
B. Few Species in each Family Survived into Cenozoic
C. Rapid Diversification of Few Survivors
D. Peaks in Cenozoic Diversity of Phytoplankton
1. Eocene
2. Miocene
E. Decrease in Phytoplankton Diversity: Oligocene
F. Main groups
1. Diatoms
2. Dinoflagellates
3. Coccolithophores
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IV. Invertebrates (no new groups appeared in Cenozoic)
A. Zooplankton: prolific and diverse foraminifera
1. Benthic and planktic forms
2. Remarkably large foraminifers: nummulitic “coin shaped”
foraminifer tests (e.g., fossil tests in the Gizeh Limestone, Egypt)
3. All types useful for extensive chronostratigraphic correlation of
Gulf Coast U.S., California, Venezuela, East Indies, Near East
B. Scleractinian corals: flourished in warm water areas
1. Early Cenozoic solitary corals: shallow-water deposits of Gulf Coast
U.S. and Europe
2. Cenozoic fossil reefs (colonial scleractinian corals)
a. most extensive in Tethyan realm (West Indies, Caribbean, IndoPacific, Panama
b. ubiquitous in Pacific atolls (ring-like reefs about subsiding
volcanic seamounts)
c. common in barrier and fringing reefs built during Pleistocene sea-level rise
C. Mollusks
1. Look like modern forms
2. Bivalves
a. clams
b. oysters
c. mussels
d. scallops
3. Gastropods
4. Cephalopods: relatively sparse fossil record
a. Nautilus (the sole surviving nautiloid)
b. squid, octopi, cuttlefish (rarely preserved)
D. Echinoderms, Bryozoans, Brachiopods, and Arthropods
1. Echinoderms: free-moving types particularly abundant
a. echinoids
b. sea biscuits
c. sand dollars
2. Bryozoans: especially common in Cenozoic rocks
3. Brachiopods: decline in abundance and diversity
a. fewer than 60 genera today
b. main groups: terebratiulids and rhynehonellids
c. inarticulate form: Lingula (living since Cambrian)
4. Crustaceans: modern forms evolved (including freshwater)
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E. Insects: over 5000 fossil species
a. Green River Formation
b. Florissant beds
V. Non-Mammalian Vertebrates
A. Fish
1. Bony fish (teleosts): fresh and saltwater
a. achieved enormous range of adaptive radiation
b. various forms included: perches, bass, snappers, seahorses,
sailfish, barracudas, swordfish, flounders, etc.
c. sites of well preserved teleosts: Green River Fm. (WY), Monte
Bolca (Italy)
2. Cartilaginous fish (sharks, etc.)
a. common as today
B. 12-m sharks
B. Amphibians
1. Small-body and smooth skin as today
2. Abundant forms: frogs, toads, salamanders
C. Reptiles
1. Sphenadon, the tuatara “lizard” of New Zealand
2. Turtles
a. shell adaptations: rib expansion to form carapace, basal body
covering (plastron), horny sheath about shell
b. toothless jaws (beaks) for slicing
3. Squamates: lizards and snakes
a. most varied of living reptiles
b. lizards gave rise to snakes
c. snake modifications: skull flexibility for swallowing prey,
multiplication of ribs
d. Miocene: diversification of snakes in general and evolution of
poisonous snakes (specialized teeth = fangs)
e. Poison type may be related to evolution of mammalian prey
(neurotoxins versus hemotoxins)
4. Crocodilians: 3 groups according to snout width
a. alligators (broad snout)
b. crocodiles (narrow snout)
c. gavials (very narrow snout)
d. caimans
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D. Birds
1. Basic skeletal structure constant over Cenozoic
a. fusion of “hand” bones as wing support
b. sternum keel for muscle support of flight
c. fusion of pelvic girdle and vertebra (rigidity in flight)
d. body covered with feathers
e. light, porous bones
f. toothless horny beak
g. four-chambered heart
h. constant body temperature (endothermy)
2. Families of birds
a. song birds (e.g., robins)
b. upland birds (e.g., pheasants)
c. forest birds (e.g., owls)
d. oceanic birds (e.g., albatrosses)
e. wading birds (e.g., plovers)
f. flightless aquatic birds (e.g., penguins)
g. flightless land birds (e.g., ostriches)
3. Best fossil lineage preserved: flightless land birds
a. Diatryma: extinct during Eocene, 3 m tall, New Zealand
b. Andalgalornis: extinct South American predatory bird
c. moas: exterminated by Maori people (New Zealand)
d. dodos: exterminated by sailors in 1700 (Mauritius)
e. extant (living) forms: African ostrich, S. American rhea,
Australian cassowaries, emus
VI. Mammals
A. Mammalian Characteristics (placental and non-placental)
1. Non-fossilized: mammary glands, hair covering
2. Fossilized: lower jaw of a single dentary bone; 3 bones of inner ear
(not commonly preserved); 7 cervical vertebra (neck); expanded brain
case; diverse teeth; separate oral and nasal passages (palate)
3. Differences with reptiles and birds
a. reptile and bird jaws have several bones
b. two of 3 inner ear bones (incus and malleus) are modified from
reptilian jaw bones
c. seven neck bones
d. large braincase
e. secondary palate
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B. Earliest Mammals
1.
2.
3.
4.
5.
Small, hard to find fossils at the outset
Evolved from mammal-like reptiles
Cusp pattern of teeth is indicative of diet
Brain enlargement relative to smell
Benefits of being mammal
a. efficient nervous and reproductive systems
b. reliable body temperature
c. larger brains
d. relatively high levels of intelligence
C. Mammal Groups
1. Monotremes: “egg layers”
a. the most primitive mammals
b. platypus (Australia, Tasmania)
c. mix of reptile and mammal genetic traits
2. Marsupials: nurture their young in pouch (marsupium)
a. Australia: kangaroo, wallabies, wombats, phalangers, bandicoots, koalas
b. Elsewhere: opossum
c. decimated by opening of Panamanian land bridge
VII. Placental Mammals
A. Insectivores: ancestral order of placentals
1. moles, shrews, and hedgehogs: modern members
2. descendants of order: edentates, bats, primates, rodents, carnivores,
herbivores, marine forms
B. Edentates
1. Extant (Paleocene-present) forms: armadillos, tree sloths, anteaters
2. Extinct in Pleistocene
a. Glyptodon (edentate with spiked knob on tail)
b. Megatherium (ground sloth)
C. Rodents
1. Characteristics of group
a. most diverse mammalian group today
b. possess continuously growing incisors in both jaws
c. outer surface of incisor is hard enamel
d. sharpened teeth (chisel-edge) due to chewing
e. diastema is gap between incisors and cheek teeth
f. taxonomy based on disposition of muscles associated with jaw
movement
2. Ancestral group: protorogomorphs
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a. ancestors of true squirrels and old-world porcupines
b. Paramys: Paleocene squirrel-like form
3. Ancestral beavers (castorimorphs): example is Paleocastor (Pleistocene form that
made corkscrew burrows)
4. Sciuromorphs: modern squirrels, chipmunks, and marmots
5. Myomorphs: hamsters, mice, rats
D. Lagomorpha (gnawers and nibblers): includes rabbits, hares, pikas
E. Bats: true flying mammals
1. Adaptations
a. elongate finger bones to support skin of wing
b. echo-location
2. Diet-based groups
a. insect-eating
b. wounding (vampire)
c. fruit-eating
d. nectar-licking
F. Terrestrial Carnivores
1. Common ancestral from: Cimolestes (a small weasel-like animal with
carnassial teeth)
2. Cretaceous-Paleocene division into two orders
a. Order Credonta: small-brained animals with short limbs, clawed
toes, and long tails
b. Order Carnivora: large-brained animals with carnissial teeth
near front jaw
3. Miocene-Holocene expansion: carnivora replace credonta resulting in
expansion of carnivora
4. Carnivore diversification: bears, raccoons, weasels, genets, hyenas,
dogs, cats, civets, mongooses, and true hyenas
5. Cat record
a. Pleistocene stabbing cat: Smilodon
b. Pleistocene-Holocene biting cats: leopard, puma
6. Dog record: Canis (dire wolf); Pleistocene-Holocene
7. Carnivore-Herbivore ecologic balance
a. competition as predator/prey for speed and agility
b. defensive adaptation by herbivores
c. carnivore effect: cull out weak, sick, deformed, counteract
degenerative mutation, and overpopulation
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G. Marine carnivores: seals, sea lions, walruses
H. Ungulates: hoofed herbivores or herbivores whose ancestors were hoofed
1. Groups whose ancestors were ungulates
a. whales
b. proboscideans (mastodons and elephants)
c. arsinotheres
2. Primitive, earliest unglates (condylarths)
a. widespread by late Paleocene-Eocene
b. Phenacodus: primitive cheek teeth; shorter two outer toes; inner
three toes longer; plantigrade stance
c. Uintatherium: six horns; canine teeth in males; plantigrade stance
3. Perissodactyls: modern odd-toed (one- or three-toed) ungulates
a. range: late Paleocene-present; Miocene peak
b. site of origin: North America
c. ancestral lineage: condylarths
d. common modern forms: horse; tapir; rhinoceroses
e. extinct forms: chalicotheres; titanotheres; hyracodonts;
amyndonts; rhinoceratids
f. distinctive characteristics: odd number of toes per foot; weight
support on middle toe; lateral toes smaller (or missing as in
modern horse); run on toes (digigrade stance)
g. oldest form: Radinskya (Paleocene, China)
h. Mesohippus (Oligocene): intermediate form with large size but
low-crowned teeth; less abrasive diet
i. Miocene trends in horse evolution: progressive size increase; leg
length increase; tooth crown height increase; brain size increase;
straightening of back; strengthening of middle toe; lateral toes
reduced; pre-molars changed to molars; increased complexity of
enamel tooth ridges
j. Miocene radiation due to spread of grassland prairie, but
conservative forms remained in forested areas
k. Merychippus (Miocene): vanguard grassland horse; single
middle toe support; reduced side toes; head deeper; face longer;
long cheek teeth
l. Hipparions (late Miocene-early Pleistocene): three-toed horses
that migrated from North America to Europe
m. Pliohippus (Pliocene): one-toed horse with lateral digits as
splints
n. Equus (Pliocene-present): non-migrating ancestor of
Merychippus that populated North America
o. Equus occidentalis and Equus giganteus (Pleistocene): small
and large horses, respectively, that spread over North America,
Eurasia, and Africa
p. Holocene extinction of Equus in North America: last horses died
few thousand years ago due to disease or over-killing by humans;
horses reintroduced by Spanish in 1500’s
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4.
5.
6.
7.
q. tapirs (Oligocene-present): primitive feet (4 toes front, 3 in
rear); low-crowned teeth; forest dwellers, leaf eaters
r. rhinoceroses (Eocene-present): primitive forms (e.g.,
Paraceratherium, 5 m tall) evolved into three groups; by end of
the “ice age” all but 5 species of rhinoceroses were extinct
s. brontotheres (Eocene-Pleistocene): pair of horns over snout;
large body size
t. chalicotheres (Eocene-Pleistocene): 3 claws (no hoofs) on feet;
horse-like; fore legs longer than rear; back slopes
Artiodactyls: even-toed (two- or four-toed) ungulates
a. range: early Eocene-Holocene
b. site of origin: Eurasia and Africa, except North American camels
c. ancestral lineage: condylarths
d. modern forms: pigs, deer, hippopotamuses, goats, sheep,
cattle, camels, peccaries, and antelope
e. extinct forms: oredonts and entelodonts
f. distinct characteristics: even number of toes on each foot (4 or
2); weight carried on middle toes (clover = split hoof); molar
and premolar are not alike; ankle and leg bones form a doublepulley system
g. success of group: most varied, abundant, numerous, and
superior of all ungulates
h. older extinct forms: oredonts (short, stocky grazers of North
American grassy plains) and entelodonts (hog-like form with
bony processes on skull and jaws)
i. Eocene trends in evolutionary radiation produced three major
groups: swine and hippopotamuses; camels; ruminants
j. swine and hippopotami: most primitive group; four toes; weight
on middle digits; swine includes pigs and peccaries;
hippopotamuses are only amphibians forms (descended from
Miocene anthracotheres)
k. camels and llamas: Eocene small common ancestor; Oligocene forms lost side
toes and gained leg and neck length; Miocene Oxydactylus (leaf browser like
giraffe); Pleistocene migration from North America (llamas to South America and
camels to Eurasia and Africa); Holocene extinction in North America (reason
unknown)
Ruminants (from rumen, the first of 4 stomach chambers): most varied
and abundant artiodactyls; two main groups, tragulids and pecorans
Bovid ruminants: Miocene common ancestors of cattle,
bison, sheep, goats; Pleistocene evolution of 7 bison species
in North America
Elephants and kin
a. proboscidean ruminants: Eocene ancestral forms
Barytherium and Moeritherium (tapir-like, Egypt);
Miocene-Pliocene split into two lines — dinotheres (tusked)
and gomphotheres (trunked forms such as mastodons,
elephants, and mammoths)
b. dinotheres (Miocene-Pliocene): proboscidean ruminants
with tusk on lower jaws; tusks curved down and back
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c.
gomphotheres (Miocene-present): proboscidean
ruminants with tusk on upper jaw; tusk curved upward and
back; Palaeomastadon (Oligocene, north Africa;
Gompotherium (Miocene ancestral mammoth, Eurasia and
North America); Trilophodon (Pliocene, “long jaw” mammoth);
Amebelodon (Pliocene, “shovel tusk” mammoth);
8. Mammoths (Ice Age Elephants): Mammut (Pleistocene, North American mammoth);
woolly mammoth extinct during Holocene
I.
Cetaceans (whales and porpoises)
1. Earliest whales (Eocene)
a. descended from terrestrial hoofed land-dwellers (artiodactyls)
b. living relatives include: cows, goats, sheep, deer, giraffes
c. these whales were able to walk and swim
d. Pakicetus (Eocene): few meters long; inhabited streams, lakes,
estuaries; ankle bone structure shows link to artiodactyls
e. Ambulocetus (Eocene): 5 m long; hind webbed feet; front
flippers
f. Basilosaurus (late Eocene): 20 m long; front flippers; useless
hind limbs
g. Eocene trend in evolution: increasing body length and
diminution of hind limbs
2. Modern whales: descendents of Basilosaurus (2 lineages)
a. toothed whale lineage (Oligocene-present): porpoises, killer
whales, sperm whales
b. whale-bone lineage (Miocene-present): titanic blue whale, right whale, Greenland
whale; plankton-feeders; toothless with ridges of hairy, hard skin from roof of
mouth; great size attained by blue whales (30 m, 3 tons)
VIII.
Pleistocene Extinctions of Large Land Animals
A. Overview
1. Maximum continental glaciation = 17,000 years ago
2. Northern hemisphere supported maximum variety and diversity of
large land animals 17,000 years ago
3. Rapid decline of populations 8,000 years ago
a. many groups became extinct
b. many groups severely reduced
B. Causes of terminal-Pleistocene Extinctions (8,000 years ago): two theories
1. Human overkill
2. Climatically controlled environmental changes (global warming)
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C. Human Overkill Theory
1.
2.
3.
4.
5.
Developed ability to hunt in social groups
Kill in excess of need (e.g., cliff slaughter)
Hunters selected best animals in herds
Predatory animals suffered due to reduced prey
Support for theory
a. large terrestrial animals only
b. marine genera not affected
c. small animals not so affected
d. extinction began in Africa where human populations were
greater
6. Criticism of Theory
a. human populations too small, nomadic
b. some non-hunted species became extinct
c. primitive hunters today do not endanger herds by overkills
D. Environmental Change Theory
1. Greatest variety and diversity during glacial intervals of Pleistocene
2. Glacial intervals averaged 100,000 years duration; interglacial stages averaged
10,000 years
3. Greater adaptation to cooler conditions versus relatively short warm
intervals
4. Curtailment of reproductive capacity in warmer climates due to
surface/volume ratio of large female animals (increased blood flow to
cooling surface versus uterus and embryo)
5. Evolutionary selection for smaller animals thus explained; explains large animal
species loss
Answers to Discussion Questions
1. Corals grow in very shallow water. Corals cannot grow out of the water, so sea level restricts
vertical growth. Scleractinian (hermatypic) corals require shallow water as they have a
symbiotic relationship with algae that live within the coral polyp and are dependent upon
getting enough sunlight to live. Atolls form when fringing reefs encircle a volcanic island. As
the island slowly sinks, the surrounding reefs grow upward keeping pace with subsidence.
Eventually, a ring of reefs called an atoll is left and no island is visible in the center.
2. Diatoms, dinoflagellates, and coccolithophores were three phytoplankton groups that
proliferated during Cenozoic.
3. Cenozoic cephalopods were much less abundant and diverse than during Mesozoic. Only
one shelled form, the Nautilus. There were shell-less forms such as squid, octopi, and
cuttlefish.
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4. The teleosts are bony fishes which were common during Cenozoic. Freshwater forms are
most characteristic of Cenozoic deposits.
5. In Pakicetus (Eocene, Pakistan), the ankle bone structure is like that of the even-toed
unglates. DNA sequencing studies, still controversial, tend to support a close genetic link as
well.
6. Mammalian teeth are diversified according to function and thus not all identical as in most
reptiles. Mammalian teeth include incisors (in front), canines (dagger-like teeth on each
side), carnassials (shearing teeth on each side), and molars (crushing teeth on each side in
the back of jaw).
7. Ruminants are the most varied and abundant modern group of artiodactyls, a category of
hoofed mammals. Ruminants had four stomach compartments and chewed to more
completely digest plant matter and these fully extract nutrients and allow bacterial action in
digestion.
8. Trunk development required progressive relocation of the external nasal opening, which
started out at the front of the skull and receded to the rear of the skull.
9. Even-toed ungulates, called artiodactyls, have been more successful than the perissodactyls
in terms of survival, variety, and abundances. Still-living artiodactyls include pigs, peccaries,
deer, hippopotamuses, goats, sheep, camels, llamas, giraffes, and many varieties of cattle
and antelope.
10. From the development of the earliest horse, Hyracotherium, during late Paleocene-Eocene to
the appearance of Equus during Pliocene, horses changed progressively by increasing in
body size from that of a fox, increasing leg length, heightening the tooth crown, increasing
brain size, straightening of the back, strengthening of middle toe for load bearing, reducing
lateral toes, changing pre-molars to molars, and increasing complexity of enamel ridges on
teeth.
11. Cenozoic herbivores adapted to the spread of prairies (i.e., grasslands and open plains
versus forests) by diversifying into many modern groups of foraging and grazing herd
animals. Specific changes due to silica content of grass and siliceous dust on grass includes:
1) changes in dentition (especially high-crowned cheek teeth that continually grow; 2)
increased length-of-face beyond the eyes to accommodate the teeth; 3) folded enamel layers
to increase grinding surfaces; 4) aligned incisors for nipping and chopping grass; and 5)
development of four-chambered stomach for grass digestion (in the ungulates).
12. Among whales in general (both toothed and whale-bone lineages), the main non-reptilian
characteristics are the great body size and limb-like structure of the fins. Among the toothed
whales, the teeth are different from Mesozoic ichthyosaurs. Among the whale-bone whales,
their plankton-feeding (toothless) mouth structure is very much non-reptilian.
13. a
14. e
15. a
16. d
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Chapter Activities
Student activities for in-depth learning.
1. View one of the animations about the Bearing Land Bridge that can be seen on-line at
http://instaar.colorado.edu/qgisl/bering_land_bridge/. Based on what you see in the animation
and what is on-line at this site and other web pages related to this topic, describe the importance
of the land bridge in the migration of animals, including early people, to the Americas. When did
all this occur and how do we know about the timing of these migrations?
2. Use the web resources at the University of California, Berkeley, Museum of Paleontology that
are related to large-size Pleistocene mammals (e.g., http://www.ucmp.berkeley.edu/mammal/
mammoth/about_mammoths.html),
compare
mammoths
(Mammut)
and
mastodons
(Mammuthus). How can you tell them apart based on fossil evidence? Compare photos of teeth
(linked to this museum page) and explain.
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