Chapter 16Chapter 16

Life of the CenozoicLife of the Cenozoic

The Age of MammalsThe 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.

Appearance of Appearance of Homo sapiensHomo sapiens

• The appearance and evolution of primates led to the ancestors of humans by the Neogene.

• Homo sapiens appeared in the Pleistocene Epoch.

• 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.

Causes of Biologic ChangesCauses 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.

Diversity of Life in the CenozoicDiversity of Life in the Cenozoic

Cenozoic Plant LifeCenozoic 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

Grasslands Expand and Grasslands Expand and Mammals RespondMammals 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.

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

Teeth Adapt to GrassesTeeth 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.

Teeth Adapt to GrassesTeeth 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).

Limbs Adapt to GrasslandsLimbs 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.

Limbs Adapt to GrasslandsLimbs 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

Cenozoic Life in the SeasCenozoic Life in the Seas

Marine PhytoplanktonMarine 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

Marine ZooplanktonMarine Zooplankton

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

Planktonic foraminifera

Significance of ForaminiferaSignificance 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.

Dominant Cenozoic InvertebratesDominant Cenozoic Invertebrates

• Sponges • Scleractinian corals • Bryozoans • Brachiopods• Molluscs

– Bivalves – Gastropods – Cephalopods

• Arthropods – Crustaceans – Insects (on land)

• Echinoderms – Starfish – Echinoids

Corals 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.

Molluscs Molluscs

Cenozoic molluscs are dominated by:

• Bivalves (clams)

• Gastropods (snails)

MolluscsMolluscs

• 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

EchinodermsEchinoderms

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.

Arthropods - CrustaceansArthropods - Crustaceans

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

Arthropods - InsectsArthropods - 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.

Cenozoic VertebratesCenozoic Vertebrates

Cenozoic vertebrates include: – Fishes – Amphibians – Reptiles – Birds – Mammals

FishesFishes

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

SharksSharks• 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.

Amphibians Amphibians

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

• Cenozoic amphibians include: – Frogs – Toads – Salamanders

Reptiles 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.

TurtlesTurtles

• 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.

CrocodiliansCrocodilians • Crocodilians appeared in

the Triassic. • Modern crocodilians

include:– Alligator (broad snout)– Crocodile (narrow

snout)– Gavial (very narrow

snout).

Lizards and SnakesLizards 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.

SnakesSnakes

• 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.

Birds Birds

Characteristics of birds include: 1. Lightweight skeleton with thin and hollow

bones 2. More neck vertebrae than most other animals

(13 to 25). 3. Jaws form a toothless horny beak 4. Keeled breastbone or sternum for attachment

of the large flight muscles leading from the breast to the wing

BirdsBirds

5. Fused collarbone (wishbone)

6. Pelvic girdle and vertebrae are fused together to provide rigidity during flight

7. Fusion of bones of the "hand" to help support the wing

8. Four chambered heart

9. Constant body temperature

BirdsBirds• 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)

BirdsBirds

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

BirdsBirds

• 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.

Mammals Mammals Mammals have the following characteristics:

1. Warm-blooded 2. Hair or fur (insulating body cover) 3. Mammary glands 4. Differentiated teeth (incisors, canines,

molars)

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

MammalsMammals5. Single bone on either side of jaw. (Reptiles and

birds have several jaw bones) 6. Ear bone-structure is derived from bones of the

ancestral reptilian jaw7. Seven neck vertebrae in most mammals, except

for manatee and sloth (low metabolic rates) 8. Large braincase compared to other vertebrates 9. Secondary palate separating mouth cavity from

nasal passages, allowing simultaneous breathing and feeding (needed for infants to nurse)

Origin of MammalsOrigin 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.

Early MammalsEarly 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.

Types of MammalsTypes of Mammals

1. Monotremes

2. Marsupials

3. Placentals a. Insectivores

b. Edentates

c. Rodents

d. Rabbits

e. Bats

f. Meat-eaters

g. Primates

h. Ungulates

MonotremesMonotremes

• 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.

MarsupialsMarsupials

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

MarsupialsMarsupials

• 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.

PlacentalsPlacentals

• Placental mammals appeared during the Cretaceous as small insectivores.

InsectivoresInsectivores

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

EdentatesEdentates

• 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

RodentsRodents• 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.

RabbitsRabbits

• 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.

BatsBats

• 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

Meat-eatersMeat-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.

PrimatesPrimates• Prosimians (lemurs and tarsiers)

• Monkeys, apes, and humans.

Lemur Male gorilla skull Chimpanzee skull &

human skull

UngulatesUngulates

• 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.

UngulatesUngulates

Odd-toed UngulatesOdd-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.

Evolution of the HorseEvolution 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.

Evolution of the HorseEvolution 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.).

Other odd-toed ungulates include rhinos and tapirs

Wooly rhino fossil

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

Even-toed UngulatesEven-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.

Even-toed UngulatesEven-toed Ungulates

Even-toed UngulatesEven-toed Ungulates

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

Even-toed UngulatesEven-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.

Proboscidians Proboscidians

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

Cetaceans 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

CetaceansCetaceans

• 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.

Whales with Whales with LegsLegs

• 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.

Whales with LegsWhales with Legs

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

Cenozoic MigrationsCenozoic 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.

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.

Panamanian Land BridgePanamanian 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.

Bering Land BridgeBering 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.

Extinction of the Large Extinction of the Large Pleistocene MammalsPleistocene 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

Extinction of the Large Extinction of the Large Pleistocene MammalsPleistocene 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