Download A Closer Look at Amniotes

26
CHAPTER
A Closer Look
at Amniotes
GETTING READY TO LEARN
Preview Key Concepts
26.1 Amniotes
Reptiles, birds, and mammals are amniotes
26.2 Reptiles
Reptiles were the first amniotes.
26.3 Birds
Birds have many adaptations for flight.
26.4 Mammals
Evolutionary adaptations allowed mammals to succeed dinosaurs as a
dominant terrestrial vertebrate.
Review Academic Vocabulary
Write the correct word for each definition.
endoskeleton
chordate
tetrapod
amniotic egg
placenta
1.
: waterproof container that keeps embryo moist
2.
: vertebrates with four legs
3.
: membrane that nourishes an embryo
4.
: support that is inside the body
5.
: animal with notochord, tail, gills, hollow nerve
cord
Preview Biology Vocabulary
Two key terms from this chapter share the same word part. Read the
definitions and guess what the word part means.
428
TERM
DEFINITION
ectotherm
animal whose body temperature depends on the
surrounding environment
endotherm
animal that can maintain
its own body temperature
WHAT I THINK -therm
MEANS
Student text pages
788–792
SECTION
26.1 Amniotes
KEY CONCEPT Reptiles, birds, and mammals are amniotes.
Before you were born, you grew and developed inside a fluid-filled
membrane, or amniotic sac. There are many different types of amniote
animals, but each of them, like you, begins inside an amniotic sac.
Amniote embryos develop in a fluid-filled sac.
All reptiles, birds, and mammals are amniotes, which means they
develop inside a sac made of layers of membranes. In some amniotes,
the sac is inside the mother’s body. In others, the tough outer shell of an
egg protects the embryo as it develops outside of the mother’s body. This
shell is semipermeable. It allows oxygen and carbon dioxide to pass
through but holds water inside, so the embryo will not dry out.
Inside the shell of an amniotic egg, four membranes protect and
nourish the embryo. The amnion surrounds and cushions the embryo
from changes in temperature or physical blows. The chorion allows gas
exchange with the outside environment. The yolk sac contains the
nutrient supply. The allantois holds waste materials from the embryo.
Visual Connection
See amniotic egg in
student text, pg. 788
How does an embryo get oxygen while it is inside an egg?
Anatomy and circulation differ among amniotes.
Over time, amniotes evolved many different body shapes and sizes. This
led to differences in anatomy and blood circulation
among species.
Anatomy The first amniotes moved like a lizard. A
lizard’s legs stick out from the sides of its body. The
elbows and knees are bent as it walks. Muscles around
the ribs move the body forward, but the contractions
make the body sway from side to side. These muscles
also inflate the lungs. Animals that move like this
cannot run and breathe at the same time.
Amniotes such as mammals, dinosaurs, and birds
evolved a more upright stance. A cat, for example, has
straighter limbs set underneath its body. When it
walks, its legs swing back and forth like a pendulum.*
* ACADEMIC VOCABULARY
The lizard has a sprawling walk. The cat walks
upright—a feature that enables the cat to
move and breathe at the same time.
pendulum an object that swings back and forth in a straight line
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The cat’s body does not sway from side to side. This type
of movement uses less energy and enables the animal to
breathe as it walks.
Circulation As amniotes evolved, their bodies required
more energy for movement and growth. To get this
energy, their tissues needed efficient ways of receiving
oxygen. This led to the development of different types of
circulatory systems. All amniotes have a centralized heart
and two circuits of blood vessels. Separate circuits allow
amniotes to conserve energy. The two circuits are the
pulmonary and systemic circuits.
• The pulmonary circuit moves oxygen-poor blood
from the heart to the lungs, and then moves oxygenrich blood from the lungs back to the heart.
• The systemic circuit moves oxygen-rich blood from
the heart to the rest of the body. It then moves oxygenpoor blood back to the heart where it can be pumped
to the lungs.
Like amphibians, reptiles have a three-chambered
heart with two atria and one ventricle. One atrium collects oxygen-poor blood from the body. The other collects
oxygen-rich blood from the lungs. Both atria send blood
into the ventricle, which pumps blood into the pulmonary and systemic circuits. The single ventricle can keep
blood away from the lungs when the animal needs to stop
breathing as it runs or swims.
Mammals and birds have a four-chambered heart with
two atria and two ventricles. These chambers keep oxygenpoor and oxygen-rich blood separate. As a result, more
oxygen-rich blood goes to the tissues. This adaptation gave
mammals and birds a large and constant supply of oxygen
for their energy needs. Eventually, this type of heart also
gave them more control over their body temperature.
AMNIOTE HEARTS
Oxygen-poor blood
Oxygen-rich blood
Three-Chambered Heart
Four-Chambered Heart
Reptiles have three-chambered hearts.
Birds and mammals have four-chambered
hearts that keep oxygen-rich and oxygenpoor blood completely separate.
How does a four-chambered heart provide more energy
for tissues?
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Amniotes can be ectothermic or endothermic.
Like all organisms, amniotes are more active when they are warm. The
term ectotherms refers to organisms whose body temperatures are
determined by their surrounding environment. Ectotherms can adjust
their body temperature by changing their external environment. For
example, many reptiles lie in the sun to warm up and move into the
shade to cool down.
Endotherms are organisms that can increase or decrease their own
metabolism to keep themselves warm or cool. When they are cold, they
shiver by rapidly contracting their muscles to make more heat. Most
endotherms also have hair, fat cells, or feathers to prevent heat loss. If
endotherms get too warm, they cool off by sweating and panting.
Endotherms and ectotherms have two different ways of managing
energy use. Endotherms can be active in all types of temperatures, but
they need more energy to keep their tissues warm. As a result, they have
to eat more. Ectotherms are less active when it is cold, but they can
survive on less food than can endotherms.
Underline three ways in which an endotherm maintains a
stable body temperature.
26.1
Vocabulary Check
pulmonary circuit
systemic circuit
ectotherm
endotherm
Mark It Up
Go back and highlight
each sentence that
has a vocabulary
word in bold.
Fill in the blanks with the correct term from the list above.
1. An organism whose body temperature depends on the environment is
an
.
2. Oxygen-poor blood moves back to the heart through the
.
3. An
26.1
can be active in a cold or warm environment.
The Big Picture
4. What are the four membranes in an amniotic egg?
5. Why can’t reptiles run and breathe at the same time?
6. How does a mammal’s heart compare to a reptile’s heart?
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SECTION
26.2 Reptiles
Student
text pages
@E;@8E8JK8E;8I;J
793–797
B.8.3
KEY CONCEPT Reptiles were the first amniotes.
The eastern water lizard may look slow, but it has a top speed of
20 kilometers per hour and strong jaws filled with sharp teeth. It may
never compare to the crocodile as a threat to humans, but it hunts, kills,
and eats in the same way. What makes reptiles unique?
Reptiles are a diverse group of amniotes.
About 200 million years ago, many of Earth’s plant and animal species
became extinct. One group of organisms that survived—the reptiles—
have thrived for millions of years. Reptiles are ectotherms that are
covered with dry scales or plates and reproduce by laying amniotic eggs
covered with a tough outer shell.
An egg allows a young reptile to develop fully before it is born. There
are two ways that reptile eggs develop.
• Oviparous reptiles put their eggs into a nest, and the eggs develop
completely outside of the adult reptile’s body.
• Viviparous reptiles hold the eggs inside of their body while the
young develop. These reptiles give birth to live offspring.
Reptiles have evolved a wide variety of shapes, sizes, and features.
Some reptiles, such as snakes, have no legs and move by rippling their
muscles. Others run swiftly on land or spend most of their time in the
water. Still others, like the turtles, carry their homes on their backs.
In spite of these differences, all reptiles share a few similar features.
All living reptiles are ectotherms. Recall that an ectotherm’s body temperature changes based on the surrounding environment. As a result,
many reptiles spend a lot of time lying in the sun to absorb heat. Their
dry scales or plates help absorb and retain the heat they need.
Underline two ways in which reptile eggs develop.
Reptiles have been evolving for millions of years.
Reptiles became the dominant vertebrate during the Mesozoic era,
which lasted from about 238 million years ago to 65 million years ago.
Synapsids, Anapsids, and Diapsids
Over time, reptile amniotes evolved into three different groups.
• Synapsids had one hole on each side of the skull. Eventually, they
gave rise to modern mammals.
• Anapsids have skulls without any holes on the sides. The anapsids of
today are turtles and tortoises.
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McDougal Littell Biology
VOCABULARY
The name reptile comes
from the Latin word
reptilis, which means
“creeping.”
• Diapsids have two holes on each side of the skull, one above the
other. Diapsid skulls came about as reptiles began to move onto land.
For the next 200 million years, diapsid reptiles ruled Earth. This
group gave rise to many of the modern reptiles and birds of today.
Skull holes may have started out as weight-reducing adaptations. Less
bone would have made the skull lighter, easier to move, and provided
more space for muscle attachment.
pterosaur
Diversity of Extinct Amniotes
• Pelycosaurs were synapsids that included meat-eaters (carnivores)
and plant-eaters (herbivores). Some of their descendants gave rise to
the mammals.
• Ichthyosaurs were some of the first diapsid reptiles. Their long
bodies, flipper-shaped limbs, and dorsal fin were similar to modernday dolphins.
• Plesiosaurs were marine reptiles. They “flew” through the water moving four limbs as flippers. Some had small heads and long necks.
Others had short necks and long heads.
• Dinosaurs were the dominant land vertebrates for 150 million years.
The largest, such as the huge sauropods, ate only plants. The carnivorous theropod dinosaurs gave rise to modern-day birds.
• Pterosaurs were the first vertebrates to evolve powered flight. Fossils
show that pterosaurs may have had hair and were endothermic.
are the
three skull
amniote
skull types?
What are What
the three
amniote
types?
JK8E;8I;J
:?<:B
ichthyosaur
dinosaur
Ancient amniotes appeared
first in the oceans then
moved onto the land.
Eventually some animals
achieved powered flight.
There are four modern groups of reptiles.
Only four groups of reptiles are alive today: turtles, sphenodonts, snakes
and lizards, and crocodilians.
Turtles Turtles, tortoises, and terrapins are the only remaining anapsid
amniotes. The rounded back of a turtle’s shell is the carapace. The
smooth belly part is the plastron. The shell is covered with tough, flattened plates made of keratin that are attached to the rib cage and vertebrae. Turtles are toothless but have sharp, horny beaks. They eat plants
and animals. Terrestrial turtles are called tortoises.
Sphenodonts The only living sphenodonts are two species of tuatara
that live on a few small islands near New Zealand. Tuataras are closely
related to lizards and snakes. They have a diapsid skull and a lightsensitive eyespot in the center of their head.
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Snakes and lizards These reptiles all shed their skins on a regular basis,
have flexible skulls that let them swallow prey larger than their heads,
and have a specialized organ that allows them to “taste” the air. When a
snake or a lizard sticks its forked tongue out of its mouth, the tongue
collects particles from the air. These particles are interpreted by the
specialized organ, called a Jacobson’s organ, found in the top of a reptile’s
mouth. The organ helps the reptile locate prey and avoid predators.
Most lizards are carnivores, but iguanas eat only plants. All snakes are
predators. Some kill by squeezing their prey, while others inject poisons
into their prey through fangs.
Crocodilians These reptiles include alligators, crocodiles, and caimans.
All are semiaquatic* predators that live in swamps and rivers in the
tropics and subtropics. They are ambush* predators. It may surprise you
to know that crocodilians are more closely related to modern-day birds
than they are to lizards and snakes.
Underline the only modern reptiles that are anapsids.
* ACADEMIC VOCABULARY
semiaquatic spends part of the time in the water
ambush lie in wait and then attack
26.2
reptile
Vocabulary Check
viviparous
oviparous
Mark It Up
Go back and
highlight each sentence
that has a vocabulary
word in bold.
1. Circle the reproduction method in which females use a nest.
2. Underline the reproductive method in which females keep their eggs
inside of their bodies.
26.2
The Big Picture
3. Why is it important that a reptile’s scales absorb heat from the sun?
4. What is the difference among synapsid, anapsid, and diapsid skulls?
5. Which extinct amniotes gave rise to some modern-day animals?
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Student text pages
798–803
SECTION
26.3 Birds
KEY CONCEPT Birds have many adaptations for flight.
The birds that you see all around you are actually related to a ferocious
dinosaur known as a Velociraptor. In fact, birds are dinosaurs that
evolved powered flight.
Birds evolved from theropod dinosaurs.
Most paleontologists agree that birds are the descendants of one group
of theropod dinosaurs. Theropods were bipedal, or two-legged, dinosaurs. Most were carnivorous and some, such as Allosaurus and
Tyrannosaurus, were huge. Birds and many theropods share five anatomical features: hollow bones; fused collarbones that form a V-shaped
wishbone, or furcula; rearranged muscles in the hips and legs that
improve bipedal movements; “hands” that have lost their fourth and
fifth fingers; and feathers.
Theropods had feathers, but they did not have wings. They
were running animals. This means that feathers did not
evolve for flight. They may have helped to keep the animals
warm or been used for courtship displays or to scare off
rivals. As birds evolved, they developed feathered wings.
The oldest fossil bird is Archaeopteryx, a chicken-sized
animal that lived about 150 million years ago. It had feathers
and a furcula. Like a reptile, it also had clawed fingers, a tail,
and teeth. Its feathers make it an important link between
The feathered Archaeopteryx is an
flightless dinosaurs, dinosaurs that could fly, and the modern important link between dinosaurs and
birds of today.
modern-day birds.
Underline five shared features of birds
and theropods.
A bird’s body is specialized for flight.
Birds have specialized adaptations for powered flight, including wings; flight muscles; a metabolism that provides a lot of
energy; hollow bones; and reproductive adaptations.
Wings Wings are structures that allow birds to fly. Their
curved shape is similar to an airplane wing. This kind of
surface is called an airfoil. An airfoil is curved down on the
top surface (convex) and curved up on the bottom (concave).
This shape makes air move faster on top than below. The
difference in air speed produces a pressure difference that lifts
the wing up.
VISUAL VOCAB
An airfoil is convex on the top and
concave on the bottom. Differences
in air pressure above and below the
airfoil create lift.
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Muscles A bird’s chest muscles provide the power for flight. In almost
all vertebrates, chest muscles attach to the arms and the breastbone, or
sternum. But a bird’s chest muscles are so big that the sternum has
evolved a larger surface called a keel, or ridge.
Metabolism Flying takes a lot of energy. Maintaining an active
metabolism to produce large amounts of ATP during flight requires a
huge amount of oxygen. To meet this challenge, a bird’s body is filled
with a series of air sacs that connect to the lungs. Air sacs store air as
the bird breathes. Inhaled air travels through the lungs and air sacs in
such a way that oxygen-rich air is always available.
Bone structure Birds have evolved bones that are hollow. Inside bird
bones there is a system of support structures. Many bones are connected to the air sacs, and air fills the cavities in the bones. This
adaptation decreases the bird’s weight and increases the amount of
air in a bird’s body to make flying easier.
Reproductive adaptations The reproductive organs of male and
female birds are active only for a few months during mating season.
The rest of the year, the organs shrink to reduce the bird’s weight.
AIR SAC
trachea
air sac
Air sacs and hollow bones
give birds enough oxygen
for flight and decrease the
bird’s weight.
What are five adaptations that help birds fly?
Birds have spread to many ecological niches.
Modern birds are the descendants of the one dinosaur group that survived when all other dinosaurs became extinct. Over time, this group
diversified into more than 9000 species. Through natural selection, birds
adapted to different habitats and gradually developed different wings,
beaks, and feet.
Differences in wing shape The shape of a bird’s wing reflects the way it
flies. Short, wide wings allow a bird to maneuver easily. Gulls have long,
narrow wings for soaring long distances over water. Hawks, eagles, and
condors have long, broad wings for soaring slowly over land. Many types
of songbirds have short, slightly pointed wings to maneuver through
tight spaces. The wings of flightless birds, such as ostriches, are too small
to lift the bird into the air.
Differences in beak shape The shape of a bird’s beak, or bill, reflects
how it eats. A bird’s beak is a layer of keratin that covers the jaw bones.
The long, spearlike beak of a heron can capture fish. The hooked beak of
an eagle can tear flesh from its prey. Thin, pointed bills can catch insects.
A parrot’s thick beak can crack nuts.
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McDougal Littell Biology
lungs
Differences in foot shape With few exceptions, bird feet have four toes.
Aquatic birds have webbed feet, with skin connecting the toes to form
paddles. Birds of prey, such as eagles, have heavy claws to capture and
kill prey. Woodpeckers have two toes pointing forward and two pointing
backward, so they can cling to vertical tree trunks.
Underline three features that help you know where a bird lives
and how it feeds.
26.3
airfoil
sternum
Vocabulary Check
air sac
Mark It Up
Go back and highlight
each sentence that
has a vocabulary
word in bold.
1. Underline the word that describes a bird’s wing.
2. Circle the word that refers to a place where chest muscle can attach.
3. Put a box around the word that refers to a structure that helps a bird
get more oxygen for flying.
26.3
The Big Picture
4. What are three adaptations that decrease weight and make flight easier?
5. How do scientists know that feathers did not evolve for flight?
6. Turkey vultures can soar for hours over the countryside. What shape
do you think their wings are? Explain your answer.
7. Would you expect to find a large keel on an ostrich? Explain.
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SECTION
26.4 Mammals
Student text pages
805–809
KEY CONCEPT Evolutionary adaptations allowed mammals to succeed
dinosaurs as a dominant terrestrial vertebrate.
You might style your hair with braids, gel, beads, and combs. Even with
no decorations or styling, however, your hair is special. It sets humans
and other mammals apart from reptiles, birds, amphibians, and fish.
What other traits make mammals unique?
All mammals share several common
characteristics.
All mammals are active, large-brained, endothermic animals with
complex social, feeding, and reproductive behaviors. Mammals are the
only group of synapsids alive today. Fossil evidence suggests that early
mammals had long noses and short legs, similar to shrews.
The ability to regulate their own body temperature gave mammals an
advantage over reptiles. Mammals could adapt better to changes in
climate. When the dinosaurs became extinct, mammals took their places
in various habitats. In time, mammals succeeded dinosaurs as a dominant terrestrial life form.
Modern mammals share four characteristics. All
species have hair, mammary glands, a middle ear
containing three bones, and a jaw that lets them chew
their food.
Hair Mammals are furry. Most species are covered
with a layer of hair that helps them stay warm. Each
hair is a long, thin, shaft or stick of dead cells with
keratin that grows out of a follicle in the skin. The hair
traps a layer of air next to the skin to provide insulation, much like a down vest insulates you.
Hair also has other uses. Have you ever seen a
frightened cat fluff up to look bigger? Colored hairs,
such as in a tiger’s stripes, can provide camouflage.
Many mammals also have long, stiff whiskers that
This mammal, a raccoon, has a thick,
collect sensory information.
furry coat of hair that helps it stay
warm in the winter.
Mammary glands All female mammals feed their
babies a fluid called milk. Mammary glands are
specialized glands that produce this milk, which contains water,
sugars, protein, fats, minerals, and antibodies. Some mammals,
such as cats, dogs, and pigs, have a series of glands along the belly
to feed several young at a time.
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McDougal Littell Biology
Middle ear Mammals have three small middle ear bones. One bone,
the stapes, is found in other tetrapods. But the malleus and incus are
unique to mammals. They were derived from reptilian jaw bones.
Sounds collect in the ear canal and vibrate against the eardrum. These
adaptations allow mammals to hear a wider range of sounds to find
prey or avoid predators.
Chewing Adaptations in the jaw enable mammals to chew their food
instead of just biting and swallowing it. A secondary palate* in the
mouth separates the nose and mouth passages so that mammals can
chew and breathe at the same time. Muscles move the jaw from side
to side.
Underline four characteristics that all mammals share.
REPTILIAN EAR BONE
eardrum
inner ear
middle ear
stapes
MAMMALIAN EAR BONES
eardrum
inner ear
malleus
incus
Modern mammals are divided into three
main groups.
More than 4500 species of mammal are alive today. They can be
classified into three groups: monotremes, marsupials, and eutherians.
ear canal
Monotremes
The monotremes are mammals that lay eggs. They have characteristics of both mammals and reptiles. Australia’s duck-billed platypus
and two types of echidna are the only species that survive today.
Monotremes have kept reptilian characteristics such as a sprawling
posture; a single external opening, the cloaca, for their urinary, digestive,
and reproductive tracts; and amniotic eggs with leathery shells that
develop outside the body. Like mammals, they have mammary glands,
but they don’t have nipples. The babies lick milk from pools on the
mother’s belly.
stapes
Unlike reptiles, mammals
have three bones in their
middle ear that help them
detect higher pitched
sounds.
Marsupials
Kangaroos, wombats, koalas, and opossums are a few species of
marsupials. Marsupials are mammals that give birth to very underdeveloped young that must grow inside a marsupium, or pouch. The
embryos spend only a short time inside the mother, attached to a
placenta. Most of them are born a few weeks after fertilization. The
tiny babies then crawl through the mother’s fur until they reach the
pouch. They attach themselves to a nipple and nurse for up to six
months before coming out of the pouch.
* ACADEMIC VOCABULARY
palate the roof of the mouth, it separates the mouth from the nose cavity
The opposum is the only
marsupial in North America.
It can sleep while hanging by
its tail.
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Eutherian Mammals
Eutherian mammals give birth to young that are far more developed. In
most eutherians, the baby is attached to the mother’s uterus by an organ
called the placenta. Through the placenta, the baby receives oxygen and
nutrients, and waste products are removed.
Gestation* in eutherians lasts longer than in most marsupials. In
some species, such as tigers, newborns are nearly helpless and need a lot
of care from the parents. In others, such as deer and horses, the young
are able to run a few hours after being born to avoid predators.
Eutherian mammals are diverse. Whales, manatees, and seals evolved
from land-dwelling mammals. Bats evolved powered flight. And one
group, humans, evolved the ability to think about their ancestors.
Underline the reptilian reproductive traits of monotremes.
* ACADEMIC VOCABULARY
gestation carrying offspring in the uterus
26.4
Vocabulary Check
mammal
marsupial
mammary gland
eutherian
monotreme
Mark It Up
Go back and
highlight each sentence
that has a vocabulary
word in bold.
Choose the correct term from the list above for each description.
1. babies grow to maturity inside a pouch
2. has mammal and reptile characteristics
3. contains milk for developing young
4. young are more completely developed at birth
26.4
The Big Picture
5. Name one difference between a mammal’s jaw and other animal jaws.
6. Describe one similarity and one difference between development of
offspring in marsupials and eutherians.
7. How does the mammalian middle ear help mammals to survive better?
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McDougal Littell Biology
Chapter 26 Review
1. Name the four membranes of an amniotic egg and describe what each
one does.
2. Describe basic amniote circulation.
3. Explain the functions of the pulmonary and the systemic circuits.
4. One possible explanation for skull holes is that they
a. help regulate an animal’s body temperature.
b. strengthen the overall structure of the skull.
c. provide more space for circulation.
d. make a skull lighter and more flexible.
5. Explain the difference between viviparous and oviparous reptiles.
6. What is the evidence that birds descended from theropod dinosaurs?
B.8.3
7. Choose the word that completes the following sentence:
When a bird breathes, inhaled air travels through the lungs and
in such a way that oxygen-rich air is always available.
a. air foils
b. hollow bones
c. keel
d. air sacs
8. How does an airfoil wing shape help a bird to fly?
9. What are four characteristics that mammals share?
10. What is a marsupium used for?
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