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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 Interactive Reader 429 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? 430 McDougal Littell Biology 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? Interactive Reader 431 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. 432 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. Interactive Reader 433 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? 434 McDougal Littell Biology 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. Interactive Reader 435 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. 436 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. Interactive Reader 437 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. 438 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. Interactive Reader 439 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? 440 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? Interactive Reader 441