Reading Essentials for Biology Glencoe (794133), страница 89
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Circlethe distinguishing features ofeach subclass.1. How has gestation helpedmammals to succeed?________________________________________________Scientists place mammals into one of three subclasses based ontheir method of reproduction. The first subclass you will study isplacental mammals. About 90 percent of all mammals are placental. Placental mammals give birth to young that have developedinside the mother’s uterus. Birth occurs when the body systems ofthe young are fully functional and they can live outside theirmother’s body. The uterus (YEWT uh rus) is a hollow, muscularorgan in which offspring develop. The young are nourished insidethe uterus through an organ called the placenta (pluh SEN tuh).The placenta develops during pregnancy.
Nutrients and oxygenpass through the placenta to the developing embryo. Wastes fromthe embryo are removed through the placenta.The time during which placental mammals develop inside theuterus is called gestation (jeh STAY shun). The length of gestation varies from species to species. Developing inside the mother’sbody is an adaptation that has helped mammals succeed. This isbecause the offspring are protected from predators and the environment during the early stages of development.________________________How is reproduction different in marsupials?Marsupials make up the second subclass of mammals. A marsupial (mar SEW pee uhl) is a mammal in which the young havea short period of development within the mother’s body, followedby a period of development inside a pouch made of skin and hairon the outside of the mother’s body.392Chapter 32READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Mammal ClassificationNameDateClassSection32.2Diversity of Mammals, continuedThere is only one North American marsupial, the opossum.Most marsupials are found in Australia and surrounding islands.The theory of plate tectonics explains why most marsupials arefound in Australia.
Scientists have found fossil marsupials on continents that once made up Gondwana (a prehistoric super continent made up of Australia, South America, Africa, India, andAntarctica). These fossils support the idea that marsupials originated in South America, moved across Antarctica, and populatedAustralia before Gondwana broke up.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What effect have placental mammals hadon marsupials?Ancestors of today’s marsupials did not have to share the landmass that became Australia with the competitive placental mammals. Placental mammals evolved in other places.
Marsupials wereable to spread and fill niches similar to those occupied by placental mammals in other parts of the world. For example, the giantanteater of Mexico, a placental mammal, has a long, sticky tonguethat it uses to collect ants and termites from their nests. Thenumbat of Australia, a marsupial, fills the same niche. The numbat has a long, sticky tongue that it uses to eat termites and ants.Since the introduction of placental mammals, such as sheep andrabbits, to Australia, many native marsupial species have becomethreatened, endangered, or even extinct.
Remember that endangered means that the number of individuals falls so low thatextinction is possible.Which mammals lay eggs?A mammal that reproduces by laying eggs is called a monotreme(MA nuh treem). Only three species of monotremes are alivetoday. Monotremes are found only in Australia, Tasmania, andNew Guinea.The platypus is a mostly aquatic monotreme with a broad, flattail much like that of a beaver. Its rubbery snout looks like the billof a duck. The platypus has webbed front feet for swimming.
Allfour feet have sharp claws for digging and burrowing. Much ofthe body is covered with thick, brown hair. Like all mammals, theplatypus has mammary glands.There are two species of spiny anteaters, or echidnas, in themonotreme subclass. The spiny anteater has coarse, brown hair.Its back and sides are covered with sharp spines that it can raiseto defend itself when threatened by enemies. From its mouth, theanteater extends its long, sticky tongue to catch insects.READING ESSENTIALS2.
Infer How does the presence of placental mammals threaten marsupialsin Australia?________________________________________________________________________3. Where are monotremesfound?________________________________________________________________________Chapter 32393NameDateClassSection32.2Diversity of Mammals, continuedArtiodactylsCetaceans220 species79 speciesCarnivores270 speciesRodents2000 speciesPrimates230 speciesChiropterans925 speciesPerissodactyls18 speciesInsectivores375 speciesProboscidsPlacentalmammals3 speciesTherapsidsmammal-likereptilesMarsupials280 speciesReptilesAmphibiansFishesMonotremesInvertebrates3 speciesSpecies numbers are approximate and subject to change pending discoveries or extinctions.Origins of Mammals4.
Why do scientists suggestthat therapsids are theancestors of mammals?________________________________________________________________________394Chapter 32The first placental mammals appeared in the fossil recordabout 125 million years ago. The oldest placental mammal fossilis Eomaia, a group of mouse-sized animals. Scientists trace theorigins of placental mammals to a group of reptilian ancestorscalled therapsids. Therapsids (ther AP sidz) had features of bothreptiles and mammals. They existed between 270 and 180 millionyears ago.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.4120 speciesNameDateClassSection32.2Diversity of Mammals, continuedThe mass extinction of the dinosaurs at the end of the MesozoicEra, the breaking apart of Pangaea (all the continents formed asingle land mass), and changes in climate opened up new nichesfor early mammals to fill.
Flowering plants appeared at the end ofthis era. They provided new living areas, food sources, and shelter. Some mammals that moved into the drier grasslands becamefast-running grazers, browsers, and predators. The Cenozoic Erais sometimes called the golden age of mammals because of thedramatic increase in their numbers and diversity.5.
How did the development of flowering plantsbenefit mammals?________________________________________________________________________◗ After You ReadMini Glossarygestation (jeh STAY shun): time during whichplacental animals develop inside the uterusCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.marsupial (mar SEW pee uhl): subclass of mammals in which young develop for a shortperiod in the uterus and complete theirdevelopment outside of the mother’s bodyinside a pouch made of skin and hairmonotreme (MAHN uh treem): subclass of mammals that have hair and mammary glandsbut reproduce by laying eggsplacenta (pluh SEN tuh): organ that provides food and oxygen to and removeswastes from young inside the uterus ofplacental mammalsplacental mammal: mammals that give birth toyoung that have developed inside themother’s uterus until their body systems arefully functional and they can live independently of their mother’s bodytherapsids (ther AP sidz): reptilian ancestors ofmammals that had features of both reptilesand mammalsuterus (YEWT uh rus): in females, the hollow,muscular organ in which offspring of placental mammals develop1.
Read the terms and their definitions in the Mini Glossary above. Circle the subclass of mammals that has the fewest species. Highlight the subclass of mammals that contains the mostmammals. Then use the term gestation in a sentence on the lines below.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________READING ESSENTIALSChapter 32395NameDateClassSection32.2Diversity of Mammals, continued2.
Use the diagram to compare and contrast the three subclasses of mammals. List characteristics of each group in the appropriate box.MarsupialAll mammalsMonotremePlacental mammal396Chapter 32Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Visit the Glencoe Science Web site at science.glencoe.com to findyour biology book and learn more about the diversity of mammals.READING ESSENTIALSNameDateClassSection33.1 Innate BehaviorSC.F.1.4.7 The student knows that organisms respond to internal and external stimuli.◗ Before You ReadYou may live in an area where you can observe animals migrating. Have you seen flocks of birds flying overhead in spring or fall? Perhaps you have seen salmon returning upstream.
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