26 A Closer Look at Amniotes

Eutherian MammalsAll the mammals most familiar to you are eutherians. Eutherian mammalsgive birth to live young that have completed fetal development. Eutherianmammals are commonly called placental mammals, but this term is misleadingbecause most marsupials also use a placenta during embryo development.In most eutherian development, an embryo is attached to themother’s uterine wall. The connection between the fetus and themother forms an organ called the placenta. Through the placenta,the mother delivers oxygen and nutrients to the embryo andremoves waste products. The placenta only forms during gestationand leaves the mother’s body following birth. The placenta isthe only example of a disposable organ.Eutherian gestation lasts longer than in marsupials—oftenmonths—and the babies are born at a more advanced stage ofdevelopment. In some species, including the Bengal tiger inFIGURE 26.20, newborns are still relatively helpless and need extensiveparental care until they can survive on their own. In others,such as deer or horses, the time shortly after birth is when thenewborn is most vulnerable to predators, so it is important thatthey are able to get up and run within hours of birth.After the extinction of the dinosaurs, eutherians quickly filledvacant ecological niches, and the modern groups of mammalsappeared quickly. Rodents and carnivores appeared about 55million years ago, and the first known species of bat, elephant,manatee, and horse appeared soon afterwards. Modern eutherians includefast carnivores such as cheetahs, and massive herbivores such as elephants.Three groups of aquatic eutherians—whales, manatees, and seals—evolvedfrom land-dwelling mammals. Bats evolved powered flight. And one groupof primates, the humans, evolved the ability to think about their ancestors.FIGURE 26.20 The cubs of thisBengal tiger will stay with theirmother for up to 18 months untilthey are able to hunt on theirown. Many eutherian mammalscare for their young after birth.To find out more about mammals,visit scilinks.org.Keycode: MLB026Compare and Contrast Compare and contrast the advantages and disadvantages ofmarsupial and eutherian mammal reproduction.26.4 ASSESSMENTONLINE QUIZClassZone.comREVIEWINGMAIN IDEASCRITICAL THINKINGConnecting CONCEPTS1. What features make mammalsdifferent from reptiles?2. How does fetal development differamong the three living groups ofmammals?3. Summarize How did the massextinction that ended the reptilereign help lead to today’s mammaldiversity?4. Classify Monotremes were confusingto early scientists because theyhad both reptilian and mammalianfeatures. How might scientists haveclassified monotremes differently?5. Ecology A sea turtle may layup to 200 eggs in a nest, thenleave and return to the ocean.When the young turtles hatchthey must fend for themselves.How do mammals differ in thenumber of offspring producedand in the role of parental careof offspring?Chapter 26: A Closer Look at Amniotes 809