192 INTRODUCTION TO EVOLUTION 



they come in close contact with the mother's blood. The embryonic blood 

 vessels in this region give rise to a series of chorionic villi (Fig. 9.12); 

 each of these villi is bathed in blood of the mother, thereby providing a 

 means for ready interchange of oxygen, food, and waste products. 



In marsupials, on the other hand, the placenta is absent or poorly de- 

 veloped. The young are born in an extremely immature, almost embry- 

 onic, condition. They complete their development while housed in a 

 pouch, or marsupium, on the abdomen of the mother. Marsupials also dif- 

 fer from placentals in a number of distinctive skeletal features. 



The opossum is the only modern, North American marsupial. Interest- 

 ingly enough, marsupials similar to the opossum were living in the North 

 America of Cretaceous times. Thus the opossum possesses unique value 

 in studies of mammalian evolution owing to its position as the most truly 

 primitive living mammal. In Australia, geographically isolated from the 

 rest of the world from late Cretaceous times, marsupials were free from 

 the competition of placental mammals and evolved into a great variety of 

 forms: kangaroos, wombats, bandicoots, koalas, and so on (see pp. 

 262-264). 



Placental mammals of Cretaceous age are at present known mainly from 

 fossils collected in Mongolia. These mammals were small insect-eaters, as 

 had been their Jurassic ancestors, the pantotherians. Thus they were the 

 ancestral forms of Order Insectivora. To this order belong such modern 

 mammals as shrews, moles and hedgehogs (not porcupines!). Early, rela- 

 tively unspecialized insectivores have long been regarded as the ancestors 

 from which other orders of placental mammals arose. Their position in the 

 center of our diagram illustrating adaptive radiation among mammals 

 will be recalled (Fig. 3.4, p. 27). The fossil record of the earliest pla- 

 centals confirms the ancestral position of the insectivores. 



References and Suggested Readings 



Colbert, E. H. The Dinosaur Book, 2nd ed. New York: McGraw-Hill Book 



Company, Inc., 1951. 

 Colbert, E. H. Evolution of the Vertebrates. New York: John Wiley & Sons, 



Inc., 1955. 

 Lull, R. S. Organic Evolution, rev. ed. New York: The Macmillan Company, 



1945. 

 Olson, E. C. "The evolution of mammalian characters," Evolution, 13 (1959), 



344-353. 

 Romer, A. S. Vertebrate Paleontology, 2nd ed. Chicago: University of Chicago 



Press, 1945. 

 Romer, A. S. The Vertebrate Story. Chicago: University of Chicago Press, 1959. 

 Simpson, G. G. "Mesozoic mammals and the polyphyletic origin of mammals," 



Evolution, 13 (1959), 405-414. 



