PHYLOGENY 



young undergo a brief development in the uterus (modified oviduct), 

 then are born in a very incomplete state of development. They are trans- 

 ferred to an abdominal pouch (marsupium), where each of the young 

 becomes permanently attached to a nipple and is thus nourished until it 

 is sufficiently developed to leave the maternal pouch and fend for itself. 

 The kangaroo is the best known marsupial, but it is also one of the most 

 highly specialized. The opossum of the United States is a much more 

 primitive marsupial. In general, the order has been rather unsuccessful 

 in competition with the placental mammals, but they have been extremely 

 successful in Australia, where they have filled almost every possible adap- 

 tive niche. But Australia has been separated from the northern centers of 

 diversity since the Cretaceous, and it is consequently almost devoid of 

 placental mammals. South America was similarly isolated during much of 

 the Tertiary, and hence a rich marsupial fauna developed there, only to 

 become extinct when placental mammals invaded from North America. 

 Today, the Australian region is the home of the only rich marsupial fauna, 

 but unfortunately very little is known of its fossil history. 



The placental mammals also appear in the fossil record in the late 

 Cretaceous for the first time, and they underwent an explosive expansion 

 at the beginning of the Tertiary, so that almost all of the orders are present 

 from the beginning. The major feature diff^erentiating them from the mar- 

 supials is that the embryos develop an efficient placenta for obtaining 

 nourishment from the mother's bloodstream. This also serves as an organ 

 of respiration and excretion. These are by far the dominant animals of 

 the world today, and this is in no small measure due to the prolonged 

 period of embryonic development which is made possible by the placenta, 

 as well as by parental care and the enlargement of the cerebral hemi- 

 spheres, diminutive in all lower classes of vertebrates. This last trend is 

 already observable in the lower mammalian groups, but is carried much 

 further by the placental mammals. 



The ancestral placentals were small animals, probably because only 

 such could compete with the reptiles of the Cretaceous. Although they 

 were potentially meat eaters, their small size restricted them largely to 

 a diet of insects, worms, and other small invertebrates, with perhaps a 

 small amount of vegetable matter. This is about what the living members 

 of the order Insectivora eat. Some of the members of this order, which 

 includes shrews, moles, hedgehogs, and tenrecs, are very primitive ana- 

 tomically. It seems probable that, of all the living orders of mammals, the 

 Insectivora is closest to the primitive placental stock. 



Because the extant ( and extinct ) orders appear so rapidlv at the begin- 

 ning of the Tertiary, it is very difficult to trace probable relationships. 

 But, on the basis of comparative anatomy, comparative serology, and 

 paleontology, certain probable relationships have been drawn up, some 

 of which are much better supported by evidence than others. Because of 

 their antiquity and primitiveness, the order Insectivora is generally re- 

 garded as the probable source from which other orders of placentals have 

 been derived. The derivation of one series of orders, the cohort Unguicu- 

 lata, from the Insectivora is fairly clear, but others vary considerably in 



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