S68 



MARSUPIALS 



xx. 6-7 



pouches, an enlarged caecum, and a reduced tail. Three distinct 

 genera of phalangers have developed extensions of the skin for pur- 

 poses of soaring; Petaurus (Fig. 351) is the best known of these flying 

 phalangers. Vombatus, the wombat (Fig. 352), is a large, burrowing, 

 tailless animal, with rodent-like grinding teeth; it eats roots. *Dipro- 

 todon was a very large marsupial, of the size and form of a rhinoceros, 

 which lived in Australia in the Pleistocene. *Thylacoleo was a marsu- 

 pial lion in which the incisors were developed as fangs. 





Fig. 352. Wombat (Vombatus). (From photographs.) 



7. Significance of marsupial isolation 



The explanation of the curious distribution of the marsupials 

 remains uncertain. The fossil evidence suggests a palaearctic distribu- 

 tion in the Eocene, followed by radiation in South America and 

 Australia with later reduction in the former. An antarctic bridge has, 

 however, been postulated by some authors. 



The opossums show that marsupial life can continue effectively in 

 competition with placentals. But it can hardly be an accident that the 

 diversification of marsupials in Australia has been accomplished in 

 isolation. It is true that there are 108 species of placentals in Australia, 

 as against 119 marsupials, but the placentals are almost all bats (40 

 species) and murid rodents. The marsupials, on the other hand, have 

 become differentiated into numerous types, arboreal, fruit-eating, 

 grazing, gnawing, digging, burrowing, ant-eating, insectivorous or 

 carnivorous, in each case with appropriate structure. It will be interest- 

 ing to see how this assemblage stands up to competition with placentals 

 in the future. Carnivores, ruminants, lagomorphs, rodents, and pri- 

 mates have recently become firmly established in Australia and it can 

 hardly be an accident that some of the corresponding marsupial types 

 are already becoming rare or extinct. 



