percent of their body weight (human testes 

 account for a mere 0.04 percent of the av- 

 erage male's weight). Honey possum sper- 

 matozoa, at 360 \xm long, are also the 

 largest in the whole class Mammalia. To 

 further prick placental inadequacy, mem- 

 bers of one subfamily of dasyurid marsu- 

 pials have two decidedly impressive erec- 

 tile organs, one in front of the other. 



True, the less spectacularly equipped 

 placentals do tend to dominate the North- 

 ern Hemisphere' — today. But this was not 

 always the case. In the last days of the di- 

 nosaurs, more kinds of marsupials than 

 placentals existed, even in the Northern 

 Hemisphere. Long after T. rex gasped its 

 last, marsupials persisted in showing off 

 their pouches and dangly bits in North 

 America until about fifteen million years 

 ago. Then after a brief period of inexplica- 

 ble absence, they reinvaded this placental 

 bastion from South America about one 

 million years ago, strong-arming placen- 

 tals all the way to Canada. In fact, marsu- 

 pials have left their bones on every conti- 

 nent. Ice probably forced them out of 

 Antarctica, but the reasons for their disap- 

 pearances from Europe (by ten million 

 years ago) and Asia and Africa (by thirty 

 million years ago) remain a mystery. 



Over the last hundred million years or 

 so, the world's placentals have indeed pro- 

 duced an impressive array of pouchless or- 

 ders. But on the single continent of Aus- 

 tralia, some of the world's most distinctive 

 mammals make their home, among them 

 noolbengers, wambengers, and wombats. 

 If we dip into Australia's fossil record, 

 such as that tumbling out of the middle 

 Tertiary sediments of Riversleigh, 

 Queensland, even more distinctive groups 

 abound, with 50 percent greater diversity 

 at the family level than survives today. Re- 

 markable marsupials have similarly 

 emerged from the fossil record of South 

 America, once home to the parrot-faced 

 groeberiids, leaping argyrolagids, tusked 

 bonapartheriids, and grizzly-sized bor- 

 hyaenids that every edible placental in the 



Twenty million years ago, the 



dense, warm rainforests of 



what is now Queensland 



were home to a strange 



menagerie of furry, pouched, 



feathered, and scaly beasts, 



among them, marsupial 

 lions, giant snakes, and flesh- 

 eating kangaroos. 



Painting by Jim Reece 



place called "Sir." While confined for the 

 most part to the Southern Hemisphere 

 today, marsupials still exhibit a range of 

 diversity nearly as spectacular' as that of 

 the world's placentals. 



The curious events of South America 

 are further humbling to the placental ego. 

 Although both marsupials and placentals 

 arrived there from North America some- 

 time between seventy and sixty-three mil- 

 lion years ago, the placentals became the 

 highly edible mammalian herbivores of 

 that land. In contrast, the marsupials be- 

 came the small- to giant-sized carnivores, 

 roles they held against almost all comers 

 until they were shouldered a bit to one side 

 by giant, meat-sucking phorusrhacid birds 



(some of which had skulls nearly three 

 feet in length). Admittedly, one group of 

 placental carnivores did manage to sneak 

 in about eight million years ago — the rac- 

 coon family, which persists today as 

 kinkajous, olingos, and coatis. The marsu- 

 pial saber-toothed "lions" may also have 

 lost out in competition with invading pla- 

 cental saber-toothed lions about two mil- 

 lion years ago. But overall, placental chau- 

 vinists can take little solace from the 

 history of South America. 



For centuries after the discovery of 

 Australian marsupials, biogeographers as- 

 sumed that the failure of placentals to 

 dominate this island continent must have 

 had to do with Australia's history of isola- 



46 Natural History 4/94 



Mr. 1 



