, p. 77-78.
Marsupials exist in North America because of migration, a process described in this very chapter, but ignored now when the authors find it inconvenient. Around 3 million years ago, a combination of continental drift and the rising Andes brought South America into contact with North America. For the first time in 50 million years, North American species and South American species came into contact. Some marsupials (like the opossum) spread north. Some placentals from North American spread south. For reasons that scientists continue to investigate and discuss, the ability of North American placentals to persist and diversify in South America was much greater than the ability of South American marsupials to diversify in the north, or to outcompete the northern invaders (Stehli and Webb, eds. 1985. The Great American Biotic Interchange. Plenum Press: New York). Today, half of South American mammals are descended from North American ancestors. South American descendants represent no more than 20 percent of modern North American species, and most of those are in Central America, near the point of initial contact (Marshall, et al. 1982. "Mammalian evolution and the Great American Interchange," Science 215:1351-1357). This helps explain the confusion of the apparently biogeographically illiterate authors of Explore Evolution about why "marsupials such as the opossum live in the northern hemisphere." It is because of migration, a process they described as uncontroversial only a page earlier. It is not a mystery, and it is unclear why the authors would regard it as such.
The claim that North American opossums are evidence against the common ancestry of marsupials bears striking similarities to a critique of biogeography by young earth creationist Kurt Wise:
There are very few examples of macrobiogeographical evidences for macroevolution, and none of them is very strong. The best-known claim is the concentration of marsupials in Australia. But there are several reasons that marsupials in Australia are actually a poor example. First, all marsupials are not in Australia. The Virginia opossum of North America, for example, is a marsupial. It is thought to have come from South America, not Australia. Thus not all similar organisms are known from every continent. Third, marsupials are the oldest fossil mammals know from Africa, Antarctica and Australia—in that order. The fossil record seems to show a migration of marsupials from somewhere around the intersection of the Eurasian and African continents and then a survival in only the continents farthest from their point of origin (South America and Australia).
source Wise, Kurt (1994) "The Origins of Life's Major Groups," ch. 6 in J. P. Moreland (ed.) The Creation Hypothesis: Scientific Evidence for an Intelligent Designer, Intervarsity Press:Downers Grove, IL, p. 223).
Wise's confusion over the status of the Virginia opossum perhaps reflects his confusion of the New World order Didelphimorphia — commonly called opossums — with the Australian order Diprotodontia — some of which are commonly called possums (without the first "o"). The groups are morphologically and molecularly distinct, with well-established paleontological histories. If the opossum truly had roots in Australia, it would indeed be a biogeographic conundrum. In fact, the only close link between opossums and Australia is Wise's typo.
Similar misunderstandings plague the discussion of the marsupial fossil record in Explore Evolution and its creationist source material. It is not surprising, the breathless tone of Explore Evolution notwithstanding, that "paleontologists have unearthed the oldest marsupial fossil of all … in China." The authors wonder "If the ancestors of the marsupials originated in the Southern Hemisphere, why has the oldest known member of the group been discovered in the Norther Hemisphere?" The answer is simple: paleontologists do not claim that marsupials originated in the Southern hemisphere, only that they migrated there.
Mammal Taxa: A phylogeny of fossil and extant mammalian taxa, combined with the known ages of fossils. The earliest fossils are found in Asia, with more modern fossils found in North America, and then spreading to South America and Australia. This pattern is consistent with known land connections between the continents at the times of apparent interchange. Zhe-Xi Luo, Qiang Ji, John R. Wible, Chong-Xi Yuan (2003) "An Early Cretaceous Tribosphenic Mammal and Metatherian Evolution", Science, 302(5652):1934-1940.
Marsupials and placental mammals separated roughly 125 million years ago, according to the most recent fossil data. Several lines of evidence indicate that the marsupials originated in Asia, spread to North America over a land bridge, which can be seen in the figure. The ages and characteristics of fossils found in Europe, South America, Africa, Australia and Antarctica suggest that marsupials spread to Europe and South America from North America. South America, Africa, Australia and Antarctica still had linkages at that time, allowing species on the Southern continents to spread easily.
Despite the feigned confusion of Explore Evolution's authors, the fossil record gives a very clear picture of the biogeographic history of marsupials, though there are many questions scientists continue to investigate. The earliest marsupial fossils (and the earliest placental fossils) are found in Asia. Fossilized marsupials are found in North America in rocks that are only a few million years younger than the Chinese fossils. During that period in geological history, plate tectonics had brought North America and Asia close enough together to forge a land bridge, allowing many species to migrate between those continents.
Cretaceous Continents: Christopher Scotese's reconstruction of the continental arrangement 94 million years ago. The land bridge between North America and Asia is indicated at the upper left. The connection between North and South America is also visible. Africa is drifting away from other southern continents, but Australia, Antarctica and South America are linked. Africa split off from this southern supercontinent beginning around 140 million years ago.
At that time, the southern continents were all connected, North America and Europe were still very close, and South America had not drifted far from North America, allowing dispersal during periods when ocean levels dropped. Marsupials related to North American species colonized Europe briefly, through a northern land bridge, and others colonized South America. Africa was in the process of separating from the supercontinent which also included Antarctica, Australia and South America, so the presence of marsupial fossils in Africa gives a good measure of how quickly they entered South America and dispersed across the supercontinent Gondwana. Fossilized marsupials in Antarctica also allow us to track their dispersal to Australia. This pattern is consistent with the fossil record of placental mammals, and with other lines of evidence (John P. Hunter and Christine M. Janis. 2006. "'Garden of Eden' or 'Fool’s Paradise'? Phylogeny, dispersal, and the southern continent hypothesis of placental mammal origins," Paleobiology, 32(3):339–344).
As the southern continents drifted apart as well, the marsupial faunas in each isolated continent followed a different path. As Antarctica drifted south towards its current polar position, it became colder and colder, ultimately driving its resident marsupials and palm trees extinct. South American and Australian marsupials produced diverse radiations which filled many of the same ecological niches occupied by placental mammals elsewhere. In the northern continents, which were periodically linked by land bridges, biotic interchanges resulted in periods of intense competition, which seem to have driven the native marsupials extinct.
When South America drifted north again and connected with North America around 3 million years ago, the Great American Biotic Interchange had the same devastating effect that biotic interchanges had on other marsupial faunas.
The same pattern of diversification and migration seen in marsupials can also be seen in other groups of plants and animals. That consistency between biogeographic and evolutionary patterns provides important evidence about the continuity of the processes driving the evolution and diversification of all life. This continuity is what would be expected of a pattern of common descent. The creationist orchard scheme gives us no reason to predict this pattern.