Evolution of Evolution — Text-only | Flash Special Report
Getting Into and Out of Antarctica
Tracking intercontinental animal migration through the ages
By Ross MacPhee
Recently, paleontologists made a remarkable find: a 10-pound, meat-eating frog so fearsome that its discoverers worked a name synonymous with Satan into its scientific label: Beelzebufo, or devil-frog. Some 65 million years ago on the island of Madagascar, nearly 250 miles off the coast of Africa, our lemur-like cousins may have been pursued by Beelzebufo. Scientists are interested in the evolution of the giant devil-frog, but even more intrigued by the fact that Beelzebufo’s closest relatives live in faraway South America rather than next door in Africa.
Making sense of such puzzles is part of the science of biogeography, the study of the distribution of organisms in space and time. Biogeography’s development is associated with Alfred Wallace, Charles Darwin’s contemporary and co-originator of the theory of evolution by natural selection. Wallace knew nothing of plate tectonics—the idea that the Earth’s surface is divided into rigid plates that move relative to each other—but he understood that the face of the planet is constantly changing. The existence of supercontinents, single landmasses existing hundreds of millions of years ago that consisted of all the modern continents, is an idea built on principles of plate tectonics. These supercontinents and their associated land bridges may have formed passageways for the distribution of organisms like the devil-frog and its relatives.
Some of the best evidence for this theory comes from the polar regions of Antarctica. Nearly devoid of terrestrial life today, Antarctica appears to have been an important animal crossroads some 45-80 million years ago, the time when mammals rose to dominance as the dinosaurs went extinct. Fossils from islands next to the Antarctic Peninsula include not just dinosaurs, birds and mammals, but also sharks, bony fishes and marine reptiles that together reveal an Antarctica with a subtropical climate. But how did these animals get there? One possibility is that they walked across a land bridge—the Scotia Portal—that connected the Antarctic Peninsula with South America. This land bridge is thought to have been broken about 40 million years ago, when the Drake Passage opened between the southern tip of South America and the South Shetland Islands of Antarctica. After this, Antarctica began heading into its deep freeze.
The Scotia Portal could also account for the presence of other mammal fossils including opossums, sloths and other distinctively South American animals on Antarctic islands. Opossums are marsupials, which today form a major part of regional mammal groupings only in Australia and nearby New Guinea. But what does that distribution imply? We know from tectonic evidence that Australia finally separated from east Antarctica around 64 million years ago. The question is, were ancestral Antarctic marsupials already on board? Almost certainly. Indeed, some groups of marsupials may have actually evolved new forms in Antarctica before traveling onward.
Well to the west, Antarctica was connected to Madagascar, India, Africa and South America until about 110-120 million years ago. When these landmasses then began to move northward, land-bound animal movements to or through Antarctica should have ended; however, analysis of other fossils from the same rocks that yielded the devil-frog indicate that some animal species may have transferred after the continental breakup. This may have been possible by another land connection—the Enderby Portal—that persisted between Madagascar and east Antarctica. There are also two other alternatives, both considered quite unlikely by biogeographers.
The controversies concerning how ancient Antarctica facilitated the distribution of animals like mammals are far from settled, but that is all to the good. Apparent conflicts between biogeographical inferences and plate-tectonic reconstructions should be regarded as places where interesting scientific problems reside, awaiting solution, not as impediments to understanding. Just as Darwin’s and Wallace’s understanding of evolution and biogeography incited great leaps forward, new evidence from Earth’s polar regions are spurring additional breakthroughs.
Ross MacPhee is former chairman of the Department of Mammalogy at the American Museum of Natural History, where he has been a curator since 1988. Known for his paleomammalogical research on island extinctions, his most recent work focuses on how extinctions occur, particularly those in which humans are thought to have been implicated during the past 100,000 years. His work includes more than 100 published papers in scientific journals and two edited major collections: Primates and their Relatives in Phylogenetic Perspective (1993), and Extinctions in Near Time: Causes, Contexts, and Consequences (1999). The National Science Foundation supports his work with vertebrate paleontology on Livingston Island in Antarctica.
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