Desert Island

How climate can promote speciation

CHRISTIAN A. SIDOR New York College of Osteopathic Medicine
A bedrock tenet of biogeography holds that organisms separated from their ancestral population will set off on their own evolutionary trajectory. Continental drift provides one such isolating mechanism, illustrated perhaps most spectacularly by the unique flora and fauna found on the island of Madagascar, which broke off from the southern supercontinent of Gondwana some 90 million years ago. Mountain upheaval and river formation can also divide populations. But a new study reveals that the barriers need not be physical. Paleontologists have unearthed fossils of giant amphibians that indicate that climate, too, can effectively isolate organisms and thereby foster endemism.

In a paper published in the April 14 Nature, paleontologist Christian A. Sidor of the New York College of Osteopathic Medicine and his colleagues announced their discovery of two new species of amphibian that lived some 250 million years ago in what is now northern Niger. The salamanderlike beasts, Nigerpeton ricqlesi and Saharastega moradiensis, are surprising not so much for their impressive size (think crocodile proportions) but because they are different from other creatures that lived at the time. For decades, scientists excavating fossils from this Late Permian interval have come across the same tetrapod forms again and again, regardless of whether they were working in southern locales such as South Africa or northern ones such as Russia. This cosmopolitanism seemed to show that tetrapods wandered anywhere they pleased during this time, when most of the earth's landmasses lay lumped together as an even larger supercontinent, Pangaea.

The new work paints a more complex picture. Whereas previous expeditions had focused on the tropical-to-temperate northern and southern latitudes, Sidor's team selected a locale closer to the paleoequator. Geologic data and climate simulations indicate that by the Late Permian a global change in climate started, shifting the planet from a so-called icehouse world, in which the polar ice sheets extended as far north as southern Africa, to a hothouse world. Ultradry conditions replaced moderate ones in central Pangaea. This shift, the authors argue, effectively isolated pockets of formerly ubiquitous tetrapods by forming a desert around them.

That would explain why Nigerpeton and Saharastega are so much more primitive than other Late Permian amphibians. Indeed, their closest relatives had lived in North America and Europe 40 million to 90 million years earlier, suggesting that Nigerpeton and Saharastega are relicts of a radiation previously believed to have disappeared far earlier. Isolation by desiccation would also explain why dicynodonts--mammallike reptiles that dominate fossil assemblages elsewhere during this time--are conspicuously absent at the site that yielded the two new amphibians.

"It is an excellent piece of work," comments Robert R. Reisz of the University of Toronto. The next step, he says, is to try to find evidence of coastal migrations during the Late Permian, which would cast light on how the assemblages in the north and south came to be so similar, despite the presence of a vast desert between them.

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