Dodging mass extinction: all around, species were dying off. But in this Devonian reef, life went on. Why?

Natural History, Dec, 2002 by Rachel Wood

Windjana Gorge, during northwestern Australia's dry winter, is a magical place. An inconspicuous cleft within steep red cliffs marks its entrance; the cliffs themselves rise abruptly from a wide, dry plain, dotted with ancient boab trees. But who, upon entering the lush gorge, could remember the barren landscape just left behind? In deep ponds--the seasonal remains of the mighty Lennard River that floods the gorge during the wet season--swim freshwater crocodiles. In the surrounding trees cockatoos chatter. Farther into the depths, the gorge widens; here, in the heat of the day, large colonies of fruit bats fidget in the eucalyptus trees, and an occasional wallaby bounces across the trail.

But Windjana Gorge is remarkable not only for its living natural history. The steep limestone cliffs also record in their sediments one of the most fascinating events in the history of life on Earth: a mass extinction of reef species that took place some 370 million years ago, at the end of the Devonian Period. Even more, the limestone bears silent witness to the subsequent recovery of an entire marine ecosystem of the time.

Mass extinctions are extraordinary natural phenomena that have surged to the fore of scientific and popular attention in the past two decades, though paleontologists have recognized their existence for more than a ;century. In part, that attention is propelled by the idea that a collision of a giant meteorite with the Earth wiped out the dinosaurs at the end of the Cretaceous Period, some 65 million years ago. Even more compelling, both the threat of nuclear war and the ongoing loss of biodiversity have raised the possibility of mass extinctions in a contemporary context. But even before the demise of the dinosaurs, the Earth had undergone no fewer than four such events.

In mass extinctions, some species that make up a community often disappear immediately; a variety of ecological effects then cascade through what remains. Yet despite the special status that evolutionary biologists now assign to mass extinctions, there is little consensus about the scope and intensity of those cascading effects. How rapidly do they unfold? To what degree can they remodel ecosystems? How important a role have they played in shaping the history of life? To help explicate the answers biologists have proposed, I have spent the past few years studying the details of the Devonian catastrophe recorded at Windjana Gorge. Surprisingly, the evidence I have uncovered suggests a less calamitous, more encouraging outcome to at least one aspect of these ecological disasters: after the Devonian extinctions, species diversity recovered much more rapidly than evolutionary biologists had thought possible.

The Devonian world offers little that we can recognize in our own. Atmospheric levels of carbon dioxide, a greenhouse gas, were between twelve and twenty times higher than they are today; as a consequence, tropical seas were considerably warmer. Tropical forests had just begun to establish. themselves, and mammals, reptiles, and birds had not yet made their appearance.

Reef communities were also quite different. Modern corals had not yet evolved, and two now-extinct groups, known as tabulates and rugosans, flourished in the reefs. Sponges, particularly the now-rare stromatoporoids, were not at all like most of their modern counterparts; they possessed a solid skeleton made of calcium carbonate, rather like that of corals (most sponges now rely on silicates). The ancient sponges provided much of the backbone of the undersea reef structures. Algae-like microbial communities were also of far greater importance than they are in modern reefs. Such microorganisms could trap sediments and precipitate limestone, and though they rarely fossilized, they did leave characteristic freestanding mounds and columns, built on the seafloor.

The catastrophe of the Late Devonian was roughly equivalent in magnitude to the event at the end of the Cretaceous that killed the dinosaurs. At least 57 percent of all species eventually perished as a result. But the fossil record shows that the extinction rate was particularly severe in the tropics--especially within reef communities such as the one at Windjana Gorge. Before the extinctions, reefs grew virtually wherever there were warm, shallow seas that fringed large continental landmasses; worldwide, they covered an area of some two million square miles. After the extinctions, that area shrank to what was probably a fifth to a tenth of its former size. Paleontologists find remains of reefs that grew after the extinction event only in rocks from what are now Canada, northeastern Russia, the northern Caspian Sea region, and the Canning Basin of northwestern Australia. In most regions the reefs simply perished.

To understand what causes mass extinctions, paleontologists and geologists must unravel the clues embedded in sedimentary rocks and in the fossil record the rocks contain. Initial analyses, which simply plotted the number of species against their age, suggested that most extinctions took place between the Frasnian and Famennian stages, at the end of the Devonian Period (stages are subdivisions of periods of geological time). In some parts of the world, layers of black shales, rich in organic matter, mark this interval, and investigators thought that such distinctive sediments might yield some indication of what caused the global disruption.