Science: A Rosetta Stone of Evolution

The tides at the closed end of the Bay of Fundy are the highest in the world, rising and falling more than 50 feet every day. For the two fossil hunters clambering over the bordering cliffs near Parrsboro, Nova Scotia, last summer, that presented a special problem. Timing their forays with the mighty ebb and flow, they often found themselves on isolated cliff faces, cut off by the surging water.

Their risky efforts paid off. The cliffs, part of a stratum of sedimentary shale and sandstone interleaved with volcanic basalt, date from between 225 million and 175 million years ago. The entire rock formation was long thought to be virtually devoid of fossils and thus of little interest to paleontologists. In fact, says Neil Shubin, 25, a graduate student in biology at Harvard, the site they discovered "looks like Rocky Road ice cream. It's dark rock absolutely splattered with bone." Says his partner, Geologist Paul Olsen, 32, of Columbia's Lamont-Doherty Geological Observatory: "We were shocked by the number of fossils."

Excavations at the Nova Scotia site have so far yielded more than 100,000 fossilized bone fragments, all dating from shortly after the mass extinction some 200 million years ago that marked the end of the Triassic period and the beginning of the Jurassic. Because of some rapid change, perhaps a catastrophic event, the fossil record shows, 43% of the animal families whose fossilized remains are found in the older Triassic rock are missing from the Jurassic layers just above it. The sudden mass extinction opened the evolutionary way for the proliferation of the dinosaurs and the emergence of the mammals. The creatures found in the rock samples, Olsen believes, were among the survivors of that event.

Last week their remains were still being painstakingly extracted from more than three tons of the fossil-rich rock that were shipped back to laboratories at Harvard and Columbia. Technicians armed with microscopes and carbide needles to pick away at the rock have already discovered some notable specimens: the world's richest collection of fossil bones of tritheledonts, the group of reptiles most closely related to mammals; a large number of sphenodonts, small, lizard-like reptiles whose only living relative is the tuatara of New Zealand; yard-long crocodiles with spindly legs, a whiplike tail and a sleek body that Olsen calls "the cheetahs of their time"; a trail of penny-size footprints left by a dinosaur no bigger than a sparrow.

Around the time of the mass extinction, the fossil site was apparently in a 300-mile-long rift valley fringed with high mountains. The climate swung between wet and dry spells every 20,000 years or so, leaving telltale alternating layers of lake sediments and sandstone visible on the present-day cliffs. "When it rained," says Olsen, "chunks of rock and mud raced down the mountainsides and buried large swaths of ground." Many of the now fossilized animals escaped the slides, only to be trapped in cracks that opened as the mud flow dried and shrank. Olsen believes the animals entered the fissures in search of water or were dragged there by carnivores occupying the cavelike spaces as dens.