The sudden arrival of a flock of crested pterosaurs shatters the eerie midday calm of Araripe Lagoon. They swoop and soar as they jostle for position over the 70-mile stretch of bubbling water, snapping for fish at the surface. Nearby, swarms of mating cockroaches cloud the sky, their dark wings whirring. A stench of rotting eggs fills the air. Mayflies, damselflies, and wasps flit through ferns.
A gigantic bubble of toxic gas seeps up from the water’s depths. Its first victim is a hapless pterosaur. Overcome by fumes, it crashes into the water. Soon thousands of insects splatter the surface of the inland sea.
When dinosaurs roamed Earth 120 million years ago, and South America nestled up against Africa, Araripe was a violent sea of death that collected evidence of ancient life. Over millions of years the changing climate dried and filled the sea many times. Today it’s a salt-saturated pile of sediment located in the northeast corner of Brazil. The lagoon was so salty that only a few fish could live at its surface in a thin layer of freshwater that flowed from a river on its eastern edge. Any animal or plant that sank into the lagoon was embalmed in a briny grave. The only inhabitants of its depths were bacteria; no predators could survive in the saline waste. As bacteria fed on the creatures that rained from above, they produced toxic gases—methane, carbon dioxide, and hydrogen sulfide—that in turn bubbled up to poison the birds and insects flying overhead.
At least that’s paleontologist David Martill’s theory. Martill, of the University of Portsmouth in England, reached this conclusion after 11 years spent unearthing an array of fossils in the dried-up lagoon, called the Crato Formation. The diversity of species he has found is extremely unusual. Fossil hunters typically find scraps of organisms—a bone here, a leaf there. Martill, however, has uncovered all kinds of intact animals and plants, and all in one place. The bottom-dwelling bacteria were able to feed on soft guts, muscle, and blood, but they could not easily digest bone, insect.
Cuticle, or plant cellulose, which explains why the fossils look flawless. Most impressive among Martill’s finds are pterosaurs with intact skin, teeth, claws, and head crests. Other specimens include fish, frogs, lizards, beetles, damselflies, cicadas, and even whole flowering plants that look as though they were picked and dried just yesterday. And last year he came upon the coup de grâce: more than a hundred fossilized mite eggs clinging to a lone feather—the oldest ever found. Many of these creatures, notably the insects, resemble their modern-day cousins. “A design that works well will not change,” Martill says.
After he had unearthed these treasures, Martill became intrigued by the preponderance of winged creatures. “The one thing that’s funny about this deposit,” he says, “is that a dinosaur hasn’t come out of the lagoon. It’s dominated by things that dropped out of the sky.” Which is how he came up with his poison-gas theory. “As bacteria ate the tissues of the animals that fell to the bottom,” he says, “they deposited a mineral called iron pyrite over the animal. The iron pyrite then preserved the fossil. The bottom sediments, rich in bacteria feeding on organic matter, would give off large quantities of gases. It’s possible that in sheltered lakes there was a buildup of gases over the surface water. And anything flying into that could have been killed by the fumes.” Earthquakes, which were common in the region, might also have forced gas to the surface of the lake.
Of course, not all the animals plunged to their death. Some, such as the odd frog or lizard, may have drifted into the lagoon from elsewhere. Plants may have arrived by a similar route, or perhaps they were blown in. However they got there, Martill is grateful they did. “The whole thing worked like a big pickling tank, preserving everything,” he says. “It’s a special window into Cretaceous life.”