For more than 150 million years, until the Cretaceous Period ended 65 million years ago, the skies of Earth were filled with flying reptiles known as pterosaurs. Yet even more than dinosaurs, their land- bound contemporaries, pterosaurs remain a mystery. Two centuries after the first pterosaur fossil was unearthed, paleontologists are still debating whether these strange beasts were more like birds, flying with slim wings and walking on the ground on two legs, or like bats, with loose, membranous wings that stretched from wrist to ankle and forced them to crawl on all fours when they weren’t aloft. The ecology of pterosaurs is an enigma, too. Their fragile bones have been found primarily in what were once ocean sediments or lake bottoms, but if they really were like birds or bats, they should have colonized many other habitats, such as mountains and forests. Perhaps most puzzling of all is the question of how pterosaurs produced their offspring. Did they lay eggs in nests, or did they give birth to live young? And did they take care of their babies?
Mike Bell may have shed some light on all these questions. Bell, a geologist at the Cheltenham and Gloucester College of Higher Education in England, has discovered a pterosaur graveyard in Chile that contains the fossils of thousands of young animals. It may once have been a pterosaur breeding colony.
Bell stumbled across the graveyard while collaborating with Chile’s Geological Survey on a study of a rock formation in the bleak deserts of the northern Andes. The formation was created 120 million years ago, when the mountains were much lower and the landscape resembled Death Valley. Mostly the rock Bell found was sandstone--featureless, lithified desert. But on the slopes of the 13,000-foot-high Cerro la Isla Mountain, his attention was grabbed by a six-foot-thick outcrop of dark, jumbled conglomerate rock. It’s something that as a geologist, you just home in on, says Bell. And then we started finding all these tiny white bones scattered along it.
The fossils included dinosaurs, crocodiles, and, most of all, pterosaurs, easily recognizable by their long, slender, and thin-walled bones. There were thousands of pterosaur bones, in fact--the most ever found in one place. In hopes of identifying the species, Bell packed up 50 or so samples and shipped them to Berkeley, California, where Kevin Padian, one of the world’s experts on these creatures, works at the University of California.
The fossils were too fragmented for Padian to identify their species or even their genus, although he did eventually estimate that the reptiles’ wingspan at maturity would have been around six feet--on the small side for a pterosaur. But the surprise came when Padian put the bones under a microscope. Like warm-blooded animals in general (but unlike, say, crocodiles), pterosaurs grew quickly while they were young and stopped when they reached adulthood. This pattern was preserved in the structure of their bones; the bone laid down during the youthful growth spurt was run through with blood vessels that fueled the rapid growth, but when growth stopped the bone acquired a hard outer layer that was blood-vessel free. All the pterosaur bones Padian looked at under the microscope lacked this outer layer. Thus they all came from juvenile animals. Bell had apparently discovered the remains of the first known pterosaur rookery.
Putting their work together, Bell and Padian have an idea of what may have happened in that Chilean desert basin 120 million years ago. The jumbled rock containing pterosaur bones scattered every which way, they think, was deposited by a giant flash flood that roared through the valley; perhaps a landslide had dammed a river, Bell speculates, and then suddenly the dam burst. The flood swept rocks and animals alike off the desert floor and dropped them miles away, at the valley’s edge. When the waters came crashing over the pterosaur colony, the adults simply escaped into the air- -but they could do nothing to save their young, who were still unable to fly.
This scenario would go a long way toward explaining why the fossils of young pterosaurs have been so rare until now. Their parents sometimes died while flying over oceans or swamps (which preserve delicate bones), but the young stayed put in their rookeries. I’ve got a sneaking suspicion, says Bell, who has since moved on to other geological sites in Chile, that if someone looked hard enough, someday they might find fragments of eggs. You’d be very lucky to find them; it’s just a dream really.
Bell’s find also demonstrates that some pterosaurs were comfortable far from water: the Pacific was 30 miles from their breeding colony. Bell thinks they lived like the gray gull of Chile, which today maintains giant breeding colonies in deserts dozens of miles inland and brings back fish from the Pacific for its young. The pterosaurs may have chosen their inland home for the same reason as the gulls: coastal real estate is scarcer, and an inhospitable desert discourages predators that might eat eggs or newly hatched young.
The Chilean colony, Padian says, buttresses his view that pterosaurs were like birds. Paleontologists who compare them to bats claim pterosaurs would have been uncomfortable on the ground and would have raised their young on cliffs or in trees, where they could easily fall into flight. Yet Bell’s pterosaurs thrived in a flat and nearly treeless desert. It’s not surprising to me, says Padian. On the other hand, it does seem hard to explain if you think they weren’t very good on the ground. You can’t think of a bat doing that.