From an Ancient Rookery in Chile
by Robert Sanders
A graveyard of perhaps thousands of flying reptiles--the largest collection of fossil pterosaurs ever found--has provided new insights into how these creatures lived and cared for their young.
The fossil bones were found strewn throughout an ancient flood deposit in Chile's Atacama desert, suggesting that they were animals or corpses caught up in a flood perhaps 110 million years ago at the beginning of the Cretaceous period.
The inland site indicates that the pterosaurs nested in colonies much like seagulls do today, says paleontologist Kevin Padian. Padian was asked to look at the fossils after they were discovered six years ago by his collaborator, C. Michael Bell, a geologist from Cheltenham and Gloucester College of Higher Education in England.
"Mike wondered if this may have been a rookery," says Padian, a professor of integrative biology and a curator at the Museum of Paleontology. "It definitely seems to be, which suggests that pterosaurs were ground nesting like birds and had parental behavior much as birds do."
Almost all pterosaur fossils found to date have been single specimens, for the most part in sediments that formed in shallow seas, quiet lagoons or lakes--environments conducive to preserving the fragile bones. The natural inference was that they lived and fed like seabirds today, such as gulls or pelicans.
The Chilean site--at the time at least a dozen miles from the ocean, and perhaps up to 30 miles from the shore--suggests that at least some species spent considerable time inland, if not nesting then at least resting together on the ground.
This theory fits well with more recent analyses of pterosaur fossil remains, which suggest that the old picture of a bat-like animal that mostly glided and needed a perch from which to launch itself is incorrect.
Padian, who has done much to change this picture, says pterosaurs could fly and maneuver with dexterity, were good runners and lived in a broad range of environments.
He argues that the great diversity of the pterosaurs has been overlooked because many of their habitats, such as forests, would not have been favorable to the preservation of their fragile bones.
Bell and Padian's analysis of the desert fossil find appears in the current edition of the bimonthly British journal Geological Magazine.
Some 60 species of pterosaur are known today from fossil bones found in sedimentary deposits around the globe, Padian says. They range from sparrow-size to the giant Quetzalcoatlus of Texas, which had a 35-foot wingspan.
Pterosaurs probably arose around 225 million years ago and existed for 160 million years, living side by side with birds for half this time.
Bell found the death assemblage in the dry Atacama desert in the Andes of northern Chile in 1989. He immediately recognized the jumble of thin-walled, broken bones as those of pterosaurs, and called Padian to discuss the circumstances that brought them there.
Bell dated the sediments to the Early Cretaceous period, which began around 135 million years ago. At the time the area was warm and dry with low, seasonal rainfall. The arid basin was surrounded by volcanoes and scattered with dune fields, saline lakes, mudflats, alluvial fans and flood plains.
The fossils are spread out sparsely over a region that once probably covered a square kilometer, leading the scientists to conclude that the deposit contained several thousand pterosaurs.
Most were probably juveniles or immature adults with few mature adults found.
The most likely scenario, Padian says, is that a flood surprised a colony of pterosaurs at their nesting site or sites, perhaps on an island in the middle of the stream isolated from land predators.
While many of the pterosaurs no doubt flew to safety, immature pterosaurs and some adults would have been caught up in the torrent and carried to their death. Some gulls today are known to stand by their nests while flood waters engulf them.
"The presence of lots of juveniles supports the idea that pterosaurs probably could run around before they could fly," Padian says.
This meshes with his conclusions from examination of thousands of pterosaur fossils over the years.