Chiappe, Luis M., Rivarola, David, Natural History
At dawn, a small flock of pterosaurs flies over a lake surrounded by sharp, craggy rocks. Vapor rises from the fresh water into the already warm air as the winged reptiles land in the shallows. They fold their wings, which span five to seven feet, submerge their foot-long, curved snouts, and rhythmically begin to filter the muddy bottom. Schools of small fishes scatter as the pterosaurs make sweeping motions with their heads, trapping tiny, edible organisms in their sievelike mouths. Soon, other pterosaur flocks arrive at the lake to feed before the heat of the day sets in.
One hundred million years ago, in what is now a corner of Argentina's central province of San Luis, this scene would have been a daily occurrence. Pterosaurs, close relatives of dinosaurs, died out along with the last of the large dinosaurs some 65 million years ago. But before their demise, pterosaurs had thrived for some 150 million years and had greatly diversified. One of the most specialized was Pterodaustro, the visitor to the Argentine lake.
In 1994, we organized a paleontological expedition to San Luis. Twentyfive years earlier, Jose Bonaparte and other Argentine paleontologists exploring the area had found the first fossils of Pterodaustro, whose skull features stand out from those of all other flying reptiles. While we were primarily interested in finding the remains of early birds, we also knew that additional specimens of Pterodaustro, one of the few freshwater pterosaurs, would help clarify the biology of this spectacular creature and that meticulous studies of the rocks containing the fossils would give us clues to the environment in which it lived.
After gathering all necessary government permits, as well as enlisting local support, we were able to take twenty people to the site for two weeks of intensive work. After quarrying some twenty tons of rock, we uncovered numerous Pterodaustro fossils, including several of the delicate skulls and jaws. We also found many specimens of fishes (some more than five feet long); clamshrimps (tiny freshwater crustaceans, enclosed in a bivalved shell of digestible keratin, and part of Pterodaustros's diet); a variety of fossil tracks, indicating an abundance of invertebrate bottom-dwellers; and plants, including the remains of an ancient flower. With all the material moved into a classroom at the Universidad Nacional de San Luis, we began to reconstruct the environment of the 100-million-year-old lake.
The presence of lava of the same age indicated that part of the lake shore had been surrounded by sharp volcanic rocks. These lava eruptions were probably related to the stresses created by the final phases of the separation of South America from Africa (continents that had been connected for more than 100 million years). The extensive lake was most likely formed when the lava eruptions dammed the waters of ancient rivers. The composition of the sediments entombing the remains of Pterodaustro suggested that these had accumulated slowly in the bottom of a shallow lake when the climate was warm and arid. …