Bones, eggs and eggshells are all that's left of a Transylvania Dinosaur-Era bird colony that made the mistake of nesting next to a big river, which wound up drowning the avian clan.
The discovery, made in the Sebes area of Transylvania, Romania, represents the first known nest colony for any Upper Cretaceous bird, according to a presentation today at the Society of Vertebrate Paleontology's 71st Annual Meeting in Las Vegas.
The birds were enantiornithines, which retained toothed beaks and had claws on their hands. Scientists now know these birds were colonial, waterside nesters, a common habit typically seen in modern birds that eat aquatic plants or animals.
Co-author Darren Naish, a vertebrate paleontologist at the University of Southampton, told Discovery News that the birds "would have looked peculiar to modern eyes." In addition to their toothiness, they had "a heavier looking tail."
The frozen-in-time avian disaster scene suggests that the birds were enjoying a peaceful end to the nesting period, with some hatchlings and their parents already in the process of leaving, when everything suddenly changed.
"Because the fossil assemblage consists only of eggshell fragments, eggs and bird bones, it is most likely that the flooding was actually a quick 'swamping' or 'drowning' where the water from the river rose by, say, a foot or two," Naish said, explaining that "it was not a massive tidal wave-style event, but most like a tidal bore-style flooding."
The fossils show that some adults were swept up by the water and drowned. Baby bird bones suggest remaining chicks died too.
"The water sweeping across the colony picked up broken eggshell, any remaining eggs and birds, and carried them a few meters across to a shallow depression, perhaps present on the other side of the colony," where the researchers found the remains buried under layers of sediment.
Gareth Dyke, who is also a vertebrate paleontologist at the University of Southampton, worked with Naish, Matyas Vremir, and other researchers on the excavation.
Dyke told Discovery News that such findings are extremely rare, since “only a handful of undisputed, isolated Cretaceous eggs are known.” He added that the Romanian scene contains “the first fossil evidence for a breeding colony of Mesozoic birds.”
Before the flooding at the site, known as Hatzeg Basin, the scene must have been quite tranquil.
"We should imagine hundreds of closely spaced individuals of these birds sitting on their nests, flying off a few times during the day to bathe or feed near or in the river," Naish said. "Seeing as colonial nesting is known in non-avian dinosaurs as well as in modern birds, its discovery in Mesozoic birds isn’t necessarily a surprise, but it’s nice to have it confirmed."
Romania was a large island during part of the Cretaceous, and "weird animals tend to evolve on islands," Naish said.
He explained that the birds’ ecosystem included dwarf cow-sized plant-eating dinosaurs, a bird-like predatory dinosaur known as Balaur, rodent-like mammals, and one of the world’s largest pterosaurs that stood about 10 feet tall at the shoulder and had a 33-foot wingspan.
"One peculiarity of this ecosystem is that big predatory dinosaurs seem to have been absent," he said.
Absent now are enantiornithines.
"For reasons that aren't yet clear, they seem to have been a dead-end," said Naish. "They died out entirely at the end of the Cretaceous, didn't leave any descendants and were not particularly close to the ancestry of modern birds."