Fossil Egg Finds Yield Clues to How Pterosaurs Lived

John Roach
for National Geographic News
December 2, 2004
The discoveries of two fossilized eggs from the ancient flying reptiles known as pterosaurs were announced Wednesday. The finds raise to three the number of known pterosaur eggs—the one other known egg was only announced last summer.

Until very recently, scientists wondered if the reptiles that filled the skies in the age of the dinosaurs laid eggs or gave birth to live young like mammals do.

"We've [now] got really strong evidence that pterosaurs laid eggs," said Christopher Bennett, a pterosaur expert at Fort Hays State University in Hays, Kansas. Bennett was not involved with the discoveries.

The first pterosaur egg, discovered in China, was announced in the journal Nature in June. A second egg from China and one from Argentina are reported in today's edition of the journal.

Fossil Eggs

Luis Chiappe is the curator of vertebrate paleontology at the Natural History Museum of Los Angeles County in California and lead author of the paper on the Argentine egg. He said the timing of the discoveries is "just a coincidence" but added that it is "amazing."

The egg from Argentina is about a hundred million years old and thought to have had a hard shell similar to those of birds and dinosaurs. Chiappe identified the egg as belonging to the flamingo-like Pterodaustro guinazui. He said the discovery firms up evidence for the species' communal lifestyle.

"Our egg was found amongst a bunch of other fossils of the same species, fossils ranging from hatchlings to teenagers to full adult individuals. … That tells us these animals nested in a colony and offered to their young parental care," he said.

Qiang Ji of Nanjing University in Nanjing, China, and colleagues described the new egg from China. The egg is about 121 million years old and reported to have had a "soft and leathery" shell similar to crocodile and turtle eggshells.

Bennett, who has reviewed the papers describing the fossils, said the eggshells of both were very thin and consistent with each other in shape and form.

"From what's available there's no evidence we have two different types of eggshell here, and I would be surprised if we did," he said. "If there was clear evidence one was hard like a chicken egg and the other soft and leathery, that would be surprising to me."

Group Living

For several years Chiappe and his colleagues have been unearthing thousands of fossils from a site called Loma del Pterodaustro in central Argentina. Every fossil, with the exception of a few fish fossils, has been identified as a Pterodaustro pterosaur.

"It gets to the point where we have such a large sample, you have to wonder. How could it be that all we find here are Pterodaustro? I would say it has something to do with the environment," Chiappe said.

He concludes that the site was unsuitable to all but this flamingo-like pterosaur. The area was perhaps filled with extremely salty lakes similar to those inhabited by flamingoes high in the Andes mountains, Chiappe speculated. Pterodaustro was a filter feeder, just like the flamingo, he added. Filter feeders get food by filtering it from water.

Chiappe and his colleagues found the fossilized Pterodaustro embryo among hundreds of other fossils, ranging from young juveniles to full-grown adults. This suggests that the reptiles nested in a colony and protected their young, he said.

"They didn't lay their eggs and leave off for somewhere else like most reptiles do—lay their eggs and say, That's it, hasta la vista," he said. "In reality they guarded the nests and presumably guarded the hatchlings."

Bennett said interpretation of the site as a large Pterodaustro colony makes sense, noting that many seabirds do the same today. "In seabirds we see large colonies. It's reasonable to suppose the same with some pterosaurs as well," he said. "But there's no reason to think they all did."

The Chinese team reports in Nature that their new fossil resembles the pterosaur known as Beipiaopterus and is "longer and narrower" than the pterosaur reported from China in June. The new fossil was found in the Yixian formation, which has become famous for dinosaur discoveries in recent years.

Both Chinese pterosaur fossil embryos are of similar age and were found in similar environments. According to Bennett, this suggests that a number of different pterosaurs may have nested around this ancient lake-filled Chinese environment.

Luis Chiappe's research is funded in part by a grant from the National Geographic Society's Committee for Research and Exploration.

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