Pterosaurs were reptiles, and the first creatures - after insects - to evolve powered flight, meaning they flapped their wings to stay aloft instead of simply jumping and gliding. They went extinct along with the dinosaurs some 66 million years ago.
Until now, scientists had found some pterosaur eggs with remains inside, including three in Argentina and five in China.
But the latest report in the peer-reviewed US journal Science is based on the biggest collection to date - 215 fossilized eggs that were found in a three-meter (yard) long sandstone block in northwestern China's Hami City, Xinjiang Uygur Autonomous Region.
Sixteen of the eggs contained fossilized remains of a pterosaur species known as Hamipterus tianshanensis.
None held a full set of pterosaur bones, likely because pieces were lost over the years due to storms and floods.
But scientists did find partial wing and skull bones, along with one complete lower jaw, which fill in aspects of pterosaurs' life cycle that has been poorly understood until now.
Using three-dimensional computed tomography scans, they discovered intact and well-developed thigh bones suggesting the creatures "benefited from functional hind legs shortly after hatching," said the report by paleontologists in China and Brazil.
But weak chest muscles indicate "that newborns were likely not able to fly" and "likely needed some parental care."
Sixteen of the eggs contained fossilized remains of a pterosaur species known as Hamipterus tianshanensis. Source: museunacional.ufrj.br
Nesting behaviour
Adult pterosaur bones were also scattered across the site, a sign that they returned to the same nesting spots over time, much the same as modern day sea turtles.
The massive numbers of eggs and bones point to major storms thrashing the site, submerging the eggs in a lake where they floated briefly before sinking and becoming buried alongside adult skeletons.
Researchers also noted that the cracked exterior of the eggs resembled the fragile softness of lizard eggs.
"All are deformed to a certain extent, which indicate their pliable nature," said the study.
One of the young pterosaurs was estimated to be "at least two years old and still growing at the time of its death, supporting the growing body of evidence that pterosaurs had long incubation periods."
An accompanying Perspective article in the journal Science, written by D. Charles Deeming of the University of Lincoln, called the study "remarkable for the number of eggs in association with adults and juvenile pterosaurs that it reports on."
However, many questions remain, including whether the size of each clutch was really two as previous studies have suggested, just how the pterosaurs concealed their eggs, whether beneath vegetation or sand or soil, and why so many of the eggs appear dehydrated.
"Hopefully additional finds of equally spectacular fossils will help us answer such questions," he wrote.
