From their eggs to heaven: how did pterosaur babies take flight?

More than 300 have been discovered so far, including 16 with preserved embryos inside. But attempts to determine the stage of development reached by each embryo have been "somehow ad hoc, just look and guess," Dr. Unwin said. He and Charles Deeming, a zoologist at the University of Lincoln in England, have undertaken to standardize the process.

The pair used fossils from this site, as well as eggs and embryos from Argentina and elsewhere in China. They first looked at the length of the limbs, as well as the size and shape of the egg. The researchers found that in general, smaller and narrower eggs represent early stage embryos, while larger and rounder ones indicate a more advanced stage.

They then examined patterns of ossification or bone hardening, examining embryos and young pterosaurs, called flaplings. Overall, they compared data from specimens of nine species of pterosaurs, Hamipterus tianshanensis – of the same type in the flooded colony, which had a snout and a wingspan of up to 11.1/2 feet – to Anurognathus ammoni, about the size of a swallow and with a stocky tail.

Because bones harden in a particular order, they can serve as "markers of development," said Dr. Unwin. They then compared these models to those observed in quail and alligator, both considered as modern analogues of pterosaurs. This made it possible to sort the embryos of pterosaurs from those newly hatched to hatching.

Along the way, they noticed something about a bone, the handwritten figure IV. Equivalent to our ring finger, it is the "wing finger" of a pterosaur, the long flexible appendix that is attached to the wing membrane and allows the animal to fly. In most vertebrates, this bone is one of the last to harden. Pterosaurs, however, "ossify very early," he said.