Over 300 have been found there so far, 16 with embryos preserved inside. But attempts to figure out the stage of development each embryo had reached have been “kind of ad hoc — just look-at-it-and-guess,” Dr. Unwin said. He and Charles Deeming, a zoologist at the University of Lincoln in England, set out to standardize the process.

The pair used fossils from that site, along with eggs and embryos from Argentina and elsewhere in China. They first looked at limb lengths, along with egg size and shape. The researchers found that in general, smaller, narrower eggs represent early-stage embryos, while larger and rounder ones indicate a later stage.

Next, they examined patterns of bone ossification, or hardening, looking at embryos along with young pterosaurs, called flaplings. Overall, they compared data from specimens of nine pterosaur species, from Hamipterus tianshanensis — the type in the flooded colony, which had a crested snout and a wingspan of up to 11- 1/2 feet — to the swallow-sized and stubby-tailed Anurognathus ammoni.

Because bones harden in a particular order, they can serve as “developmental markers,” Dr. Unwin said. They then matched these patterns to those observed in quails and alligators, both considered modern analogues of pterosaurs. This helped to sort the pterosaur embryos, from newly laid to about to hatch.

Along the way, they noticed something about one bone, the manus digit IV. Equivalent to our ring finger, this is a pterosaur’s “wing finger,” the long, flexible appendage that is attached to its wing membrane and allowed the animal to fly. In most vertebrates, that bone is one of the last to harden. Pterosaurs, though, “ossify it very early,” he said.