Mrs T and her egg (Image: Junchang Lü/David M. Unwin and colleagues/Science)

A spectacular fossil apparently of a pterosaur and its egg may help to unlock the mystery of how the winged reptiles reproduced. The fossil’s discoverers think it provides evidence that the beasts produced clutches of young and provided no parental care – essentially suggesting that hatchlings could fly.

Pterosaurs were the first vertebrates to take to the air, first appearing in the fossil record some 220 million years ago in the late Triassic period. Before their demise 65 million years ago the group evolved to include the largest flying animals ever to live – some had a wingspan of 10 metres.

Yet much remains unknown about their biology and behaviour. Birds and crocodiles are pterosaurs’ closest living relatives, but the three groups have diverged enormously. Fossils had already shown that pterosaurs laid eggs – but few have been found, and we have no direct evidence of how they raised their young. The new fossil provides some tantalising clues, however.


The end of Mrs T

Found in China, it is the remains of a Darwinopterus pterosaur – a genus with a wingspan of up to 1 metre. The new specimen – dubbed “Mrs T” or “Mrs Pterodactyl” – fractured a wing about 160 million years ago and sank to the muddy bottom of a lake. There the corpse’s decay apparently caused it to expel a single egg, says David Unwin of the University of Leicester, UK. Sediment then buried the pterosaur with the egg between its legs (see picture).

The egg’s width matches that of the pterosaur’s pelvis, says Unwin – evidence that the animal could have laid the egg, and that the “mother” and egg didn’t simply coincide by chance on the lake bed. Further analysis revealed that the eggshell is not composed of calcium carbonate as birds’ eggs are, although calcified fossils in the surrounding sediment show that conditions were suitable for preserving such material. Unwin says this is strong evidence that the shell was flexible and parchment-like, like those of lizards and other scaled reptiles, confirming earlier theories.

He and co-author Junchang Lü of the Chinese Academy of Geological Sciences in Beijing, China, also note that the pterosaur egg is relatively small. They estimate the pterosaur weighed 110 to 220 grams and the egg 6 grams. Modern birds of a similar weight typically lay eggs two to three times heavier.

Unwin suggests that pterosaurs, like modern reptiles, may have buried many small eggs in ground where moisture could seep through their parchment-like eggs during incubation, nearly doubling their mass before hatching.

“When pterosaurs hatched, they were tiny versions of adults,” Unwin told New Scientist, and “probably” could fly and survive on their own. In contrast, modern birds hatch with undeveloped wings, and require parental care to survive.

Where’s the crest?

Mrs T may crack another mystery of pterosaur sex – how to tell the males from the females. Some pterosaurs had crests on their heads, but others didn’t. The 30 Darwinopterus fossils so far identified by researchers in China show a sharp distinction between two types – one with large crests and one without.

A piece of Mrs T’s skull is missing, but enough remains to show that she had no crest, says Unwin. He takes that as evidence that the crests were sexual displays for males, and notes that the crested Darwinopterus fossils tend to have small pelvises, as expected for males.

Kevin Padian of the University of California, Berkeley, remains to be convinced on either count, however. He points out that crest size might have changed as the pterosaurs grew, as the skull ornamentation on horned dinosaurs did: Mrs T might lack a crest because she was young, not because she was female.

“It is not unusual for animals to be able to reproduce before being fully grown,” he says. “Everything does that, except small mammals and small birds.”

Padian also doubts pterosaurs could have laid many eggs. Mrs T’s egg “was taking up the entire body cavity not occupied by the lungs”, he says, so the pterosaur probably laid only one egg a time – although whether this influenced the amount of parental care invested in the young remains unclear.

Journal reference: Science, DOI: 10.1126/science.1197323

When this article was first posted, we incorrectly said that researchers had identified 30 to 40 Darwinopterus fossils in China