Egg helps determine gender of 160-million-year-old reptile fossil

A rare discovery in China of a nearly complete fossilized skeleton of a pterosaur – along with a never-before-seen pterosaur egg – has allowed scientists to assign gender to a fossil specimen of this species for the first time. What’s more, details of the egg itself provide evidence that pterosaurs reproduced as reptiles do, rather than as birds do, which had been widely assumed prior to this discovery.

The fossil – brought to scientists’ attention by a Chinese farmer – is approximately 160 million years old. The fossil shows the skeleton of the pterosaur preserved alongside her egg. Paleontologists reported the discovery in late January 2011 in the journal Science.

Artist\’s concept of a pterosaur

Pterosaurs were flying reptiles that lived and went extinct alongside the dinosaurs. But these creatures were not exactly dinosaurs. That term is reserved for a certain group of reptiles with an upright stance. Instead, pterosaurs – sometimes called pterodactyls – were flying reptiles, the earliest known to fly under their own power.

The fossil has been dubbed “Mrs. T,” or less cleverly, M8802, and she tells an interesting story, according to the study. The paleontologists believe that the expectant pterosaur fractured her forearm and became unable to fly. She apparently plunged into a body of water. She drowned and sank to the bottom, and as her body decayed, the egg was somehow expelled from her body.

Image courtesy of Science/ AAAS

In the photo, you can see that the oval-shaped egg is right below the fossil’s pelvis (pointed out as “ie” in image B). The scientist believe that the egg’s closeness to the body, a very rare event, makes the specimen unmistakably female. This means that male pterosaurs had small pelvises but large crests, which are interesting-looking extensions of bones on top of their heads. Female pterosaurs had wider hips, for reproductive purposes, but no fancy crests, as shown in the photo below.

Image credit: Mark Witton, University of Portsmouth

Looking at the egg, the scientists discovered it likely started out with soft surface, somewhat like parchment. In that way, the egg might have resembled reptilian eggs, which have leathery surfaces. The softness of this pterosaur egg suggests that – like reptiles – pterosaurs might have buried their eggs and left them. If so, the eggs would have soaked up water and nutrients from the ground, and the newborn pterosaur would have emerged relatively well developed, maybe even able to fly out of the nest, according to scientists.

Of course, it didn’t work out like that for Mrs. T.’s egg, but now it seems – for our human purposes at least – this ancient pterosaur and her egg perished for a good cause. Their 160-million-year-old remains have allowed paleontologists to crack open some of the mysteries of pterosaur reproduction.

February 5, 2011

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