Just a few hours after yesterday's post on dinosaur embryos went up, another major egg-based discovery was announced, in the journal Science.
In October of 2009, paleontologists first described the flying reptile Darwinopterus, a pterosaur that lived in what is now China over 160 million years ago. Since then, multiple other specimens have been found, including a well-preserved specimen purchased by the Zhejiang Museum of Natural History from a local farmer. This slab, given the designation M8802, preserves the nearly complete skeleton of this pterosaur, but what makes it truly remarkable is that it also contains a pterosaur egg.
The egg can be seen directly behind the pelvis of the Darwinopterus specimen—confirming that this individual was a female—but this pterosaur was not fossilized in the act of egg-laying. Her skeleton tells of a more tragic end. Based upon a break in the skeleton and the way in which the slab formed, the scientists state:
We suppose that this individual experienced a violent accident that fractured the forearm, rendering the pterosaur incapable of flight and precipitating her into a water body. After this, she drowned, her carcass became waterlogged, sank to the bottom, and, as decay processes began, the egg was expelled from her body.
It is rare that we can glean such stories from the fossil record, but the real significance of this discovery is that it gives us a few new insights into the biology of these long-dead animals. For one thing, this find may allow paleontologists to determine the sex of these pterosaurs. The hips of M8802 and another specimen from a different museum (YH-2000) are wide and have a relatively large canal that would have allowed the egg to pass through. Other specimens, which may be males, have narrower hips, and this is consistent with the idea that females would require larger hips to lay eggs.
The difference between the sexes may be apparent in the headgear of these pterosaurs, as well. Both of the female specimens lacked crests on their heads, while the narrow-hipped, possibly male specimens had crests. A larger sample size will be needed to test this idea, but the presence of crests does seem to be associated with the more narrow-hipped specimens.
As for the egg itself, close examination showed that it had a leathery shell. It did not have a hard, mineralized outer coating like a chicken or dinosaur egg. Rather than being closed off from the outside world, then, the paleontologists state that the pterosaur egg would have increased in volume by taking up water after being laid.
This same mode of reproduction is seen among lizards and snakes today, and the authors of the new paper argue that, like these reptiles, pterosaurs did not provide care for their newly-hatched young. Rare pterosaur hatchlings show that they were well-developed very early on and may have been able to fly out of the nest quickly, although these inferences about behavior can only be approached indirectly for the moment. Further discoveries will inform our still-growing understanding of how these flying reptiles reproduced.
Lu, J., Unwin, D., Deeming, D., Jin, X., Liu, Y., & Ji, Q. (2011). An Egg-Adult Association, Gender, and Reproduction in Pterosaurs Science, 331 (6015), 321-324 DOI: 10.1126/science.1197323