Fossil, physiological, and genetic evidence discovered over the past decade has shown that birds are probably the descendants of dinosaurs. As a result, the nature of the relationship between pterosaurs, reptiles, and modern birds has grown extremely controversial, with no scientific consensus in sight. Wading into this controversy, a 2011 study has shown that pterosaur eggs had more in common with reptile eggs than with bird eggs. Thus, they may be more closely related to some modern reptiles than to modern birds.
Differences between reptile eggs and bird eggs
One of the defining characteristics of birds is that all birds lay hard-shelled eggs. These calcereous shells consist primarily of calcium. They are almost completely impermeable to water, which keeps the egg from dehydrating. However, they are permeable to gas exchange, which allows oxygen to enter the egg and carbon dioxide to leave the egg.
Not all reptiles lay eggs. For example, rattlesnakes always give birth to live young. To make things even more confusing, some mammals also lay eggs. The eggs of monotremes, such as the spiny anteater, are usually soft-shelled when they are first laid, which grow more rigid over time.
Based on the properties of their eggs, egg-laying reptiles can be divided into 3 groups. The first group lays flexible, leathery eggs with little or no calcereous layer. The second group lays eggs which are harder but less flexible.
The third group is the crocodilians, which lay eggs with calcereous shells comparable to bird eggs. Unlike bird eggs, crocodilian eggs tend to develop microcracks which increase their permeability to water.
However, crocodilians represent an early branch away from the rest of the pterosaur-dinosaur-bird family tree. Thus, reptiles which lay soft eggs are more closely related to birds than reptiles which lay hard eggs.
There are also several other differences between bird eggs and reptile eggs. All birds lay eggs which are larger compared to the adult than reptile eggs. As a result, most reptiles lay more eggs per clutch than birds do. Most reptiles don't stay around after they lay their eggs, while most birds stay with the eggs and young well after they hatch.
In most cases, the temperature at which a reptilian egg incubates determines the sex of the offspring. In birds, the sex of the offspring is independent of the incubating temperature. However, this kind of difference would be nearly impossible to identify from the fossil record alone.
How pterosaur eggs measure up
Pterosaurs laid soft-shelled eggs with a parchment-like eggshell. These eggs were highly permeable to water. This in itself is clearly reptilian, and not birdlike at all.
To make the case in favor of reptiles even stronger, the size ratio of the pterosaur egg to the adult is similar to that of reptiles. By comparison, the pterosaur egg is half the size it would be if it conformed to typical bird size ratios.
Thus, egg analysis provides strong evidence that pterosaurs were more like reptiles than birds. Thus, most scientists agree that pterosaurs are the distant cousins of birds, and not their direct ancestors.