After the discovery of at least 4 and maybe 5 pterosaur eggs, now we know their pattern. Based on pelvic diameter, we know the maximum diameter of an egg. The length and shape varies: longer for long-snouted taxa. Hatchlings were 1/8 the size of adults, based on egg size and the example of the embryo Pterodaustro. Hatchlings were virtual copies of adults (contra traditional thinking).
A Hypothetical Egg for the Largest of all Pterosaurs
Quetzalcoatlus, the largest pterosaur, would have laid the largest pterosaur egg. Figure 1 portrays the Q. sp. smaller version and a 2.3x larger hypothetical Q. northropi pelvis associated with a 0.12x hatchling tucked into an egg shape. It’s no surprise that the hypothetical diameter of an egg that would contain the hatchling exactly matches the pelvic opening. The egg of Quetzalcoatlus would have been elongated to contain the elongated skeletal elements. Within the pelvis, such an egg would have extended anterior to the prepubes.
Figure 1. Quetzalcoatlus northropi and Q. sp. to the same scale alongside hypothetical eggs and hatchlings one-eighth as tall as the adult in each case. Q. northropi was 2.3x as large as the smaller, more complete, unnamed species. This image is to the same scale as the ostrich in Figure 2.
Compared to the Ostrich
The largest living bird, the ostrich, provides some comparison. The ostrich is the largest bird and it produces the largest egg, but that egg is the smallest relative to adult size. Figures 1 and 2 are to the same scale. The large Quetzalcoatlus egg would have been smaller in diameter than the ostrich egg, but 2.5x longer. Longer eggs are possible when the shell is extremely thin and relatively pliable, like those of snakes and lizards.
Figure 2. The ostrich is the largest bird and it produces the largest egg, but it is the smallest egg relative to the adult size.
Hypothetical Details of the Quetzalcoatlus Embryo
With the examples of other pterosaur eggs, we should expect the proportions of the Quetzalcoatlus hatchling to match those of the parent. In order to cram in the long beak, long, stiff neck and elongated metacarpals, the containing egg has to be long. As in other reptiles, the jaws would have been tipped down, pressed against the ventral neck. The eyes would have been relatively no larger than in the adult. The jaws would have been no shorter than in the adult. The legs would have been tucked up against the torso and the feet would have been hyperflexed.
Figure 3. Quetzalcoatlus embryo and egg. The elongated shape and soft, thin shell were needed to encompass the elongated beak, neck and metacarpals.
The Benefit of Being a Lizard
As lizards, pterosaurs could have retained their eggs within the mother until embryonic development was complete. Perhaps only one was produced at a time. A hatchling would have been large enough and well-developed enough (following the pattern of several smaller pterosaur embryos) to be able to fly.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
References
Kellner AWA and Langston W 1996. Cranial remains of Quetzalcoatlus (Pterosauria, Azhdarchidae) from late Cretaceous sediments of Big Bend National Park, Texas. – Journal of Vertebrate Paleontology 16: 222–231.
Lawson DA 1975. Pterosaur from the latest Cretaceous of West Texas: discovery of the largest flying creature. Science 187: 947-948.
wiki/Quetzalcoatlus
Tetrapod Zoology blog; Why-azhdarchids-were-giant-storks
The Pterosaur Heresies 
Birds obtain oxygen thanks to extremely efficient lungs. Did pterosaurs have similar lungs or could they have used their large and thin wing membrane to oxygenate blood, like I hypothesize bats do?
Hollow, pneumatic bones suggest that air sac extensions of the lung invaded their cavities.
With regard to bats, if you see them acting like gills, that should already be in the literature. I would Google that.
It seems more logical that the hollow bones transported oxygen from the membranes to the flight muscles than from the lungs to the membranes without muscles.
Flying requires an enormous amount of energy. the bat Pallas has the fastest metabolic rate of any mammal (including shrews), requiring a lot of oxygen. Mammalian lungs simply cannot supply such large amounts of oxygen, being far less efficient than bird lungs. For example, the Mexican free-tailes bat can reach 60 mph for a few seconds and its lungs simply cannot provide the huge amount of oxygen required for that much power. Their low wing loading (relative to birds) may perhaps be due to respiration. During flight the underside is at a higher pressure, enhancing oxygen uptake and the top side is at a lower pressure, enhancing CO2 release.
Your arguments make perfect sense, Sam. A quick google revealed this report: http://www.ncbi.nlm.nih.gov/pubmed/17971117 that concludes, “…in [the bat] Epomophorus wahlbergi, the wing web has structural modifications that permit a substantial contribution to the total gas exchange.”
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