The Biggest Pterosaur
There are a few questions to resolve about which pterosaur qualifies as the biggest. It is definitely a member of the Azhdarchidae and it is probably Quetzalcoatlus northropi with a wingspan of up to 12m (39 feet).
A Russian pterosaur specimen, Azhdarcho lancicollis, is close to the basal form of the Azhdarchids and that is a big pterosaur. However, it is known only from partial remains and they are not sufficient to give a good estimate of the wingspan of large individuals. Azhdarcho may have had a wingspan in excess of 10m.
Quetzalcoatlus is a Late cretaceous form, mostly identified from US specimens and they are generally of good quality, covering almost all of the skeleton in detail. The present reconstruction of Quetzalcoatlus is considered to be very good.
Pteranodon ingens is probably the most studied of the big pterosaurs and that has a wingspan up to 7m, so it is not a contender for the biggest pterosaur. Geosternbergia sternbergi is another pteranodontid with a wingspan up to 9m.
Quetzalcoatlus and Spitfire to scale
With a wingspan approaching 12m, Quetzalcoatlus must be close to the limit of what is possible in a flying animal. It is unlikely that a larger pterosaur will be found. Such animals would be limited in their flying ability close to the ground and probably only took to the air when they needed to. It is thought that Quetzalcoatlus was a scavenger, very much like a modern day Vulture.
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Nesov L. A., 1984, Pterozavryi i ptitsyi pozdnyevo myela sryednyei Azii (Upper Cretaceous Pterosaurs and birds from central Asia). Paleontologisheskiy Zhural. (1) pp. 47–57.
Averianov A. O., 2010, The osteology of Azhdarcho lancicollis NESSOV, 1984 (Pterosauria, Azhdarchidae) from the late Cretaceous of Uzbekistan. Proc. Of the Zool. Inst. RAS vol. 314, No. 3, 2010, Pp 264-317.