Azhdarchid pterosaurs are known from the Late Cretaceous mostly of North America, Europe, and Africa, with fossils found in Alberta, Montana, Texas, Romania, the Baltics and more. Several genera are known ranging from 2.5 m wingspan Montanazhdarcho all the way up to 10-11 m Quetzalcoatlus and Hatzegopteryx.
Pterosaurs have been shown to increase in size over the Cretaceous, with the largest pterosaurs ending up at the end of the Cretaceous in the azhdarchids. In the mid-Jurassic, birds started to appear, giving the pterosaurs competition in the skies. Some studies have suggested that both groups were able to survive by occupying different niches [1], others have shown that pterosaur diversity did not decrease until the Late Cretaceous so this was unlikely to be caused by birds [2], while recently it has been suggested that birds outcompeted small pterosaurs, driving them to the gigantic sizes seen in Late Cretaceous azhdarchids [3]. Of course, this is not agreed upon, and other studies suggest it's a preservational bias that is keeping the small Late Cretaceous pterosaurs from being preserved, not that they didn't exist at all [4].
There is some evidence of smaller pterosaurs from the Late Cretaceous including those of Montanazhdarcho and Eurazhdarcho, but also some smaller fragments around that point to some very small pterosaurs. Another small pterosaur is one called Piksi, known from small fragments from the Two Medicine Formation. First described as a bird, only a few small wing fragments have been found including the distal humerus, and the proximal radius and ulna [5]. It had an estimated wingspan of just 1 m, and is thought not to be an azhdarchid, but an ornithocheiroid, a group not really present in the Late Cretaceous. This is the first example of a possible small pterosaur in the Late Cretaceous, but there are also potential examples of small pterosaurs from other parts of the world, but no clear examples yet.
While small pterosaurs are obviously not common in the Late Cretaceous, there do seem to be some examples which suggest there is something else going on than just them being outcompeted by birds. Another interesting fact is that there are no confirmed records of azhdarchid juveniles from this time either. Obviously for there to be giant pterosaurs, they have to be juvenile at some point. While there are reports of other pterosaur juveniles at different time periods around the world (e.g. "Nemicolopterus", Pterodactylus, Hamipterus) and even embryos (like Pterodaustro), we would expect to find juvenile azhdarchids, yet these haven't been found. Often in the fossil record we find that that are biases in what is preserved and what isn't, and these biases change between environments, throughout time, and in different localities. One such bias is known from Dinosaur Provincial Park in Alberta, where azhdarchid pterosaurs are known from. Very few small dinosaurs or animals in general are found in this area, and this is especially seen in juveniles. Juveniles are extremely rare. If this is known from other areas and taxa, why not for pterosaurs?
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| "Nemicolopterus" which may be a juvenile Sinopterus, a tapejarid from the Lower Cretaceous of China. Image copyright John Conway. |
The fact that no juvenile Late Cretaceous pterosaurs have been found makes me extremely reluctant to agree that no small pterosaurs existed during this time. There must be some kind of preservational bias keeping small pterosaurs and juveniles from being preserved. As we find more localities and more fossils, more evidence is suggesting that there were more than just giant pterosaurs around in the Late Cretaceous, we just need to find them!
Next week I'm off to Romania on the hunt for these tiny pterosaurs (and the big ones too)... Wish me luck!
References
1. McGowan, A. J., and Dyke, G. J. 2007. A morphospace-based test for competitive exclusion among flying vertebrates: did bats, birds and pterosaurs get in each other's space? Journal of Evolutionary Biology 20: 1230-1236.
2. Butler, R. J. et al. 2009. Estimating the effects of the rock record on pterosaur diversity patterns: implications for hypotheses of bird/pterosaur competitive replacement. Paleobiology 35: 432-446.
3. Benson R. B. J. et al. 2014. Competition and constraint drove Cope's Rule in the evolution of giant flying reptiles. Nature Communications 5: 3567.
4. Vremir, M. et al. 2015.A medium-sized robust-necked azhdarchid pterosaur (Pterodactyloidea: Azhdarchidae) from the Maastrichtian of Pui (HaĊ£eg Basin, Transylvania, Romania). American Museum Novitates 3827: 1-16.
5. Agnolin F. L. and Verrichio D. 2012. Systematic reinterpretation of Piksi barbarulna Varricchio, 2002 from the Two Medicine Formation (Upper Cretaceous) of Western USA (Montana) as a pterosaur rather than a bird. Geodiversitas 34:883-894.