Pterosaur diversity at the very end
I started this blog by presenting why the popular notion of pterosaur decline at the hands of birds is incorrect in the face of the evidence we have; while some points are to be corrected, the following still stand intact:
– Pterosaur loss of diversity is not the result of a steady decline, but of sudden extinction events that wiped out several other tetrapod taxa, most notably the Turonian extinction events.
– Avian diversity doesn’t seem to have claimed niches previously held by pterosaurs; most large Cretaceous birds such as hesperornithids and gargantuavids moved into niches previously held by marine reptiles and theropods respectively, and most flying birds remained relatively small. A few flying enantiornithes reached larger sizes, but they don’t seem to have occupied any new niches; seabird like enantiornithes and confuciusornithids are known from the lower Cretaceous, when pterosaur diversity was in it’s peak, while large terrestrial flying birds included the bizarre omnivoropterygids.
– Lazarus taxa situation are known among upper Cretaceous pterosaurs, most notably Gwawinapterus.
That said, what exactly was alive at the Maastrichtian? We don’t have a lot of evidence because the fossil record of the very end of the Cretaceous is extremely poor; we already have no idea about the dinosaur taxa of many landmasses, let alone pterosaur taxa.
Most of the followng is speculative work, but then again speculation is nigh-omnipresent in the study of palaeobiology.
Of all pterosaurs, azhdarchids were the ones most obviously present in the late Cretaceous, and the only ones truly confirmed to have been present in the Maastrichtian. At least five genera can safely by attributed to the Maastrichtian fossil record, and several remains unassigned to a particular genus are also known. And we know that not only they were alive, but were thriving.
Azhdarchids were very clearly cosmopolitian, having been found in nearly all landmasses but Australia (which had no upper Cretaceous fossil record) and probably South America (see below), and had virtually no known competitors; unenlagiine deinonychosaurs were the only other known animals to occupy a similar ecological niche, and they seemed to had preffered wading niches, there being a niche partitioning between both groups as in modern storks and herons. The disappearence of non-tyrannosaur large theropods from Laurasia also favoured azhdarchid expansion; the giraffe sized Quetzalcoatlus is the second (or third, depending on the validity of Nanotyrannus) largest land predator (ecologically speaking; by size alone, it was bigger than the expected adult size of Nanotyrannus) in Maastrichtian North America, with a recently discovered azhdarchid genus being the third/fourth largest predator, and the european Hatzegopteryx and Bakonydraco were the undisputed apex predators of their island habitats.
Indeed, the upper Cretaceous was the golden age of azhdarchid pterosaurs, which were fairly low key species until the Turonian, indicating that they replaced their closest relatives like chaoyangopterids and thalassodromedids. Even if other pterosaurs might have had an unfortunate fate, the adaptability and success of these terrestrial predators suggests that, should the KT event never had occured, azhdarchids would probably have kept diversifying. The fact that these pterosaurs competed directly with theropod dinosaurs, avian and non-avian, suggests a possible eventual advantage over their dinosaurian contemporaries, much restricted by their flightlessness. That there are no large predatory birds in the Cretaceous seems also to imply that azhdarchids filled this niche rather well, the only other bird of prey analogues being volant deinonychosaurs like microraptorines.
Whereas azhdarchids are clearly present in the Maastrichtian, the other iconic group of upper Cretaceous pterosaurs, the ornithocheiroids, are a subject of extreme controvery.
These pterosaurs, mostly represented by the Campanian pteranodontians, as well as several possible ornithocheirid/boreopterid remains and the istiodactylid Gwawinapterus, are virtually absent from the Maastrichtian except for a possible nyctosaurid fossil. This seems to be suggestive of their demise at the Campanian/Maastrichtian boundary, which did claim several american dinosaur taxa.
In the case of pteranodontids, this might be the case. These pterosaurs seem to have been almost exclusively north american; a possible nyctosaurid from the Barremian was found in Romania, the possible genus Ornithostoma is known from Britain and Asia, and “Nyctosaurus” lamegoi was found in Brasil, but none of these fossils have been established to be pteranodontians. Many supposed pteranodontian remains have turned out to be azhdarchids or chaoyangopterids, something that can only be the result of lack of diligence at pterosaur examination, since the expected anatomical disparity between both groups, as to be expected from animals with different lifestyles.
Thus, unless we’re lucky and “Nyctosaurus” lamegoi, Ornithostoma and the romanian remains are in fact pteranodontians, we’re looking at clade that was appearently restricted to the Western Interior Seaway that split apart North America during the Cretaceous. While there is no indication that this seaway closed until the Paleocene, the relatively restricted range would had made them vulnerable to local extinction events; on the other hand, unlike the distinct decline of terrestrial dinosaurs, there doesn’t appear to be an equivalent event in the marine ecosystems of the Western Interior Seaway. Indeed, from the fossil record associated with the Maastrichtian, the marine ecosystems seem to have remained fairly conservative in terms of tetrapod fauna; the present avifauna was still dominated by the dolphin like hesperornithes, with no larger variation of aerial seabirds (in fact, the iconic ichthyornithids are every bit as absent as pteranodontians, with the only evidence for their presence being teeth, which might belong to hesperornithes instead).
Pteranodontian survival, therefore, depends on whereas the group occured outside of the Western Interior Seaway, and even then mere lack of preservation might had ensued.
Other ornithocheiroids, however, are less ambiguous. Istiodactylids are present as the only canadian pterosaur Gwawinapterus, which lived in the late Campanian. There is a gap of over 45 million years between this animal and it’s closest relatives, indicating that at least istiodactylids are prone to Lazarus taxa situation, having avoided being preserved in the fossil record. This makes sense; unlike pteranodontians, istiodactylids lived inland, being vulture like scavengers, and unlike azhdarchids they were dedicated soarers, spending most of their lives on air. Therefore, lack of preservation is only normal; in fact, sans Gwawinapterus and Istiodactylus, all istiodactylids have only occured in lagerstätten. Thus, I think it is safe to say that istiodactylids were defenitely present, although north american species might had disappeared during the slight extinction event that decimated half of North America’s dinosaurs.
But pteranodontians and istiodactylids were not the only ornithocheiroids to have in consideration.
Several supposed ornithocheirid remains have been recovered from the post-Turonian Cretaceous; I’m not exactly familiar on how common they are on even on where they occur (european and moroccan remains seem appearent, some of which found alongside azhdarchids, but I’ve also heard of Ross Island ornithocheirid fossils), and since these animals were cosmopolitian their range could easily had extended into the Maastrichtian, not being limited and occuring in areas with relative ecological stability.
Indeed, the issue is not whereas these pterosaurs were present; it’s whereas they were ornithocheirids or boreopterids.
According to Mark Witton, the similarity between ornithocheirid and boreopterid anatomy leads to the possibility that many so called ornithocheirids are in fact boreopterids instead. One such case is with Aetodactylus, the supposed texan ornithocheirid. The paper describing it by Myers essencially rendered the animal the sister taxon to Boreopterus; given that we now know that boreopterids are not ornithocheirids, it seems that Aetodactylus probably should be included within Boreopteridae.
Overall, boreopterids would have higher chances of surviving than ornithocheirids, occuring primarily in freshwater environments while the latter were generally marine (although some ornithocheirids did occur in freshwater deposits). However, they would had been less likely to not be preserved, although the post-Turonian Cretaceous fossil record of many landmasses, such as Africa, is almost non-existent.
Many pterosaur taxa are restricted to lagerstätten fossil sites. Pterosaurs, like birds, have hollow bones, making preservation harder, but while birds still tend to have quite some bone density (specially in the hindlimbs, as bipedal take off demands), pterosaurs were capable of extending pneumatisation, to the point that, in most pterodactyloids (the exceptions being the dsungaripteroids, which had robust bones), the bone walls were less than a milimeter thick. In life, the bones would have remained solid thanks to the internal honeycomb structure, but after death disintegration would be common. Large pterosaurs like azhdarchids managed to be preserved thanks to their sheer size, but several other pterosaur taxa were only found in lagerstätten fossil sites, where the bones could had been preserved.
Among such taxa are ctenochasmatoid pterosaurs, which were generally small, filter feeding pterosaurs unheard off from non lagerstätten fossil sites except for the largest species. Notably, filter feeding birds are only known from the Cenozoic; some presbyornithids are known from the late Cretaceous of Mongolia, but it is not known if they were dedicated filter feeders, and in any case they were most likely did so facultatively, being omnivores like modern ducks, while ctenochasmatoids were most likely flamingo like in habits, being specialised in this type of lifestyle. It is possible that the less epcialised filter feeding boreopterids replaced ctenochasmatoids, however.
More likely, however, are anurognathid pterosaurs. These pterosaurs are already considered the most basal pterosaurs known, and are suspected of being a ghost lineage dating back to the mid-Triassic. Being so small and fragile, they are only known from only the most exceptional of lagerstätten, some of which dating to the Cretaceous, and it is very likely that they survived undetected for the remaining of the Mesozoic. Being small insectivores, there would be little to no reason for them to become extinct until the KT event. Cenomanian non-pterodactyloid pterosaur remains have been found; while these don’t belong to anurognathids, they show that non-pterodactyloid pterosaurs survived well into the Cretaceous, and therefore anurognathids likely didn’t go extinct until the asteroid impact destroyed the established terrestrial ecosystems.
As previously noted, many remains of supposed Maastrichtian non-north american pteranodontians have been found. However, many supposed lower Cretaceous pteranodontians have been reclassified as chaoyangopterids. Therefore, rather than Maastrichtian pteranodontians, there could had been Maastrichtian chaoyangopterids.
Turonian chaoyangopterids were found in Africa, and there is a poor, if non-existent, fossil record of azhdarchids in South America. Therefore, it is possible that chaoyangopterids retained dominance in gondwannan landmasses, while azhdarchids dominated Laurasia, and “Nyctosaurus” lamegoi is the possible representative of such a clade of upper Cretaceous chaoyangopterids.
The list of pterosaurs probably present at the KT event.
Azhdarchidae (established to be present)
Istiodactylidae (inferred by ecological role/Lazarus taxa situation)
Ornithocheiridae/Boreopteridae (evidence present, but no set classification)
Pteranodontia/Chaoyangopteridae (evidence present, but no set classification; possibly azhdarchids instead)
Ctenochasmatoidea (inferred by ecological role/preservation patterns)
Anurognathidae (inferred by ecological role/preservation patterns/ghost lineage situations)