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Pterosaurs and birds: The stupidity lives on

April 15, 2012

Azdarchid standing vigilant over the flaplings by Mark Witton.

As we have previously explored, the idea that pterosaurs were declining and being replaced by birds in the late Cretaceous is bullshit of the most equisite decaying rot of a flavour. For starters, pterosaurs did not decline. Sure, they were much less pterosaurs when the KT event hit, but it wasn’t as much a decline as a sudden die off.

When animals are facing an ecological decline via competition, they disappear over a large period of time, their diversity decreasing over millions of years. This was the fate of hyenodonts, who began declining in a matter of million years after their apex in the early Oligocene, and eventually disappeared by the middle Miocene, as carnivorans took hold of their niches. This was the fate of sphenodontians, which reached an apex in the Jurassic, but follow an agonisingly slow decline across the remainer of the Mesozoic, culminating with their complete absence from most landmasses by the Maastrichtian. This is going to be the fate of mousebirds, which were very common a mere few million years ago, but are now slowly walking to extinction.

Pterosaurs did not follow this pattern. Pterosaurs did not have a slow lost of diversity. Instead, most of their diversity went out with a bang.

As previously discussed, most pterosaur clades were victims to an ecological event known as the Cretaceous Thermal Maximum. This event wiped out nearly all pterosaur clades with the exception of neoazdarchians and euornithocheiroids (and possibly Gwawinapterus, if an istiodactylid). This event wiped out many other sauropsid taxa on water and on land, so we know it was a significant extinction event. Therefore, if birds diversified after this, it was because there were vacant niches to explore.

Except they didn’t.

Evolution of avian diversity

Gargantuavis by Hyrotrioskjan. Giant flightless birds were among the late Cretaceous novelties, but they weren’t occupying any niches left behind by pterosaurs.

If birds replaced pterosaurs, logic would dictate that we would see a wider diversity of avifauna after the CTM. This doesn’t appear to be the case, however.

In Liaoning, we see a vast menagerie of bird taxa, ranging from primitive jeholornithids to ornithurines. We have fowl like hongshanornithids, kinglerfisher like longipterygids, grouse like omnivoropterygids and even tropicbird like confusuisornithids. In short, birds were already a very diverse group, rivalling modern avifauna in terms of sheer diversity in ecological niches, and most defenitely being far more diverse in terms of taxa. These birds are not a small minority opressed by the equally diverse pterosaurs, but equals; both lineages shared the ecosystem of Liaoning, in a far more egalatarian way than the modern bird/bat dichotomy.

By the late Cretaceous, we see changes in avifauna diversity. Most of the bird clades seen in Liaoning become extinct, being replaced by more modern taxa; all non-ornithothoraces are gone, and even within these only the more derived enantiornithes and ornithurines remain. If this was a consequence of the CTM or if they were actually outcompeted by more modern birds, we may never know.

Within the remaining birds, we see familiar forms. Shorebird, fowl and sandgrouse like ornithurines are omnipresent, while passerine, falcon and osprey like enantiornithes are known. In the seas, loon and cormorant like Hesperornithes began competing with the already established marine sauropsid groups, some of them growing to the well known large flightless forms, while terrestrial ecosystems see the first ratite like birds in the form of Gargantuavis and kin.

However, we have a consistent pattern in Mesozoic avifauna, true in both before and after the CTM: the lack of large flying birds.

The late Cretaceous saw the arrival of large birds, but these were all flightless. The largest known flying birds were a few derived Euenantiornithes, most specifically within the Enantiornis + Avisauridae clade, but these didn’t reach wingspans beyond 1.2 meters. The fact that the early Cretaceous already had similar sized birds in the form of omnivoropterygids makes their feat even less impressive.

In addition, there just wasn’t that much of a new radiation of avian niches. Nearly birds we know in the late Cretaceous had analogues in Liaoning. The only exceptions occured in regards to aquatic niches, in that there is indeed a higher diversity of aquatic birds in the post-CTM Cretaceous than before. However, most of these birds are diving birds like the Hesperornithes, simply replacing ecological niches left by marine sauropsids destroyed in the CTM. Flying seabirds like ichthyornithids not only were not very diverse, but actually quite uncommon, compared to pterosaurs occupying similar ecological niches such as pteranodontids. Indeed, even in fossil sites without ornithocheiroids, seabirds are rare.

Therefore, birds were not moving into any niches left by pterosaurs. This crushes the whole premise that birds were replacing pterosaurs.

Evolution of pterosaur diversity

Azdarchoid skulls. The last specimen belongs to an azhdarchid recently discovered in North America; as you can see, it bears quite some similarities with thalassodromedid skulls.

After the CTM, pterosaur fossil reccord is primarily dominated by azhdarchids and euornithocheiroids. The first retain a stable diversity all the way to the KT event, and indeed actually become more diverse (see below); the latter are mainly present in the form of pteranodontians, which are very common since the times of the CTM itself all the way to the mid-Campanian, though the presence of pteranodontians afterwards is controversial.

As previously seen, azhdarchids were becoming diverse. As the post-CTM Cretaceous passed, they increased in terms of body types and potentially ecological niches; forms similar to thalassodromedids such as Bakonydraco are found in North America and Europe, while giants like Quetzalcoatlus and Hatzegopteryx not only became the largest flying animals of all time, but also became macropredators, analogous to theropods in terms of ecological niches. While we have a higher number of named specimens from the Santonian and Campanian, the number of unnamed remains is consistent across the late Cretaceous, and combined with their cosmopolitian distribution, we know these were very successful animals. Indeed, given the absence of unenlagiines and large troodontids in azhdarchid dominated areas, it’s perhaps possible to state that azhdarchid pterosaurs were actually replacing bird-like theropods, not the other way around. If azhdarchids had survived the KT event, it is very likely that we’d see further diversity explosions.

Euornithocheiroids, on the other hand, were on a less privileged position. Pteranodontians were very common in the million years succeeding the CTM, but by the mid-Campanian they’d all but vanished. The same applies to nyctosaurs, though they arguably never had the same success pteranodontians had, since they were much more scarce. A brazillian fossil known as “Nyctosauruslamegoi might indicate that these pterosaurs survived until the KT event, but it is possible that it belongs to a chaoyangopterid instead (see below). Why exactly these pterosaurs disappeared is not clear, since we observe little to no changes in other contemporary marine tetrapod fauna, but it might coincide with the closing of the Western Interior Seaway, that somehow damaged the habitat of these animals in some way.

Do note, however, that, even after pteranodontians and nyctosaurs die off, there’s no change in flying seabird diversity, suggesting that they did not occupy vacant niches. This might indicate that they were still around, just not preserved in the fossil reccord, or that birds simply couldn’t occupy the same ecological niche for some reason.

Most notably, these weren’t the only pterosaurs around. Ornithocheirid and possible lonchodectid remains have been found in the post-CTM Cretaceous, but how successful these animals were is not clear, nor how long they lived. Vega Island remains implies that they might have endured to the very end. Meanwhile, remains associated with chaoyangopterids have been identified as well, but I am currently waiting more information on them. From what I’ve seen so far, they appear mostly on south american fossil sites, indicating a Gondwannan refuge for pterosaur taxa, which might imply more species survived in Africa and Australia, which have pratically no post-CTM fossil sites.

Once more, I bring the speculation on anurognathids. The fact that non-pterodactyloid remains have been found in the Cenomanian greatly increases the possibility of these pterosaurs having survived unpreserved until the KT event.

Conclusion

It appears very obvious the myth that birds replaced pterosaurs is busted.

I think the appeal of this idea comes from a general dislike towards pterosaurs that media has.

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