Ex Nihilo Pterosaurs

On all my years of trying to figure out what the hell was going on with pterosaurs in the last few million years they were on this earth, a question occasionally pops up in my mind: where did Pteranodon came from?

This is a very iconic THE pterosaur to be sure, but it seems very odd when you look at the bigger picture. Through most of the Cretaceous, the dominant carnivorous/piscivorous pterosaurs were “ornithocheirids”*, which lasted for over 55 million years until they disappeared in the Cenomanian. Then, almost as soon as they kick the bucket, two brand new clades of close relatives show up: the toothless pteranodontids, and the also toothless but also wing claw-less nyctosaurids. These would take over the fish-eater business, though juveniles might have lived on land.

*Yes, I’m aware of all that taxonomic mess, I’m usuaing it as a shorthand for “is related to Pteranodon and has teeth”

With the other major lineage of Late Cretaceous pterosaurs, the azhdarchoids (azhdarchids, tapejarids like Bakonydraco and Caiuajara and probably even chaoyangopterids like Argentinadraco), we get a clear origin in the Lower Cretaceous, and frankly they easily mesh with the earlier pterosaur faunas, rich in azhdarchoids. But we don’t really have anything resembling a Lower Cretaceous antecedent of pteranodontians: they just magically poof into existence in the Turonian.

Putative Antecedents

There are exactly two Lower Cretaceous taxa that might, MIGHT be pteranodontians. The oldest is MTCO 17.738/1628 from the Berriasian of Romania, a humerus that has been compared to that of nyctosaurids. If this is the case then this represents one hell of a ghost lineage of at least 60 million years between it and the first unambiguous nyctosaurids in the Campanian. Pterosaur ghost lineages are far from uncommon and I suspect we’ll find more Late Cretaceous taxa on the vein of ctenochasmatoids or even anurognathids, but this is still exceptionally incredible, especially if nyctosaurids really are specialised marine soarers.

The other taxon is Ornithostoma, an animal that has more or less ping-ponged between being considered an azhdarchid or a pteranodontian but seems confidently recovered as a pteranodontid or a stem-pteranodontian in several latest phylogenies.

Ornithostoma is known from pretty fragmentary remains but it seems to already have a toothless beak, which checks out on it being an ancestor to Pteranodon. If this identification is correct, then it shows pteranodontians already lost their teeth in the Lower Cretaceous much like azhdarchoids did. Much like with them, an intermediary form between “toothed pterosaur” and “toothless pterosaur” remains to be found, suggesting that the process was probably fairly quick.

The fact that putative pteranodontians are found far back might justify the usual trend in phylogenetic studies to diverge them from the various “ornithocheirid” pterosaurs rather than nest them deeply among them as one might expect.

Ascent

Given their overall rarity one can perhaps expect them to have been fairly rare members of pterosaur faunas until the Cenomanian turned their fortunes, likely killing off their “ornithocheirid” relatives. Pteranodontids and nyctosaurids alike were quick to expand into the vacant niches; pteranodontids seem to have had some initial success, given the iconic Pteranodon and co from the Campanian, but nyctosaurids quickly picked off the slack and became the dominant non-azhdarchids during the Maastrichtian, with North African beds showing 3 nyctosaurid genera and one solitary pteranodontid.

Typically these animals are invisioned as pelagic soarers. Pteranodon has abundantly been proven to be a piscivore with massive wings akin to an albatross, while Nyctosaurus was an extreme animal with no fingers aside from the wing finger that likely spent most of its life on the wing like a frigatebird; the same also likely applies to their Maastrichtian relatives mentioned above, barring Alcione which had short wings and was probably a diver like an auk. This hardly sounds like something that would be more apt to survive an extinction event than he less specialised “ornithocheirids”, yet that’s how it happened.

Ornithostoma at least appears to be a fairly small, possibly generalised animal, perhaps like a seagull or maybe not even marine at all. This could explain why pteranodontids at least survived, and there’s the added adage that juvenile Pteranodon were non-marine, likely occupying ecologial niches on land. This would make pteranodontids deceptively specialised: only adults lived a seabird-like ecology, with juveniles likely being generalists, basically like a flying version of the crocodile lifecycle from insect eater to aquatic carnivore. I’m quite willing to bet that the mysterious Piksi barbarulna was a juvenile pteranodontid.

This, however, does not explain the apparent longevity of nyctosaurs, which not only seem to be more specialised but, in Nyctosaurus‘ case at least, taking to life at sea early on. We can perhaps be generous and assume nyctosaurids were more diverse than previously imagined, given the sheer amount of North African taxa, some of which in dissimilar ecologies like Alcione. Hell, I’m starting to wonder if nyctosaurids were as specialised as typically believed: their most notable feature, the loss of fingers aside from the wing fingers, was simply a more extreme take on the condition seen in pteranodontids and azhdarchoids aside from tapejarids, which have small digits that connect to the end of the fourth metacarpal as metacarpals 1–3 lost contact with the wrist and disappeared.

As to why they survived and not “ornithocheirids”, perhaps their toothlessness had something to do with it. Its been long speculated that modern birds survived while their Cretaceous toothed relatives did not was due to their toothless beaks allowing for more omnivorous diets, though this has recently been contested.

A reimagining?

Barbaridactylus in the desert in Prehistoric Planet.

The overall evolution and origins of pteranodontids are surprisingly udnerdiscussed in literature, and when they are it seems to paint them as specialised relics. However, it is now clear they underwent a massive adaptative radiation in the Late Cretaceous from humble beginnings, so while the iconic Pteranodon and Nyctosaurus are unambiguously fish eaters I wonder if other members of their group might have had more diverse ecologies, at least at younger stages of their lives.

Nyctosaurids at least were doing just fine until the asteroid hit. Who knws whatw a sin store had their survived?

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