Musings on Nyctosauridae

CM 11422, a Nyctosaurus gracilis specimen.

A new study vindicates the high diversity of pterosaurs in the final stages of the Cretaceous, so as someone who has seen the patterns of pterosaur diversity for a long time I naturally had to write something on it. Something that did take me by surprise however was the high diversity of nyctosaurids, with at least three new genera being named, one of which having possibly more than two species and there being new taxa hinted at like a femur from the Maastrichtian of Maryland.

Nyctosaurids are a lineage of marine pterosaurs, best exemplified by their type species, Nyctosaurus gracilis, from the Campanian of North America and most likely beyond. This animal is well known for two things:

• Its ridiculously enormous, antler-like head crest. Once thought to be a sail, it is now best understood as a horn-like crest, since the bone encasing is smooth and does not have the porous surface indicative of keratin extensions or soft tissue membranes (Witton 2013). It hardly makes it less impressive, however, since its still two massive prongs much longer than the animal’s head and body.
• Its speciations to life on the wing, such as its massive wings (the largest in proportion to its body size of any animal), small and pneumatized hindlimbs and the complete loss of any digits but the wingfinger on the forelimbs. In particular, the strong ossification of its forelimb tendons implies that it spend most of its life on the wing (Witton 2013), and it was likely the most aerial vertebrate that has ever lived, as pterosaur superprecociality meant that it didn’t have to brood its eggs and that young could fly as soon as they were born, a luxury that birds like swifts — which can spend years on the wing — do not have.

Life restoration by Julio Lacerda. A flying deer-thing that almost never had to touch the ground seems almost mythical, doesn’t it?

The latter point in particular has been used as a demonstration of speciation, as something that spends most of its life flying is a rather extreme ecological niche. Nyctosaurus gracilis is known from deep sea marine deposits, indicating that it was a pelagic piscivore perhaps not unlike modern frigatebirds. Unlike the contemporary pteranodontids such as Pteranodon itself, which are known only from adult specimens, Nyctosaurus gracilis is represented by both adults and juveniles, indicating that it did spend most of its life away from shore.

Until recently, other nyctosaurids have been assumed to have followed the same trends. Muzquizopteryx coahuilensis does not appreciably differ from Nyctosaurus gracilis in its wing elements, and other nyctosaurid remains from the Maastrichtian of Mexico, Brazil and Jordan all come from similar deep sea marine deposits, and are rather rare and implete at that. Therefore, the whole clade could be constructed as a lineage of open sea soarers with little niche differentiation, which combined with their low diversity all indicated towards a high specialized clade, perhaps not even ecologically relevant anyway. Just a group of weird relics, pushed into the extremes of aerial ecology by the success of the newly arisen seabirds, just clinging on to life before the KT event struck.

A perfect representative of what the last pterosaurs were depicted as. Even as the discovery of the large diversity of azhdarchids and other terminal Cretaceous pterosaurs revealed that the “flying reptiles” as a whole were doing well at the end of their reign, nyctosaurids would still clearly be shown as oddities on life-support.

Then came these new North African taxa.

Joschua Knüppe’s depiction of the Maastrichtian North African pterosaurs. With the exception of a soaring Tethydraco regalis (yellow crest) and an egg-stealingPhosphatodraco mauritanicus (a pteranodontid and azhdarchid respectively), everything in this picture is a nyctosaurid.

Not only was it revealed that a large number of nyctosaurid species co-existed at the same time, these also came from coastal deposits, and show some rather intriguing morphological differences. While Nyctosaurus gracilis and Muzquizopteryx coahuilensis can probably be constructed as flap gliders and thermal soarers (Brower 1983, Witton 2013), the Alcione species have shorter wings and probably were more avid flappers, while the larger Barbaridactylus grandis probably relied on more extensive thermal soaring, as probably did the related “Nyctosaurus” lamegoi.

Probably the most intriguing news is the possibility that the Alcione species were divers. Their shorter wings are taken as a possibility that these animals relied on underwater flight, much like tropicbirds, gannets and auks. The Maryland nyctosaurid alluded to seems to further indicate diving speciations among some nyctosaurids, as it is described as particularly robust. This is the first time in modern times that a diving lifestyle is suggested for aquatic pterosaurs, though as Mark Witton noted before swimming pterosaurs probably weren’t good floaters like modern birds are, so diving was probably more common than previously assumed.

This strikes me as particularly significant because Witton compares nyctosaurids to pteranodontids in his book in regards to swimming abilities (Witton 2013). Pteranodontids possess several adaptations in the humerus and shoulder girdle that suggest a capacity for water based launching; this, combined with their large size and the lack of speciations in their neck vertebrae that would help in frigatebird-like dip-feeding, would mean that they probably foraged while swimming. Nyctosaurids lack these speciations, and are thus rendered as more likely to have fed on the wing. However, nyctosaurids are also compared to rhamphorhynchids, which share with them similar hatchet-like deltopectoral crests and proportionally massive wings and the lack of purported aquatic launching speciations seen in pteranodontids. However, he more recently argued that these were aquatic foragers.

Rhamphorhynchus muensteri by Mark Witton. Rhamphorhynchids resemble nyctosaurids in several aspects of their humeri and large wingspan, so they are likely close functional analogues.

As such, a case for nyctosaurids not being particularly suited for swimming seems less well found, and the line between swimming and aerial piscivores becomes more blurred among pterosaurs.

I still think that forms like Nyctosaurus gracilis itself probably were indeed aerial frigatebird analogues, given the sheer size of their wingspan as well as their highly ossified forelimb tendons, but other taxa like the aforementioned Alcione species and the Maryland nyctosaurid were most certainly foraged in the water. Perhaps they launched differently from pteranodontids — maybe they just flapped their way out of water, instead of more elaborate arm motions? — or maybe their hatchet-shaped deltopectoral crests simply indicate a different solution to the same problem.

Ornithocheirus simus launching from water by Mark Witton, representing the inferred mechanism for aquatic take-off in pterosaurs. Pteranodontids have similar humerus and shoulder girdle adaptations to ornithocheirids, but nyctosaurids do not. Different launching strategy, or simply another set of adaptations to the same goal?

Speaking of pteranodontids, the relationship between them and nyctosaurids also gets an upheaval. Traditionally, both groups have been considered as closely affiliated, being fairly similar marine, crested, toothless pterosaurs overall. Several phylogenetic studies, however, have recovered them as distant relatives, with nyctosaurids being the most basal ornithocheiroids (here used in the traditional sense, not including azhdarchoids) while pteranodontids nested closer to forms like ornithocheirids, anhanguerids and istiodactylids. Unwin seems to have settled a monophyletic Pteranodontia, including both nyctosaurs and pteranodontids, but starting with Meyers 2013 nearly all phylogenetic studies have recovered nyctosaurids as basal and pteranodontids as having diverged immediately after them. Pteranodontia was thus used to apply to what are normally called ornithocheiroids.

This study, however, does recover a monophyletic Pteranodontia solely of pteranodontids and nyctosaurids, excluding the rest of the ornithocheiroids:

Pteranodontia according to Longrich et al 2018. Notice the new additions, like Volgadraco bogolubovi.

Personally, I think a Nyctosauridae + Pteranodontidae at the expense of the rest of Ornithocheiroidea is most parsimonious, given not only the sheer amount of similarities they have but also because both groups more or less appear at the same time in the fossil record. True, ghost lineages are a thing and I’ve used extensively in my rhetoric, but two ghost lineages of relatively large pterosaurs seems a bit too much.

And yes, it is clear that nyctosaurids and pteranodontids have been here for a while. After all, they are the most basal ornithocheiroids, so they diverged before the massive variety of ornithocheirids, boreopterids, anhanguerids, istiodactylids and lonchodectids that have been flying around in the earliest Cretaceous. However, they are notoriously rare in the early Cretaceous: a humerus very similar to that of a nyctosaurid has been found in the Berriasian (aka early Cretaceous aka Ichthyoconodon turf) deposits of Romania, while a pteranodontid-like feet has been found in the Aptian of Australia. I doubt that these are actual nyctosaurids or pteranodontids, with the former easily also being a relictual rhamphorhynchid (remember, similar deltopectorals) and the latter just another ornithocheirid foot.

However, if they are they paint a rather curious pictures on early nyctosaurid diversity. The Romanian specimen occurs in a riverine deposit, suggesting that it didn’t in fact live at sea, but rather along freshwater lakes or even inland altogether. Pterosaurian Limnofregata, anyone?

These early pteranodontians must indeed have been primarily terrestrial and freshwater species, explaining their rarity in marine deposits while other ornithocheiroids became some of the most common fossil remains in their strata. However, by the Cenomanian, ornithocheirids, boreopterids and anhanguerids disappear suddenly, probably killed off by the climatic turnovers of this epoch. This opened the niche for the “shy pteranodontians, that underwent a massive adaptive radiation, and therefore became the dominant marine pterosaurs.

During the initial stages of this late Cretaceous pteranodontian adaptive radiation, pteranodontids probably had the lead. In places like the Coniacian-Campanian Niobrara Formation, pteranodontids vastly outnumber nyctosaurids both in terms of both individual diversity and possibly species diversity as well (its unclear if Pteranodon had more than one species, or if other purported genera like Geosternbergia and Dawndraco are even valid and not). Nyctosaurids were then only represented by Nyctosaurus gracilis and the possibly contemporary Muzquizopteryx coahuilensis, both small, open sea soarers while the absence of juveniles in the deposits indicates that pteranodontids also occupied coastal and possibly even freshwater habitats in at least early stages of their lives.

By the Maastrichtian, however, nyctosaurid remains far outnumber pteranodontid ones across the globe, and these North African sites showcase that nyctosaurids vastly outnumber pteranodontids, not only in species diversity (at least five nyctosaurids, only Tethydraco regalis among pteranodontids) but in morphology and niches as well, with nyctosaurids being presented as both marine soarers and auk-like species while Tethydraco regalis is probably a generalised coastal soarer. I think its too early to suggest outright competitive displacement, but that’s what it looks like to me at least.

Compared to nyctosaurids, pteranodontids certainly do seem generalised, not only still retaining clawed digits in their forelimbs but also preferring coastoal biomes. However, it appears that the very speciation that one assumed rendered nyctosaurs niches specialists apparently also did them favour in other environments besides the open sea. One can only guess how things would have turned out if the KT event didn’t happen, but I would assume that nyctosaurids would continue to expand into niches previously held by other ornithocheiroids, while pteranodontids would either remain in a generalised niche or disappear altogether.

So there you have it. A long lived lineage of the most specialised flying vertebrates of all time, not weird relics of the open sea but as a highly diverse lineage from inland soarers to auk-like divers. Nyctosaurids can be considered in a way the “heirs” to the ornithocheiroid diversity of the earlier stages of the Cretaceous, and wait further finds eagerly.

Referens

Longrich, N.R., Martill, D.M., and Andres, B. (2018). Late Maastrichtian pterosaurs from North Africa and mass extinction of Pterosauria at the Cretaceous-Paleogene boundary. PLoS Biology, 16(3): e2001663. doi:10.1371/journal.pbio.2001663

Witton, Mark (2013). Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press. p. 51. ISBN 978–0691150611.

Andres, B.; Myers, T. S. (2013). “Lone Star Pterosaurs”. Earth and Environmental Science Transactions of the Royal Society of Edinburgh: 1. doi:10.1017/S1755691013000303.

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