Life in the Age of Pterosaurs

 

Tethydraco from Apple TV+'s Prehistoric Planet.

If you grew up with BBC's landmark documentary Walking With Dinosaurs like I did, you might remember a scene from its last episode, "Death of a Dynasty", featuring a pterosaur called Quetzalcoatlus. It swoops down, skims for a fish from a small lake, and then lands by the shore. On the ground, its gangly form almost seems to limp, forecasting its coming extinction. Kenneth Branagh's narration solemnly informs us that this species of "delicate glider", with its 13-meter wingspan, are the last of their kind, pterosaurs having entered a mortal decline. As it flies away, scared off by a large crocodilian, we are told that the skies of the future will belong to the birds.

This was the popular image of pterosaurs at the end of the Cretaceous period for most of the 20th century: a dying breed, dwindling in numbers due to competition from modern birds. This was not a bad guess, per se, given what little information was available during much of that time. For decades, poor sampling limited our understanding of the evolution and life history of Late Cretaceous pterosaurs, and what representatives we did have were few, and mostly very large, seeming to imply that pterosaurs had abandoned the small flyer niches that birds dominate today.

Needless to say, a lot has changed in the last 20 years, and the image of the sad Quetzalcoatlus counting the days until its extinction is now a thing of the past. I was inspired by the impressive diversity of pterosaurs that I recently saw in Prehistoric Planet to write this post. The last of the pterosaurs were more diverse than we could have imagined a few decades ago, and it is my hope that this post will serve to paint you a picture of the world that they lived in.

The Lifestyles of Dragons

Were one to take popular culture at face value, one would get the impression that there were only a couple types of pterosaur (if you're lucky enough not to just get a bargain-bin pterodactyl for your trouble) and that they all did basically the same things. Typically these movie monsters only come in a "Rhamphorhynchus" variety or a "Pteranodon" variety, the latter often with teeth. It's also uncommon to see them exhibiting any behaviors other than skimming for fish or else picking up hapless prey with their feet like oversized ospreys. (The latter, incidentally, is a behavior which wasn't possible for any pterosaurs that we know of; their feet simply weren't built to grasp and bear a great deal of weight.)

Pterosaurs resembling Rhamphorhynchus and Pteranodon as glimpsed in the webcomic "Homestuck" by Andrew Hussie.

This limited pop-culture snapshot of pterosaurs does them a bit of a disservice. They arose at roughly the same time the dinosaurs did, during the Late Triassic, and disappeared with the mass extinction event at the end of the Cretaceous. During their more than 160 million years on our planet, they achieved a great degree of morphological and ecological diversity, making up a few hundred known species so far.

Across this large spread of different species, scientists have inferred a great number of possible lifestyles, some of which are now disproven. These include insectivory, frugivory, durophagy, terrestrial stalking, carrion probing, diving, macropredatory hunting, suspension feeding, and yes, skimming. Ultimately, each pterosaur group responded to the needs of their environment in greatly varying ways, and the clues to how they did this lie in the fossils they left behind.

One thing which does seem to unite all the pterosaurs, aside from their ability to fly (shared with only two other vertebrate groups, the birds and the bats), are the circumstances of their birth. As far as we can tell, all pterosaurs began life in the same way, within pliable-shelled eggs incubated under substrate, as so many reptiles do today. All archosaurs, as far as we know, retained the egg-laying behavior that's ancestral to all tetrapods (although Dinocephalosaurus, a marine Triassic reptile which might be an archosauromorph, is known to have given live birth).

Upon emerging from their eggs, pterosaur hatchlings, often called by the more whimsical term flaplings, would most likely have had to fend for themselves. Although a case has been made for a period of limited parental supervision or outright parental care, the evidence as it stands currently favors a strongly precocial development for pterosaur flaplings, based on the fact that pterosaurs seem to have hatched with their flight adaptations already highly developed. This is an ecological strategy which is employed by many modern animals, including megapodes and several other bird groups. Considering that the first pterosaur eggs and embryos weren't actually known to science until this century, I think it is possible that a bigger sample size may eventually show that some pterosaur groups developed altricial behaviors instead. Nevertheless, I'd also wager that precocious flaplings were the ancestral state, and indeed the norm.

This has interesting implications for the roles pterosaurs played in ecosystems. After all, a flapling with a half-meter wingspan striking out on its own is probably not going to eat the same things, or eat as much, as an adult sixty times its size. Pterosaurs apparently grew fast - more so in the later, more derived forms than their earlier relatives - and would have most likely filled a number of ecological roles throughout their lives. In this way, flaplings could have avoided direct competition with their adult counterparts by not pursuing the same food sources.

In light of all this, we can see two types of diversity that pterosaurs would have displayed at different times: taxonomic diversity and ecological diversity. At some times in the history of the group, the former would have been less than the latter, as we'll see in the next sections. It's time to leave the big picture of pterosaurian life history behind, and to have a closer look at a successful group which dominated the skies of the late Cretaceous world.

The Reign of the Azhdarchids

Hatzegopteryx hunting the Romanian dinosaur Zalmoxes, by Mark Witton. Originally published in Naish & Witton 2017. Retrieved via Wikimedia Commons. CC BY-SA 4.0

In 1971, the world of paleontology was undergoing a rapid evolution. The science had, during the 1960s, just emerged from a lengthy hibernation that spanned the Great Depression and Second World War. Pioneering studies of the active, birdlike attributes of dinosaurs were just emerging onto the scene. That year, in Big Bend National Park, Texas, something new would likewise mark a leap forward in the study of pterosaurs.

Douglas A. Lawson, then only 23 or 24 years old, found something peculiar in the park's outcroppings of the Javelina Formation: the partial left wing of a truly massive pterosaur. First named Quetzalcoatlus northropi in 1975, after the Aztec god Quetzalcoatl and the aircraft designer John K. Northrop, this animal wasn't just remarkable for its truly immense size (with a wingspan estimated somewhere in the 11 meter range), but also for the manner of its fossilization. Up to that point, pterosaur fossils were known from marine deposits, such as the Blue Lias of England or the Solnhofen Limestone of Germany, but almost unheard of in terrestrial environments. Although an inland sea crawled up the American interior at the time Quetzalcoatlus lived, the Big Bend would still have been a great distance from the nearest shoreline. Clearly, something about the idea of pterosaurs exclusively being fish-eaters from marine and seaside habitats was amiss.

A related animal, Arambourgiania, had actually already been identified from a cervical vertebra found in the phosphate deposits of Jordan in the 1940s, and studied in the 1950s. Aside from its size, however, scientists of the time could glean little from this single bone. In the 1980s, the Soviet paleontologist Lev A. Nesov connected the dots with his description of another pterosaur from the Cretaceous of Uzbekistan. This animal, named Azhdarcho, a Persian word for "dragon", would in turn give its name to a family uniting all three of these genera, and even more in the years to come.

Most work on the Azhdarchidae, ultimately, would occur in the 21st century, with over a dozen new genera being named in the last 22 years. They were a cosmopolitan group, with specimens of azhdarchids, or of closely related animals, being found on every continent except Antarctica. Even that exception may be overturned eventually, as large pterosaur bones, albeit not verifiably azhdarchid in nature, are known from the latest Cretaceous of the Antarctic Peninsula. As a better picture of these animals has been painted by more complete fossils, we've started to understand a lot more about the shape and lifestyle of the azhdarchids. Although there have been several competing hypotheses, many researchers now believe that they lived a terrestrial lifestyle, stalking and opportunistically hunting small prey like storks or ground-dwelling hornbills of the family Bucorvidae do today. They certainly wouldn't have limped; in fact, they would have been pretty confident striders, moving upright on all fours.

 Quetzalcoatlus, Arambourgiania, and Hatzegopteryx are all in approximately the same size range, with wingspans of 10 meters or more, and represented the largest flying animals ever known to have lived. They weren't all giants, however, and many species are much smaller. Their wide distribution during the Late Cretaceous suggests that they were an adaptable group which were at home in many different habitats. It has often been assumed that azhdarchids were long-distance gliders, and would have made cross-continental flights, but recent studies have cast some doubt on this. The fact that Romanian pterosaurs like Hatzegopteryx are so unique among the azhdarchids for their short necks and robust bodies implies that azhdarchid species had distinct regional differences, and likely were making long-distance voyages only rarely. Contrary to this hypothesis, there is a cervical vertebra from Tennessee which has been attributed to the otherwise Jordanian Arambourgiania, but I'm personally inclined to wait and see whether that actually pans out.

Azhdarchids had a long temporal range too, existing for at least 40 million years, and possibly quite a bit longer. Despite previous suggestions that they were in decline, their success only seems to have been abruptly halted by the end-Cretaceous extinction event. For a long time, three or four azhdarchid species were the only pterosaur fossils known from the last few million years of the Cretaceous, but this has also changed. In the 2010s, a remarkable fossil assemblage from Morocco gave us another piece of the puzzle and proved that azhdarchids weren't the only pterosaur group to still be around when the asteroid hit.

A Life at Sea

Nyctosaurus, illustrated by Dmitry Bogdanov. This genus was extinct by the Maastrichtian, but the relatively complete fossils we have for Nyctosaurus may help us to reconstruct its close relatives. Retrieved via Wikimedia Commons. CC BY-SA 3.0

In the second half of the 19th century, a remarkable fossil formation was first identified and explored in the American Great Plains. Most famous for its outcroppings in Kansas and Nebraska, the Niobrara Chalk was deposited during the middle of the Late Cretaceous, approximately 87 - 82 Ma, under the waves of the Western Interior Seaway. I mentioned this inland sea briefly in the last section. At the end of the Cretaceous period, 66 Ma, the Seaway was still present but receding. In the timeframe of the Niobrara Chalk, however, the sea was at its widest extent and teeming with life.

Among the many fossils known from the formation, one can find the remains of two distinct but closely related pterosaur lineages which are relevant for this part of our story. The first is Pteranodon, perhaps the most famous pterosaur, which was discovered there in the 1870s. As the first pterosaur found outside of Europe, it quickly gained notoriety and by the 20th century had cemented its place in popular culture. Its less famous cousin, Nyctosaurus, was discovered a few years later. It wasn't until the 21st century that the first fossils of its magnificent, almost antler-like head crest were described, giving us the picture we have of this genus today.

The Pteranodontidae and Nyctosauridae are usually united in a group termed Pteranodontia, as they are each other's closest cousins. They share the same toothless beaks (similar to, but evolved separately from, those of azhdarchids) and the same preference for marine habitats. Despite how common they were in the Coniacian, Santonian, and Campanian stages of the Cretaceous, it was thought until quite recently that both groups were extinct before the beginning of the Maastrichtian.

This changed in 2018, when pterosaur fossil findings from the Ouled Abdoun Basin in northern Morocco were published by Nicholas Longrich and colleagues. These phosphate beds had previously offered up the azhdarchid Phosphatodraco in the 2000s, and are dated to approximately 66 Ma, the latest Cretaceous. The new pterosaurs described from there included a pteranodontid, Tethydraco, as well as three nyctosaurids, Alcione, Barbaridactylus, and Simurghia. This has shown conclusively that these two families of pteranodontian pterosaurs did, in fact, persist to the end of the Mesozoic after all.

Although these finds are mostly quite fragmentary, we can extrapolate from their better-known, earlier relatives to gauge how they lived. Looking at Tethydraco first, the well-studied Pteranodon gives us a pretty good idea of its lifestyle. Pteranodontids were among the largest pterosaurs, with large male Pteranodon reaching a wingspan as large as 7.25 meters. They seem to have exhibited size dimorphism between the sexes, with females being considerably smaller. Pteranodontids, like azhdarchids, had a relatively upright bodily carriage and would probably have been able to move comfortably on the ground.

Unlike the shorter wings of azhdarchids, however, pteranodontid wings were more akin in their proportions to those of soaring seabirds like albatrosses, and would have enabled long, energy-efficient flights over the water surface. It was originally proposed that pteranodontids were skimmers, but they don't seem to have been particularly well adapted for this. Instead, it's more likely that they bobbed on the water surface while dipping or diving for fish, or perhaps dove for fish while on the wing. It's generally accepted now that pteranodontids would have had little issue launching into flight from the water's surface.

Nyctosaurids were similarly adapted for long-range soaring over water, but had a peculiarity of their wings which they did not share with pteranodontids. The membrane of a pterosaur's wing was supported by one, superelongated finger. The other fingers would have remained as a hand of sorts at the front of the wing, acting as a surface for walking or other functions. Nyctosaurids were missing this, however, having lost all of their fingers except the ones supporting their wings. This would suggest that nyctosaurids were much more awkward on the ground than their pteranodontid relatives, and might imply in turn that they spent less of their lives on the ground.

What would such a life have looked like? If nyctosaurids followed the precocial pattern like other pterosaurs seem to have done, they would have hatched with at most a minimum of parental supervision, would have started flying almost immediately, and would have grown rapidly to their adult size (over 2 meters in the largest nyctosaurids) in just a year. It's fascinating to think about how these pterosaurs lived, and equally fascinating to learn that pterosaurs as different from one another as the azhdarchids and the nyctosaurids both lived to the end of the Cretaceous.

Aside from the Ouled Abdoun pterosaurs, other pteranodontians now known from the Maastrichtian include nyctosaurid remains from Maryland and Brazil, suggesting that they were still widespread. There is also the matter of a small pteranodontian from the famous terrestrial Hell Creek formation, known as the Triebold pterosaur. Since this specimen has not yet been properly described, it's hard to say any more about it than that.

It is, indeed, also still a possibility that more pterosaur groups persisted up to the K-Pg boundary, although there are not yet any examples to point to. Although the Javelina azhdarchid, from the same Big Bend deposits as Quetzalcoatlus northropi, has been interpreted as a thalassodromid/thalassodromine (depending on whether you think that group is a subset of Tapejaridae or not) the evidence would seem to support an azhdarchid identity for this animal instead. Despite this, I don't think it's all too unlikely that evidence will one day turn up of tapejarids and/or thalassodromids surviving to the end of the Cretaceous. We'll have to wait and see.

The Bird Quandary

A figure from Longrich et al. (2018) comparing body size in marine (blue) and terrestrial (brown) Maastrichtian pterosaurs and birds. Retrieved via Wikimedia Commons. CC BY-SA 4.0

There is a point which I've been haphazardly poking at throughout this entire post: the ecological diversity of Maastrichtian pterosaurs. My master plan all along was to write this section, analyzing the old assumption that birds outcompeted pterosaurs in the Cretaceous period, leading to the pterosaurs' terminal decline. This hypothesis was implicitly referenced in the Walking With Dinosaurs segment I brought up at the start of the post, and has been bouncing around the literature for a long time. In light of all this new evidence, how does it hold up?

In light of new evidence, there are now a couple of chief points which, in fact, make this theory look more unlikely than before. These are:

1. The fact that Maastrichtian pterosaurs were more taxonomically diverse than previously assumed.
2. New evidence of prenatal development suggesting widespread precociality in pterosaurs, which implies that the young filled different niches than the adults.

Should we put credence in the hypothesis that pterosaur flaplings occupied small-bodied niches that the adults did not, it doesn't seem so likely that birds were outcompeting pterosaurs in these niches after all. Consider how abundant azhdarchids were in the Maastrichtian world; their young would have been all over the place, and even if birds were growing more diverse at this time, it hardly seems like the pterosaurs were fighting for their lives. The idea that the last pterosaurs found refuge in huge body sizes to avoid competing with birds, in light of all this, seems to be unsupported, especially when adult azhdarchid relatives no bigger than 2.5 meters in wingspan are now also known.

The bird competition hypothesis was never a bad one; for a long time, there really did seem to be a significant drop-off in pterosaur diversity in the last stages of the Cretaceous, accompanied by an apparent rise in the diversity of birds. Time was against the pterosaurs, but not necessarily in the way that 20th century paleontologists assumed. The very same lightweight, fragile bones which made them such superb fliers also meant that relatively few pterosaurs would have been preserved, and those that were would generally be fragmentary. Time was also against them in the sense that, as a general maxim, the very first and very last members of a species or group will tend not to be fossilized. This is known as the Signor-Lipps effect, which proposes that extinctions of animal groups will often appear more protracted than they actually were. This could very well be the case with pterosaurs, which would explain why this decline looks less pronounced as we get more fossil data.

Thus, the evidence as it stands now suggests that pterosaurs were doing just fine up to a point, only to abruptly go extinct along with the non-avian dinosaurs at the end of the Cretaceous, most probably because of the Chicxulub impact event. More evidence may back this up, or it might turn this hypothesis entirely on its head. Either outcome would be equally exciting to discover more about; that's the beauty of science!

The Age of Pterosaurs

Our knowledge of pterosaurs is growing by leaps and bounds at a rate we've never seen before. This doesn't just go for the very last pterosaurs either; it goes for the earliest pterosaurs in the Triassic, the period in the Late Jurassic and Early Cretaceous when they flourished, and everywhere in between. Pterosaurs first became known to science in the late 18th century, but the vast majority of all published studies about them have come out since the year 2000. Entirely new families are being identified, gaps are being filled in, and the trajectory still seems to be upward from here.

We live in an age of pterosaurs, but more generally, we also live in an age of paleontology. If the 1960s - 1980s were a renaissance, then I don't know what to call the period we're in now, where the field has never been more vibrant. What I do know is that this blog post is probably going to be outdated in just a couple of years, and I couldn't be happier about it.

Barbaridactylus from Apple TV+'s Prehistoric Planet.

Thanks for reading. Our next post will be about sauropod dinosaurs, a group which I've so far neglected to mention very much. After that, who knows? The sky is the limit...

Sources:

• Andres, Brian and Wann Langston, Jr. (2021) "Morphology and taxonomy of Quetzalcoatlus Lawson 1975 (Pterodactyloidea: Azhdarchoidea)." Journal of Vertebrate Paleontology 41.1: 46-202. DOI: 10.1080/02724634.2021.1907587
• Bennett, S. Christopher. (1995) "A statistical study of Rhamphorhynchus from the Solnhofen Limestone of Germany: Year-classes of a single large species." Journal of Paleontology 69.3: 569 - 580. DOI: 10.1017/S0022336000034946
• Bestwick, Jordan, David M. Unwin, Richard J. Butler, Donald M. Henderson, and Mark A. Purnell. (2018) "Pterosaur dietary hypotheses: a review of ideas and approaches." Biological Reviews 2018.93: 2021 - 2048. DOI: 10.1111/brv.12431
• Butler, Richard J., Paul M. Barrett, Stephen Nowbath, and Paul Upchurch. (2009) "Estimating the effects of sampling biases on pterosaur diversity patterns: implications for hypotheses of bird/pterosaur competitive replacement." Paleobiology 35.3: 432 - 446. DOI: 10.1666/0094-8373-35.3.432
• Kellner, Alexander W. A., Taissa Rodrigues, Fabiana R. Costa, Luiz C. Weinschütz, Rodrigo G. Figueiredo, Geovane A. De Souza, Arthur S. Brum, Lúcia H. S. Eleutério, Carsten W. Mueller, and Julian M. Sayão. (2019) "Pterodactyloid pterosaur bones from Cretaceous deposits of the Antarctic Peninsula." Anais da Academia Brasileira de Ciências 91.2: e20191300. DOI: 10.1590/0001-3765201920191300 
• Longrich, Nicholas R., David M. Martill, and Brian Andres. (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
• Martin-Silverstone, Elizabeth, Mark P. Witton, Victoria M. Arbour, and Philip J. Currie. (2016) "A small azhdarchoid pterosaur from the latest Cretaceous, the age of flying giants." Royal Society Open Science 3:160333. DOI: 10.1098/rsos.160333
• Naish, Darren, and Mark P. Witton. (2017) "Neck biomechanics indicate that giant Transylvanian azhdarchid pterosaurs were short-necked arch predators." PeerJ 5:e2908. DOI: 10.7717/peerj.2908
• Naish, Darren, Mark P. Witton, and Elizabeth Martin-Silverstone. (2021) "Powered flight in hatchling pterosaurs: evidence from wing form and bone strength." Science Reports 11.13130. DOI: 10.1038/s41598-021-92499-z
• Smith, Roy E., Anusuya Chinsamy, David M. Unwin, Nizar Ibrahim, Samir Zouhri, and David M. Martill. (2021) "Small, immature pterosaurs from the Cretaceous of Africa: implications for taphonomic bias and palaeocommunity structure in flying reptiles." Cretaceous Research 130:105061. DOI: 10.1016/j.cretres.2021.105061
• Unwin, David Michael and D. Charles Deeming. (2019) "Prenatal development in pterosaurs and its implications for their postnatal locomotory ability." Proceedings of the Royal Society B 286:20190409. DOI: 10.1098/rspb.2019.0409
• Witton, Mark P. and Darren Naish. (2015) "Azhdarchid pterosaurs: water-trawling pelican mimics or 'terrestrial stalkers'?" Acta Palaeontologica Polonica 60.3: 651 - 660. DOI: 10.4202/app.00005.2013