For such popular prehistoric animals, Pteranodon and it’s brethren are rather surprisingly rare not only in pop culture – most “cartoon pterodactyls” with the iconic headcrests are frankly never actually called Pteranodon, so… -, but also in documentaries. I literally can’t think of any appearences by these pterosaurs in documentaries aside from both Sea Monster documentaires – the BBC series and the National Geographic movie -, and amusingly enough the only accurate depiction of these animals on CGI is by Disney’s infamously generic movie Dinosaur. While Quetzalcoatlus has rather unsurprisingly been featured more frequently thanks to it’s massive size and actual co-existence with Tyrannosaurus, when much less popular pterosaurs like Nemicolopterus and Eosipterus have had their shot at (admitely very horribly depicted) documentarydom, you know something’s really, really wrong.
Regardless, pteranodontids have started to captivate me, ironically given the amount of “new”, more exotic pterosaurs being discovered and the fact that they represent what pterosaur lovers have long struggled against, the stereotypical seabird like animal. However, pteranodontids are fascinating and beautiful animals on their own right, and while Pteranodon longiceps is possibly one of the most well understood pterodactyloids known, other members of this clade are woefully understudied, adding a layer of mystique to a supposedly characteristic clade.
The Ultimate Flying Animals
Pteranodontidae is part of a clade of specialised pterodactyloids known as Ornithocheiroidea, which quite possibly may represent the vertebrate clade most specialised for an aerial living. With high shoulders, warped dectopectoral crests, large sternal keels, extensive pneumatisation of the skeleton, well developed notariums, broad and robust scapulocoracoids and proportionally massive wings – correspondent to porportionally small hindlimbs -, as well as casque-like crests without fibrous bone and thus keratinous extensions, ornithocheirioids stand well amidst other pterodactyloids as oddities, and indeed they are thought to have diverged early in monofenestratan evolution.
In pteranodontids and their close cousins, the nyctosaurs, these specialisations kept being perfected to a further extreme: skeletal pneumatisation became so extensive that even the hindlimbs are pneumatised, the sternal keel among the proportionally largest of all pterosaurs, the wings have become even larger via the extreme elongation of the fourth metacarpal, and the toothy maws characteristic to pterosaurs – very impressive in ornithocheiroids like ornithocheirids themselves – were replaced by a toothless, keratinous beak. The metacarpals aside from the one supporting the wing finger became astonishingly short, barely more than the digits themselves, becoming attached to the end of the fourth metacarpal, a trait also shared with azhdarchoids, though whereas this is a specialisation for flight or the inevitable consequence of the thinning of the I-III metacarpals is unknown. Nyctosaurs went further, adapting the dectopectoral crest and scapulocoracoid anatomy to stranger forms and loosing the non-wingfinger digits altogether, but pteranodontids kept conservative to ornithocheiroid anatomy in these regards, except for the development of strangely long tails with biforcated end caudals.
Because of these extreme anatomical changes and the rather sudden appearence of these animals in the middle-Cretaceous, how did these pterosaurs evolve is a mystery. The most probable explanation is that pteranodontids are actually highly derived ornithocheirids, rendering Ornithocheiridae paraphyletic – indeed, Guidraco has been suggested as being a sister taxon to Pteranodontidae. Another possibility suggested to me in a conversation was that pteranodontids evolved from animals akin to boreopterids, which already show a few pteranodontid like traits like the long tails, and have rather weak, fragile teeth, probably of little use besides preventing small fish from escaping, which could rather rapidly be replaced by a keratinous, toothless beak. Starting as inland foragers, these early pteranodontians would then have expanded into marine ecosystems, replacing ornithocheirids or simply filling their vacant niches – with pteranodontians reaching their golden age not long after ornithocheirids largely disappeared from the fossil reccord, this may be a blatant case of ecological replacement.
In turn, it is equally possible that Nyctosauridae may be a derived clade within Pteranodontidae, rendering the latter polyphyletic, though with little examination of the relationships between the known taxa this is of course difficult to attest. It is also likely that nyctosaurids are simply an entirely different branch of Ornithocheiroidea, having diverged from other ornithocheiroid branches and simply converged with pteranodontids, though both clades do share several traits in common at the exclusion of other ornithocheiroids.
How Many Are There?
Traditionally, Pteranodontidae is composed of two named genera, Pteranodon and Ornithostoma. The former, the first known north american pterosaur, is one of the best known flying reptiles, at least 1000 specimens having been uncovered, many of which astonishingly compete. The latter, on the other hand, is very fragmentary, and it’s identity as an actual pteranodontid is nothing short of controversial, with at least two specimens attributed to it being recovered as azhdarchoids, though the holotype may be a genuine pteranodontid.
Pteranodon has had many species attributed to it in historical times, but since the nineties that only two species have been recognised: P. longiceps, with it’s iconic thinner crest, and the hatcht crested P. sternbergi. However, more recently two new genera have been distinguished from Pteranodon (Kellner 2010): P. sternbergi became it’s own genus, Geosternbergia (composed, in turn, of both G. sternbergi and G. maysei), while a P. longiceps specimen was elevated to Dawndraco kanzai. These two genera haven’t gone uncontested, however (Mark Witton 2013), and indeed they may very well be all within Pteranodon, as they don’t have many significant anatomical differences beyond crest and beak shapes. The genera/species have been informally suggested to form a temporal species complex, with one form replacing another, but all co-exist in the Niobrara Formation at seemingly the same time period.
Remains attributed to Pteranodontidae and even Pteranodon itself have also been found in other parts of the world, including Japan, Australia, Argentina, Brasil, Romania, France and Sweeden (Allt om Vetenskap 2012, Barrett et al 2008), spanning all the way from the late Albian to the Maastrichtian. Little work has been done on these non-north american pteranodontids, but they clearly indicate that the clade was both cosmopolitian and long lived, lasting for at least 37 million years from the Lower Cretaceous to the very end of the Mesozoic. This indicates that Pteranodontidae, alongside Nyctosauridae, Azhdarchidae and whatever the hell Navajodactylus, Piksi and the New Zealand pterosaur are, was among the last pterosaur clades to have ever existed, perishing only due to the KT event.
While we know little about how large most pteranodontids grew, the north american forms at least were positively gigantic animals. Pteranodon longiceps reaches a wingspan of 6 meters, while Geosternbergia maysei might reach a wingspan of 7.25 meters, being among the largest flying animals to have ever existed: only giant azhdarchids like Hatzegopteryx and Quetzalcoatlus, the teratorn Argentavis and the ornithocheirid Coloborhynchus can claim larger sizes; pelagornithid seabirds and some ornithocheirids come close in size. The exact weight of these animals is unclear, but based on estimates of birds and bats – as exposed to historically “super light weight” nonsense -, a healthy full grown animal should weight around 93 kilos, mass effordlessly vaulted from the ground by the pterosaurs’ powerful forelimbs. Even without the unique pterosaurian launching mechanism, however, such sizes would be affordable anyways, as the pelagornithid seabirds and teratorns show, contrasting previous studies on weight limits for flying animals.
Most impressively, only the males reached these massive sizes. Females remained much smaller at wingspans of around four meters or even lower, implicating that the males’ gigantism was the result of extreme sexual selection, observed nowhere else among pterosaurs (though possibly due to the lack of such a large sample in many other species), though the possibility of simply different ecological niches occupied by each gender cannot be dismissed. Animals generally reach large sizes because they can, and being the dominant flying vertebrates over the Late Cretaceous oceans, with seabirds remaining considerably smaller and even rarer in several cases, and with nyctosaurids and other marine pterosaurs consistently also smaller and less common, pteranodonts presumably wouldn’t have been forced into larger sizes by competition by either birds or other pterosaurs, as the “average” females show.
As Coloborhynchus was indeed larger than the largest known pteranodontids, it’s unlikely that the animals’ unique adaptations contributed much to their large sizes. Indeed, the even more specialised nyctosaurs are generally rather small animals, on average with 2 meter wingspans, excepting “Nyctosaurus” lamegoi‘s estimated 4 meter wingspan, which might implicate that these adaptations for flight have ironically also restrained their sizes.
In general, the flight style of pteranodontids has been probably the most well documented among pterosaurs. It’s almost universally agreed that pteranodonts were oceanic soarers akin to modern seabirds, specialised in utilising the marine wind currents and thermals for gliding effordlessly for hours with minimal wing flaps. Their wings are exceptionally long, with high aspect ratios and indeed comparable in shape to those of modern soaring seabirds like albatrosses, if wider (keep reading, though). The sternum nonetheless has a fairly deep keel for pterosaur standards, and the wing musculature appears to have been quite extensive and robust, certainly allowing for powerful flapping when needed, perhaps even more so than in modern seabirds. Like all pterosaurs, pteranodontids were probably capable of altering their wing shape, using the muscle fibers in the patagia to contract and expand the membrane, allowing wider wings when relaxed and thinner wings when contracting, something particularly useful when transitioning from stactic soaring to dynamic soaring, and vice versa, allowing for effective soaring in both marine and inland settings.
As stated previously, pteranodontids extended the pneumacy of their ornithocheiroid ancestors, with even the hindlimbs being pneumatised. While some bones may be rendered lighter with pneumacy, it has been made clear again and again that animals with hollow bones are not significantly lighter than non-pneumatised animals, and indeed Coloborhynchus managed larger sizes with less pneumatisation. Pneumacy instead seems to actually serve to make the bones stronger, allowing them to gain larger sizes simply by expanding the airsac into the bone like an air baloon with minimal extra resources wasted, and with density equally distributed. As such, pteranodontids had thicker bones elements than many other pterosaurs, allowing for both more extensive wing muscle attachment sites as well as a skeleton even more apt to deal with the pressures of powered flight.
It seems that pteranodontids, like most ornithocheiroids, were not very good at walking, with rather short hindlimbs in proportion to the massive forelimbs. The long fourth metacarpal was probably very close to the maximum limit for effective terrestrial locomotion: any longer, and the forelimbs wouldn’t have been able to be used as effective propellers in coordination with the hindlimbs. Worth of note is that Pteranodon was the main focus of early hypothesis about pterosaur terrestrial locomotion: all the insane ideas, from dragging itself by it’s belly to upright bipedality, were attributed to this pterosaur, before quadrupedal walking was realised.
Over The Sea
With adaptations for oceanic soaring, generally found in marine formations and with north american specimens associated with fish remains, including regurgitated fish scales, it’s clear that pteranodontids followed in the lines of ornithocheirids and rhamphorhynchines as the classical piscivorous pterosaur model, soaring over the Cretaceous’ oceans and feasting on teleosts, cephalopods, crustaceans and other small marine animals in the manner of seabirds, both contemporary to them and modern. However, unlike many earlier aerial piscivores, pteranodontids might have been especially like seabirds, landing on the ocean surface and swimming instead of catching prey on the wing: unlike dip feeding pterosaurs, pteranodontids have small anterior cervicals, indicating that their neck was not robust enough to withstand the stresses of dip feeding, nor that it had neck musculature strong enough for carrying the necessary neck motions. Instead, pteranodontids have characteristics thought to have expressed adaptations for water based launching, with reinforced scapulocoracoid systems, robust shoulders and possibly expanded and reconfigured downstroke muscles, allowing these animals to catapult themselves out of the water in a series of row-like motions from the arms. Additionally, pteranodontids have fairly large feet, as opposed to the miniscule feet of other ornithocheiroids, and are associated with small prey items, as opposed to the general tendency of aerial dip feeders of capturing large, easy to spot prey.
Once in the water, pteranodontids would have captured their morsels in a myriad of ways. Perhaps they simply dipped their long beaks, capturing prey near the surface as they floated, or maybe they dabbled like modern ducks and many seabirds, capturing prey slightly deeper whilst still floating in the surface. Maybe they selectively picked prey with their bills, or passed their beak through the water, closing it quickly as soon as they felt prey. Maybe they weren’t restricted to the surface after all, and could dive, aquaflying with their powerful forelimbs; when even modern albatrosses can dive for at least as deep as 6 meters, this is a serious possibility. Maybe they even plunge dived. When individual seabird species have varied foraging techniques, it’s likely pteranodontids employed many of these too. Dawndraco has a longer, thicker bill than other known pteranodontids, so it may have foraged differently. At any rate, pteranodontids in general have long, stork like bills, like the long bills in modern tropical/subtropical seabirds, allowing them to reach prey at deeper depths.
In at least north american pteranodontids, only adults and subadults are found in the fossil sites, thought to have once been the sea bottom of the open ocean. This means that juvenile pteranodontids probably had coastoal or even terrestrial habits, and only when they reached a large enough size did they venture into the open sea; by contrast, young nyctosaur fossils indicate that these pterosaurs started their pelagic lifestyle very early on. Some non-north american remains, like the romanian pteranodontid, occur in non-pelagic fossil sites, and indeed these animals probably landed, on land or on sea, to roost like most modern seabirds, unlike the seemingly perpetually aerial nyctosaurs.
It’s made very clear that pteranodontids were sexually dimorphic animals, with males being much larger than the females and having more impressive head crests and distinct overbites. Sex has been determined by the width of the pelvic cannals, which in the smaller, pratically crestless individuals is much wider, which is consistent with a female interpretation (though, once again, I give this warning). Much like modern pinnipedes, pteranodontids probably were lek strategists, the animals gathering for brief periods of time on coastoal rookeries where males would have maintained an harem of females. As to be expected from pelagic animals, potential mates would perhaps rarely encounter each other, so these short lived affairs would prommote extensive sexual selection, with larger, more flamboyant and more agressive males being favoured. The overbite in particular may imply the use of the beak in interspecific combat between males, though this is of course yet unattested.
Another, more dark interpretation is that, like modern mallards and dolphins, pteranodontids probably focused on a more direct form of sexual selection, with the males being larger and stronger to forcefully subdue females; mallards in particular are also exceptionally flamboyant in the males, and given the extensively documented cases of males ganging up on females (and other males) to mob rape them, such signalling might have been used to attract fellow males rather than repell them.
Regards of the strategy, it’s very unlikely males had much to do with the offspring. Or the females, for that matter, as like most pterosaurs baby pteranodontids would probably have been superprecocial, though parental care can’t at the present be dismissed. Female pteranodontids would have laid their eggs on sandy beaches like modern sea turtles, but given the soft eggshells of pterosaur eggs, ovovivipary is also a possibility, something of particular consideration given the wide pelvic cannals of these animals. The young, if not raised by the parents, presumably would have lived on the beach or in inland habitats for the few months of their lives, before growing large enough to venture into the sea.
Pteranodontids as a whole probably were the heirs to the ornithocheirid empire that lasted for over 63 million years until the Cenomanian, spreading across the world in the final stages of the Cretaceous. They seemingly were a common component of the world’s marine faunas, and probably ecologically analogous to their ornithocheiroid predecessors, cementing the domain of toothless pterosaurs after the Turonian. After their own 37 million year dynasty, they met their own extinction in the KT event, they were probably replaced ecologically by the pelagornithid seabirds, which only recently have become extinct.
For a group of pterosaurs bearing the most famous pterosaur of all, Pteranodontidae still has a lot of mysteries to unveil, all the while invoking a sense of wonder matched by few living animals. They were not as big as azhdarchids and didn’t have tall necks, but they were still massive animals with wings casting immense shadows over ancient seas, so similar yet so alien compared to modern seabirds. They were quite possibly the very last inovative pterosaur design, having appeared seemingly well after other known pterosaur clades were well established, and they reached impressive dichotomies in sizes between members of their own species.
Above all, they are an iconic symbol of pterosaur research.