Pterosaur Ecological Bias
Pterosaurs have gone through an epic renaissance in the beginning of the 21th century, unfortunately only very recently noticed by the public. In 2000, we knew they were elegant, endothermic flyers not unlike birds or bats, but we thought these animals were restricted to aquatic biomes, occupying the niches of modern aerial seabirds (how nobody questioned the absurdity of such a gigantic number of species occupying the same niche remains unanswered), the exceptions being the tiny insectivorous anurognathids (because frankly, even then you’d be called an idiot for suggesting small, bat like flyers were seagull analogues), and the terrestrial, mollusc eating dsungaripterids (which were still connected to aquatic environs). We also had no idea how they reproduced, and even less about many other details about their anatomy. We weren’t even sure of how they flew.
By 2007, our knowledge had radically changed. We know knew pterosaurs were lizard like in terms of reproductive habits, laying soft shelled eggs that hatched juveniles already capable of flight. We learned that their wings were not mere leather, but the most complex membranes amidst tetrapods, with tens of millimetre (or even half-millimetre) thick layers of muscle and collagen fibers. We learned several details of their internal organ anatomy, such as a complex air sac pulmonary system akin to that of birds, that even extended into the wings. We even learned how they flew, and how they took off.
Most importantly, we also learned that pterosaurs weren’t just giant, weird seagulls. Pterosaurs were most definitely not restricted to aquatic environs, taking forms as diverse as the marten like Dimorphodon, the stork like azhdarchids, the antelopine tapejarids, the kite/vulture like istiodactylids and even the boar/bear like dsungaripterids. Pterosaurs didn’t occupy many niches birds colonised, but they branched off in ways that birds never did; in fact, pterosaurs might have been fierce competitors with non-avian dinosaurs, and many forms resemble modern mammals in terms of ecological roles.
The Ecological Bias
In the end though, the newly discovered ecological niches that pterosaurs occupied were not sufficient to dispell the public idea that most pterosaurs were aquatic, that there was an ecological bias towards waterbird/seabird like niches. When new pterosaurs are discovered, more often than not the bias is mentioned, comparing the new terrestrial taxon to the “seabird like majority”.
In the end, no matter how chaotic nature is, there are still patterns, and nothing better indicates this than ecological biases towards certain ecological niches. All bovids are herbivores, but the vast majority are still “antelopes”, with only a few select clades producing bizarre mountain forms, robust beasts or horse and deer mimics. Carnivorans include such bizarre forms as the short skulled felines, the marine pinnipedes and the cursorial dogs and hyenas, but the vast majority still gravitates towards mustelid and viverrid like niches. Gruiformes include stork like cranes, grouse like bustards, loon like sungrebes, and, according to recent genetic studies, the extremely passerine like cuckoos (!), but they still are mostly rail like forms. Pseudosuchians included theropod like macropredators, ostrich like omnivores, armadillo like omnivores, dolphin like piscivores, duck billed omnivores, whale like filter feeders and even various types of herbivores, but a bias towards the iconic amphibious lifestyle was already occuring in the Triassic (although, through most of the Mesozoic and well into the Cenozoic, the basal monitor like terrestrial forms were equally common).
As obvious, the more speciose and diverse a clade is, the smaller is the presence of a bias. You can pinpoint the niches that individual placental clades gravitate towards, but you can’t accurately pinpoint the ecological trends of the entirity of Eutheria.
Pterosaur Niche Trends
Pterosaurs, being a clade of specialised aerial tetrapods, were highly diverse in terms of lifestyles, producing forms ranging from the small insectivorous anurognathids to massive terrestrial macropredators like azhdarchids and dsungaripterids to the bizarre filter feeding ctenochasmatoids and boreopterids. They were more diverse than modern chiropteran mammals, and while nowhere as speciose as birds, they were just as diverse in terms of body plans and ecological niches, and in fact branched in ways birds never did (although birds also colonised niches likely never occupied by pterosaurs). All in all, they weren’t degenerate specialists, but a lineage that competed fiercely with their dinosaurian relatives, and simply had the misfortune of not being able to cope with the KT event (and neither did most birds; 90% of all Cretaceous avian taxa was gone alongside the pterosaurs, thus discrediting any inherent “superiority”).
That said, volant vertebrates, while inherently extremely adaptable, are indeed limited by their aerial evolutionary path. Modern bats, while the second most speciose mammal lineage, are after all almost entirely comprised of aerial insectivores, with a few specialised forms. Neornithes, although a doubtlessly titanic clade of over 12000 species (and those are just the ones that weren’t wiped out due to anthropogenic influence), still follows follows ecological trends; most “generic” neornithes are forms similar to Charadriiformes, being mostly terrestrial or semi-aquatic omnivores. Within Neoaves, however, passerine like forms are also prevalent.
Pterosaurs, for the most part, seem also to follow a distinct ecological bias. However, contrary to what the media claims, or to what we subconsciously gravitate towards due to our upbringing, this bias is not towards aquatic, seabird like forms; within Pterosauria, only three lineages became specialised in piscivorous niches, said lineages being Campylognathoidea, Rhamphorhynchidae (note that, while often classified as rhamphorhynchids, scaphognathines may not belong within that clade, and in any case they appearently weren’t strictly if at all piscivorous, being terrestrial predators) and Euornithocheiroidea; Ctenochasmatoidea was comprised of aquatic forms, but they resembled more closely modern waterfowl in habits, rather than seabirds.
After Mark Witton’s gift to the world in showing that azhdarchids were terrestrial predators, the public seems to have taken this to the extreme and claimed that pterosaurs in general were terrestrial predators. And indeed the ecological bias pterosaurs followed seems to have been that one: most pterosaurs were, in fact, terrestrial predators.
Within non-pterodactyloid pterosaurs, dimorphodontids, scaphognathines and wukongopterids are all thought to have been predatory animals; their jaws exhibit teeth designed to pierce like those of their piscivorous cousins, but they considerably more robust, less designed to hold slippery prey and more to crush through bones. Likewise, their inland habitats make a piscivorous lifestyle suspicious at best, and both dimorphodontids and some scaphognathines show a decrease in their flight capacities, having become more specialised to stalk prey on the ground.
In pterodactyloids, the development of better adaptations for terrestriality without sacrificing flight capacities allowed this type of lifestyle to become rampant. The most basal ornithocheiroids, isitodactylids, had short, serrated teeth, like those of a shark, indicating that they fed on flesh, presumably being scavengers like modern kites and vultures. “Generic pterodactyloids”, like Pterodactylus itself and basal dsungaripteroids, are pretty much crow/seagull like animals, being small sized generalistic carnivores that foraged on the ground (or, in Pterodactylus‘ case, also in the water) in search of small tetrapods, invertebrates or carrion. Azhdarchoids became even more specialised in hunting small prey on the ground, having developed long necks and toothless beaks, while the limbs also elongated and in fact developed similar proportions to those of modern ungulate mammals, indicating their ability to run efficiently. Their wings also became shorter, as they became further specialised to hunt on the ground.
Derived dsungaripteroids, however, took this lifestyle to it’s logical extreme. Their skeletons became more robust, to the point that their flight capacities were barely superior to those of modern Galliformes. Their jaws, meanwhile, became progressively more robust, and developed a sharp rhamphoteca at the tips, with the lower jaw consistently bending upwards in a hook (in Dsungaripterus itself, the upper jaw also forms an upwards oriented hook); generally, this was interpreted as specialisations towards molluscivory, but the inland habitat of these pterosaurs casts doubt about this. Most likely they were generalistic carnivores, feeding on not only mussels, but also carrion and living vertebrates, up to the size of small dinosaurs. The poor flight capacities, alongside the robust, upwards bent jaws (remaniscent to some extent of the jaws of the pseudosuchian aetosaurs), would have meant a lifestyle probably not too dissimilar to that of modern boars and small bears, or even that of small to medium sized terror birds.
With each new discovery, the idea that pterosaurs were seabird analogues becomes further an artifact of ignorance. Some pterosaurs were indeed piscivores, but in the context of the clade as a whole, these were aberrant forms, as their relatives occupied inland biomes.
Julio Lacerda deserves special credit, because he was the inspiration for this post.I plan to use his pterosaur pics for a greater purpose than this post, and I greatly thank him for his permission on the matter. Go check his blog:
Mark Witton is about to publish his pterosaur book, which I hope that it will offer an even bigger impact to the public than the London pterosaur exhibition. Go check his site:
Robert Black is not doing anything important in relation to this post, but go to his gallery anyway: