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How pterosaurs became airborne

November 24, 2012

Scleromochlus reconstruction by Jaime A. Headden

While it’s easy to generalise birds, bats and pterosaurs as the result of very lucky gliders, the truth is that we know very little about how these animals became airborne. Aerial dinosaurs are notoriously subjected to intense debate, especially considering that forms like Microraptor are amazingly highly unsuited for both climbing and WAIR, but birds are hardly the only mysteries. Bats, often credited as derived from gliding mammals, might actually be the result of a highly unusual cave dwelling lifestyle, though predictably this is highly criticised as well.

In both birds and bats, therefore, can can negate origins from tree climbing gliding ancestors, the usual models for flying animal ancestors. So, in that light, how did pterosaurs, the other group of flying vertebrates, became airborne?

What we currently know

Jeholopterus by Mark Witton. Animals like these are among the most basal known pterosaurs.

More controversial than any other thing about them, is where pterosaurs fit in the tree of life. The most valid suggestion, which is currently the “official” one, is that pterosaurs evolved from archosaurs closely related to dinosaurs, and that both Pterosauromorphs and Dinosauromorphs form clade Ornithodira, or “bird necks”.

And within Ornithodira, pterosaurs are often considered to be closely related to the archosaur Scleromochlus, a little sauropsid that is pretty much the archosaurian answer to Sharovipteryx. Even then, this is not certain; some consider Scleromochlus to be a more “primitive” ornithodiran, or even outside of the clade all together. The fact that it has long hindlimbs and short forelimbs, as opposed to the long forelimbs and short hindlimbs of pterosaurs, doesn’t help matters, though it’s worth to note that gliding squirrels do have more developed forelimbs than other squirrels, so such a reversal is not unheard off.

Within Pterosauria itself, the most basal taxa don’t help matters. Currently, the most basal known pterosaurs are anurognathids. Anurognathids were highly different from the stereotypical pterosaur: with very short and wide toad-like jaws, massive eyes and short, broad wings covered in specialised pycnofibers, these were a far call from the elegant, diurnal, colourful pterodactyloids of the Cretaceous, being basically the closest a sauropsid ever came close to a bat. More importantly, however, they provide clues as to how pterosaurs evolved.

Anurognathid morphology is consistent with that of arboreal, nocturnal/crepuscular insectivores. Their short and wide snouts were highly remaniscient of not just amphibian mouths, but also of the beaks of nightjars, swifts and swallows, all acrobatic aerial predators of moths and other insects. Their wings were proportionally short by pterosaur standards, being quite adequate for flying among dense forests, and the wing membranes were bordered by long pycnofibers similar to the barbs of silent-flight birds like owls, indicating that these animals had quite silent wing beats. It’s very likely that they were ambush predators, relying on the cover of darkness and their silent wings to stalk their prey undetected, and likely to avoid potential predators like arboreal mammals and, latter on, larger pterosaurs. We also know that they had unique crests on their arm bones, implying that these were also among the few known pterosaurs to be capable at hoovering.

Like all non-pterodactyloid pterosaurs, anurognathids were also highly adapted to climbing. Their claws were robust and curved, impairing movement on flat surfaces, but allowing efficient climbing. Of what we know about their wing membranes, they were more extensive on the legs than in more derived pterosaurs, also making running cumbersome. These were, therefore, highly arboreal animals, presumably avoiding the ground as much as possible.

Therefore, based on Anurognathidae, we can speculate that the first pterosaurs were nocturnal, arboreal gliders/flutterers. The tree tops of the Triassic had a menagerie of diurnal arboreal reptiles like drepanosaurs, and back then there was virtually no competition for the numbers of flying insects. With competition on the ground and availiable prey on the air, flight was an inevitability.

There are modern analogues!?

Ptychozoon gecko gliding.

Most surprising, is that there is an echo of the old ornithodires that produced pterosaurs, in the dense tropical rainforests of the Holocene.

Gliding or flying geckos are curious squamates distributed across four genera. The most famous, and ostensibly the most aerial, are the Ptychozoon geckos from Southeast Asia, but there is also the closely related Luperosaurus and the distantly related Thecadactylus.

All these gekkotans are nocturnal, arboreal animals just like most geckos, but thanks to the extensive flaps of skin that stretch along the limbs, in the flanks and between the toes, they can glide efficiently. As said before, Ptychozoon, with the most extensive membranes and wider tail, is the most aerial of these geckos; they are frequently captured in bat nets, suggesting that they engage in aerial locomotion frequently, and perhaps even to capture aerial prey.

Most interestingly, unlike other gliding squamates, which rely on ribs to support the wing, the flank membranes of these geckos are unsupported. Indeed, these animals rely a lot on the webbed toes to form the wing surface; such a reliance on the limbs, combined with already present flank membranes, makes the evolution of powered flight the logical conclusion, and the ancestors of pterosaurs likely followed the exact same path, especially when marks of membranes between the pterosaur clawed fingers have been found.

The evolution of “new pterosaurs” from Ptychozoon is probably sadly postponed, as bats and nocturnal birds rule the skies of their environment. However, long ago, in the Triassic, there were no flying dinosaurs or mammals, and as such animals very similar to Ptychozoon were free to further develop their wings.

(EDIT: Before proceeding with the comments’ section, read Darren Naish’s post on the phylogenetic controversies raised by David Peters)

15 Comments leave one →
  1. November 24, 2012 11:25 am

    Very interesting post. Exactly, why did you choose anurognathids for looking to the ancestry of Pterosauria? I mean it’s true that Kellner classified them as one of the most basal groups of pterosaurus, but they could also be the sister taxa of Pterodactyloidea (as in Andres et al., 2010). Would species such as Dimorphodon and Preondactylus be a better analogue?

    • November 24, 2012 2:44 pm

      Ñot really. Andre’s study is not taken very seriously, as it was more of an attempt to justify not making them a ghost lineage.

      Dimorphodontids might be more basal, but the most recent cladograms I’ve seen make anurognathids the most basal pterosaurs, with dimorphodontids sister taxa to Campylognathoidea.

      At any rate, while not as … aberrant, as anurognathids, dimorphodontids might had led a similar lifestyle, also having extreme arboreal adaptations and wide mouths. Of course, Dimorphodon itself was more of a pine-marten analogue.

  2. March 4, 2013 12:30 pm

    Why not look to the ancestry of pterosaurs in Sharovipteryx and kin, a taxon you briefly mentioned? If you’re looking for good evidence for the ancestry of pterosaurs you can find it at reptileevolution.com and pterosaurheresies.com. Not sure why paleontologists, including yourself, are avoiding the tritosaurs, like Huehuecuetzpalli and Macrocnemus and the tritosaur fenestrasaurs, Langobardisaurus, Cosesaurus, Sharovipteryx and Longisquama as pterosaur ancestors when this is the only sequence of taxa with a gradual and increasing list of pterosaurian traits.

    • March 4, 2013 1:28 pm

      Considering that Tritosauria is polyphyletic, considering that your bias towards Sharovipteryx ignores several recent morphological examinations, and considering that your character examination is dubious at best, I am not going to waste time arguing about that.

      Note, however, that Sharovipteryx could ostensibly have led a similar lifestyle to modern gliding geckos. It shares several adaptations for arboreality, and it was a quadruped as well.

      • March 4, 2013 1:32 pm

        Polyphyletic? First I’ve heard. Please send refs. Sharovipteryx examinations? Please send refs. davidpeters@att.net

      • March 4, 2013 5:29 pm

        Its been considered as such since the mid-2000’s, it’s strange you’ve never heard before. If you must, see Modesto, 2004. As for Sharovipteryx, Darren Naish covers it pretty extensively.

      • March 4, 2013 5:39 pm

        Urls or titles please, if possible. Googling is not bringing these up. Tritosauria (literally the third clade of squamates) was a term invented by me within the last two years, so it is doubtful that Modesto 2004 could have described it as polyphyletic.

      • March 4, 2013 7:01 pm

        What you claim to be the bulk of Tritosauria, however, was pretty much rendered highly polyphyletic in that analysis.

      • March 4, 2013 7:17 pm

        What is the name of the taxon in Modesto 2004? That may help me to find it.

      • March 4, 2013 7:31 pm

        By any chance are you referring to Modesto and Sues 2004? And the tree they found published online here: http://en.wikipedia.org/wiki/Protorosauria

      • March 4, 2013 7:45 pm

        Yes, pretty much.

      • March 4, 2013 7:56 pm

        Good! Then our problems are solved. The large reptile tree at http://www.reptileevolution.com/reptile-tree.htm includes a magnitude more taxa providing that many more opportunities for taxa to nest. Given those opportunities the tree topology is different and better resolved without suprageneric taxa, like “squamates.” Also missing from the Modesto and Sues list are the more derived spheondontids that would nest with Trilophosaurus and rhynchosaurs. More taxa documents the convergence between tanystropheids and protorosaurs, which is quite remarkable, but still a case of convergence. This is a test that anyone can duplicate, like a good science experiment. I encourage you to test the large reptile tree any way you want to. It’s quite robust. I will post on this topic in the next few days at pterosaurheresies.com. Thanks for the impetus.

Trackbacks

  1. Pterosaur Tails, woohoo! « Gwawinapterus
  2. On pterosaur wing development | Gwawinapterus

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