Skip to content

Pterosaurs and bats, hands and wings

May 2, 2017

Two diagrams by alphynix, showcasing the evolution of pterosaurs and bats. In both cases, the “missing link” is indeed missing.

As discussed previously, flying vertebrates most likely did not evolve from gliders. Not only does gliding not lead to powered flight (see this podcast for further examination on this in regards to birds), but we have evidence that bats evolved from fluttering, not gliding, ancestors (Padian 2011, Jepsen 1970), while pterosaurs evolved from ground-dwelling hopping ancestors (Witton 2015). The discussion on avian flight (as well as other dinosaurs like Yi) is still ongoing and far from complete, so we will not focus on that here.

Instead, we’re going to focus on how pterosaurs and bats got aloft and how this resulted in two radically different wing structures. By association, we will discuss how this relates to speculative evolution, as well as how to understand the mysterious volaticotheres.

High contrast

Picture1

The basic bat and pterosaur wing models.

Although chiropteran and pterosaur wings are frequently compared, in truth they could be more functionally different.

The former evolved around the notion of grasping. Bat flight relies on the fingers bending as the wings move, an extension of the way mammal fingers normally flex (Cooper 2008, Swartz 1992).

To accomplish this task, bat wings are truly specialized hands, with all non-thumb fingers supporting the wing membrane. The digits themselves are elongated and highly flexible, individual bones even being able to bend considerably. Most interesting, most of the same tendon groups seen in other mammalian fingers remain, but they are thinned out to an extreme degree, hence the “skeletal” appearance of the wings.

Pterosaur wings, by contrast, are determined by the polar opposite, rigidity.

Picture

Pterosaur wing muscles.

Only one finger, the fourth digit, became a wing finger. Not only is this finger almost completely rigid – though some pterosaurs like anurognathids (Witton 2013) might have been able to bend the wing finger – its also oriented in a lateral way. It was unable to grasp the air.

Furthermore, the rest of the forelimb was also unable to grasp anything. All metacarpals are tightly pressed against each other, being unable to move significantly. The clawed fingers themselves have few phalanges (just one in the first digit, as in all sauropsids), rendering them rigid as well. The individual tendon systems associated with the digits also appear rather degenerate, except for the wing finger’s tendons, which are massive and would have made the hand area look rather fleshy in life.

Probably the best way to summarize this is to compare how both animals fold their wings:

MicrDav4

fossiloflarg

Bats fold their wing fingers, flexing them like closed fists.

Hatz and Aram Witton 2015 low res

Azhdarchids by Mark Witton.

Pterosaurs, on the other hand, couldn’t do this, and their wing fingers stuck out like blades.

But why?

3292920307_f0c19ac4ab

Iguana hand. Unlike mammals, which ancestrally have the same number of phalanges in all digits aside from the thumb, sauropsids have a gradient number of phalanges, from 1 in the first digit to as many as four in the fourth digit.

There are possibly various factors as to why we ended up with two wildly different wing types.

One could simply be the way sauropsid and synapsid hands are structured. While mammals have ancestrally the same number of phalanges in all digits (3) aside from the thumb (2), making them generally more evenly sized, in sauropsids the number of phalanges is gradient, increasing from the single phalange in the thumb, two in the second digit and so fourth until a maximum of five in the fifth (Witton 2013, Linzey 2012). Accordingly, in many reptiles like lizards this corresponds to a gradient in finger length.

To these ends, flexible, grasping hands are less common in sauropsids than they are in mammals. Most reptilian climbs prefer to use their digits as hooks, rather than grasp. Thus, a sauropsid flutterer might have been stressed to develop a more rigid wing, rather than a flexible one. The fact that rigid wings are present in pterosaurs, birds and scansoriopterygids would seem to prove my point.

However, I must admit that I’m not entirely satisfied with this explanation. A few sauropsids like chameleons have developed grasping hands (and, in the case of birds, grasping hindlimbs, which have the same phalangeal pressures as the forelimbs), so I think that, hypothetically, a reptile could develop grasping bat-like wings.

Rather, I think that the second possible factor is more likely. Simply, how bats and pterosaurs got airborne.

Bats, as mentioned before, became airborne by fluttering down. Baby bats still perform the hypothetical ancestral behavior: when approached from a high perch by a predator, they lunge themselves into the air, flapping their wings as to break the fall. Bat ancestors, likely arboreal or cave dwelling, likely flapped their forelimbs the same way. Boarder hands meant more efficient flapping, and over time this lead to wings.

Pterosaurs… are a total mystery. Its frankly impossible to say how exactly they became airborne.

However, the fact that non-pterodactyloid pterosaurs have hindlimbs specialized for hopping (Witton 2015), and that the hopping, kangaroo-like Scleromochlus is the closest known relative of pterosaurs (Witton 2013), it seems to suggest that pterosaurs evolved from a terrestrial hopping ancestor. This makes sense: at no point in pterosaur evolution did they ever develop arboreality, with even basal pterosaurs being more at home in the ground.

So, whatever process lead to pterosaur flight was likely ground focused, a true “ground up” scenario. My personal hypothesis is that pterosaur ancestors took the kangaroo-like hopping into a more lagomorph-like scenario, and the added boost provided by the forelimbs lead to flight. This hypothesis is based on the digitigrade nature of pterosaur forelimbs, which has been compared to ungulate mammals (Witton and Naish 2008).

Significance

vh8msc8

Ichthyoconodon commission by Dylan Bajda. Here, the animal is depicted with styliform-based pterosaur-like wings.

Animal flight is often oversimplified. Its often interpreted as a sort of homogenous process, functionally identical in at least vertebrate animals, be them birds, pterosaurs or bats alike.

As such, its common in creature design to just depict it randomly when it comes to speculative evolution. Critters might have either bat or pterosaur-like wings at random, it being simply a matter of preference as far as the artist is concerned. I am myself guilty of this, having made up many flying animal clades in various projects.

However, it seems as though flight was unique in locomotion methods in that various groups developed it in radically different ways. While you can easily mistake the flippers and fins or whales, marine reptiles and fish or the digitigrade hindlimbs of various mammals and dinosaurs, the wings of pterosaurs, birds, bats and insects are nothing alike besides just being airfoils.

Structurally, they are very different organs impossible to be distinguished for one another. And as we find new winged animals like scansoriopterygids, we see that this uniqueness continues.

Which brings us to a group I’ve talked about a lot, the volaticothere eutriconodonts.

As I’ve mentioned before, there is evidence that these animals might have been capable of powered flight. And, to add to the ambiguity, the most complete specimen so far, the Volaticotherium antiquum, bears a rather incomplete hand (Meng 2006 additional materials), rendering the conversation all the more frustrating.

What little do we know suggests an odd forelimb. The metacarpals are similar to those of the hindfoot, but the proximal phalanges are longer than them and distally expanded. These details make the hands somewhat distinct from the feet, so while a conventional paw can’t be ruled out, it might also hint at a more unconventional forelimb.

Given the proposed grasping function of the forelimb phalanges, if it is a wing then it might have functioned somewhat like bat wings. However, unlike in bat wings the metacarpals were not significantly elongated, while the phalanges were. This would have implications both in functionality as well as how flight evolved.

If it was a more conventional paw, there’s the possibility that it had a long styliform element. Styliforms evolved multiple times in mammals and archosaurs (Xu 2015), so a wing structure dependent on styliforms, while speculative, is definitely plausible. If this was the case, then we might be looking at an entirely different way of acquiring powered flight.

Either situation would be a vindication of the established pattern: each flying vertebrate group is unique.

References

http://blogs.nature.com/news/2011/11/new_theory_on_bat_flight_has_e_1.html

Glenn L. Jepsen, Bat origins and evolution, 1970

https://core.ac.uk/download/pdf/12172599.pdf

Cooper, K. L., & Tabin, C. J. (2008). Understanding of bat wing evolution takes flight. Genes & development, 22(2), 121-124.

Swartz, S. M., Bennett, M. B., & Carrier, D. R. (1992). Wing bone stresses in free flying bats and the evolution of skeletal design for flight. Nature, 359(6397), 726.

Witton, Mark P. (2015). “Were early pterosaurs inept terrestrial locomotors?”. PeerJ. 3: e1018. doi:10.7717/peerj.1018. PMC 4476129 Freely accessible. PMID 26157605.

Mark P. Witton (2013), Pterosaurs: Natural History, Evolution, Anatomy, Princeton University Press, ISBN 978-0-691-15061-1

Witton MP, Naish D (2008). McClain, Craig R., ed. “A reappraisal of azhdarchid pterosaur functional morphology and paleoecology”. PLoS ONE. 3 (5): e2271. doi:10.1371/journal.pone.0002271. PMC 2386974 Freely accessible. PMID 18509539.

Donald W. Linzey, Vertebrate Biology, JHU Press, 2012

Meng, J.; Hu, Y.; Wang, Y.; Wang, X.; Li, C. (2006). “A Mesozoic gliding mammal from northeastern China”. Nature. 444: 889–893. doi:10.1038/nature05234.; specifically, the associated appendix

Xu, X.; Zheng, X.; Sullivan, C.; Wang, X.; Xing, L.; Wang, Y.; Zhang, X.; o’Connor, J. K.; Zhang, F.; Pan, Y. (2015). “A bizarre Jurassic maniraptoran theropod with preserved evidence of membranous wings”. Nature. 521: 70–3. doi:10.1038/nature14423. PMID 25924069.

2 Comments leave one →
  1. May 2, 2017 6:43 pm

    Lizards usually have the fourth digit larger, not the last. And isn’t their first pdigit typically two phalanges?
    The idea that fliers didn’t evolve from gliders is thought provoking. But if that is the case, why we don’t see any intermediate flier today? We don’t have either animals that flap their limbs while jumping, or others who flap their limbs while falling from trees, even though it would be advantageous. The only things we have are either true fliers or gliders. So could gliders be indeed the ancestors of fliers?

    • May 2, 2017 8:42 pm

      Some do, but the usual formula is that the fifth digit has the most phalanges. Weirdly, it is rarely longer than the fourth digit, breaking the gradient.

      Some fish like hatchetfish and butterfly fish do use their forelimbs to flap themselves out of the water. Otherwise, I’m simply going to assume we’re simply unlucky.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: