Tuesday, 15 December 2020

The evolutionary origin of dragon flight

Perhaps some of you know the ‘dragon’ series of books by Marie Brennan. They are very enjoyable fantasy novels, describing the consecutive adventures of a woman in a fictional world resembling late 19-century Earth quite a bit. There are of course disparities, such as the existence of dragons. 

The protagonist is a young woman, or she is at the start of the first book; in later volumes she is older. She wishes to study Natural History, not at all a proper endeavour for a young woman in her rather Victorian world. To find out more about her and her adventures, you should read the books for yourself, because this post mostly deals with the cover art of one the books: 'Within the sanctuary of wings'. 

You can have a look at all the covers on the site of the artist, Todd Lockwood, where you can also buy prints and have a better look at the covers. 


Click to enlarge; copyright Todd Lockwood

Here is the cover in question. I think you can guess why it caught my interest, well, other than this being creative and high-quality art. The four creatures form an obvious succession, starting with a six-legged dog-lizard wingless beastie in front. The distance between its hind and middle legs is larger than between the middle and front legs, which is interesting, but the really important detail is the little membrane connecting the hind part of the middle leg to the torso. 

In the next stage, that membrane is a lot larger, and that second 'protodraco' seems to be gliding as much as it is running. The middle legs stick out sideways a bit; do they even hit the ground? The third stage has undeniable wings, and the fourth stage is all about wings. That one is a proper dragon, with four walking legs and two wings. I think this cover is a diagram of the evolutionary history of flight in dragons. 

The covers of all these books betray an obvious biological interest, which is completely in keeping with the content of the books: the heroine builds her career on the scientific study of dragons, and the readers get to hear interesting snippets here and there. When I saw the cover, I was curious whether this apparent evolutionary history also featured on the books, but I did not find that there. 

Would this progression from a running hexapod with six walking legs to a clade with four walking legs and two wings work? Regular readers will know that Furahan hexapods gave rise to flying forms as well, so I had to devise an evolutionary background for them, just as had been done for these dragons. Actually, hexapods did not only gave rise to the four-winged tetrapterates, but also to two-winged bipterates. On paper there is even a third flying clade, but I haven't worked much on that one yet. 

A basic concept in evolution is that an organ or feature will not develop just because some future descendent, a few million times removed, may make good use of whatever the organ does. That structure must convey some advantage in its early stage, or why would it be there? That advantage is unlikely to be true flight. But the future wing could help in gliding down from a tree or a cliff, or it could help the animal to jump higher and longer, or to run better. If you read about the evolution of flight in pterosaurs, birds and bats, you will find such explanations. I think that the winglets in the 'protodracones' on the cover show that, in dragons, flight evolved 'from the ground up', starting with more efficient running. 

Click to enlarge; copyright Gert van Dijk

At present, that is not how Furahan tetrapterates or bipterates got into the air. They took the 'down towards the ground' route. The rough sketch above was my first try at finding a shape for such animals. This is an arboreal species with folding leg flaps on all six of its legs. They crawl up into trees and then jump out of them and glide away, somewhat clumsily. 

Once the evolution of such gliders is jump-started (sorry for that one), the next stage is likely to involve optimization. The animals will probably not be very good at flying in the beginning, so it may pay for them to have an aerodynamically stable body plan. A stable flight scheme would make them less maneuverable, but at least it should keep them airborne without sophisticated neural control. The easiest way to achieve a stable shape would be to have the centre of lift directly above the centre of gravity, and in turn the easiest way to get there is probably to use the middle limbs as the main or only wings. 

There are intriguing other possibilities though, because the hexapod Bauplan in principle allows  not just one, but two or even all three pairs of limbs to be used for flight. With less than three pairs the next question is which pairs of limbs should be used for that? Any such design must also look into how the remaining legs must be adapted, so the animal will make aerodynamic sense while also being able to move about on the ground. 

Such things might be good material for another post, but I would not be surprised if readers run away with these and better ideas long before I will ever get around to writing that post.