Why Did Pterosaurs Fly with Their "Ring Fingers"? | The Secret Hidden in the Reptilian Blueprint

Pterosaurs were flying reptiles that lived during the same era as dinosaurs. Rhamphorhynchus, Pteranodon, and Quetzalcoatlus are likely well-known examples.
While this is a famous story for those interested in paleontology,the majority of a pterosaur's wing was supported solely by the fourth digit (the ring finger in humans).
Looking at the wing skeleton, it has developed to be unusually thick and long—so much so that it is hard to believe it was originally just one of the fingers—and it serves as the frame that supports the wing.

So, why on earth did pterosaurs adopt such an eccentric structure?
In this article, I would like to explore this based on the body structure of reptiles and examples from living animals.

The Structure of Pterosaur Wings

First, let's compare the wing structures of the three representative flying animals: pterosaurs, bats, and birds.

• Bat: Extends the second through fifth digits significantly and stretches a membrane between them

• Bird: Extends and spreads feathers from the first to third digits (the second digit is the most developed)

• Pterosaur: Develops only the fourth digit to be unusually long and supports the membrane with that single finger

Perhaps it is just because we are used to them, but the wing structures of bats and birds have a certain sense of stability, and there is a feeling of satisfaction that "they are well-made."
In contrast, the more you look at the structure of a pterosaur's wing, the more you cannot stop feeling a sense of discomfort, thinking,
"How did it end up like that!?
"

Why the Fourth Digit?

Why did pterosaurs adopt the eccentric structure of supporting their wings only with an abnormally developed fourth digit?
Actually, it seems that the basic design of the reptilian body is related to this.
Generally, in reptilian hands, the first digit (thumb) is small and has low mobility, while the third digit (middle finger) and fourth digit (ring finger) tend to be relatively long and have higher mobility.

For example, in iguanas, the third and fourth digits are the most developed, playing an important role in gripping tree branches.I think you can clearly see the difference in finger length by looking at the iguana photos posted on the following site:

Also, a similar trend can be seen in the fingers of living crocodiles and primitive dinosaurs like Coelophysis.

In this way, reptiles are designed so that the outer fingers of the hand are naturally prone to development. This is related to developmental factors such as the axial structure of the skeleton and how the muscles are attached.

From this, the seemingly eccentric structure of pterosaurs "developing only the fourth digit" can actually be considered a natural evolutionary flow for reptiles.

The Path from Fourth Digit Development to Flight

We now understand that pterosaurs, being reptiles, had a foundation where the fourth digit was prone to development.
Then, from there, through what process did it evolve into a flight-capable wing with the fourth digit as a support pillar?

In reality, evidence regarding the origin of pterosaur flight is scarce and has not yet been elucidated.
Both (1) terrestrial origin (evolution from leaping to flight) and (2) arboreal gliding origin are being considered, and while the terrestrial origin theory seems to be the mainstream, it appears that the matter has not yet been settled.

Arboreal Gliding Origin Theory

As a personal feeling, I think the arboreal gliding origin makes more sense.Regarding the step-by-step evolution of pterosaurs originating from arboreal gliding, I would like to state my own arbitrary view below.

• Initial stage: The fourth digit, which is advantageous for grasping branches and leaping, developed to be long for arboreal life

• Middle stage: The fourth digit developed further because it worked advantageously for posture control during leaping and gliding

• Late stage: The fourth digit develops into a wing strut, completing a wing structure that enables flight structure.

We do not know if pterosaurs truly went through this exact evolutionary process. However,
this pattern of evolution, where the same parts and structures are repeatedly reused and repurposed for different functions at each stage, is a natural flow observed in features like feathers and the jawbones of fish.
Pterosaurs did not suddenly become capable of flight; it is thought that they followed these evolutionary principles, surviving by utilizing the cards they were dealt (the fourth digit) at each step, which ultimately led to the acquisition of flight capabilities.

Furthermore, the evolution of finger structure for arboreal life, as shown in the initial stage, can also be seen in a creature living in the forests of Central and South America today: anole lizard. If you look at the hind limbs, the way the fifth digit grows is clearly different from the other digits, showing structural and functional differentiation. In fact, this fifth digit plays an important role in gripping tree branches and jumping.

Summary

Behind the pterosaur's structure of "supporting the wing with a single ring finger" lay the basic reptilian blueprint where the "fourth digit is prone to development." The seemingly strange wing structure of the pterosaur was actually the result of a natural evolutionary flow for a reptile. Also, it could be said that pterosaurs were magnificent beings that fully utilized the card they were dealt—the fourth digit—to acquire flight capabilities in a unique way different from other creatures.

🗒️ I have written other articles related to dinosaurs. Please take a look.

List of Image Credits

1. Article header image
・Title: Ptero-condor
・Author: Frederic A. Lucas
・Source: https://commons.wikimedia.org/wiki/File:Homology.jpg?uselang=ja
・License: Public Domain
・Modifications: Added margins to the left and right

2. Skeletal diagram of pterosaurs, birds, and bats
・Title: Homology
・Author: John Romanes (1848-1894)
・Source: https://commons.wikimedia.org/wiki/File:Homology.jpg?uselang=ja
・License: Public Domain
・Modifications: None

3. Photo of an anole lizard
・Title: Anolis carolinensis by Jeff Heard
・Author: Jefferson Heard
・Source: https://commons.wikimedia.org/wiki/File:Anolis_carolinensis_by_Jeff_Heard.jpg

・License: CC BY 2.0
・Modifications: None