[Image: Mark Witton’s interpretation of HyPtA C, the hypothetical third major stage of proto-pterosaur evolution.]
The fossil record has yet to reveal any obvious proto-pterosaurs, but that doesn’t mean we can’t speculate within reason about what these ancestral pterosaurs may have looked like. In his book Pterosaurs, Mark Witton refers to these animals as HyPtA (“Hypothetical Pterosaur Ancestors”) and divides early pterosaur evolution into major stages A through E, which we’ll take a brief look at below:
- HyPtA A: “Hyperactive Ground Hopper”
These animals would have resembled Scleromochlus. They already held their limbs beneath the body when walking. A coat of primitive pycnofibers helped them manage their body temperature, an advantage for creatures with high metabolism which would otherwise have lost more of their heat to the environment. They spent most of their lives on the ground, perhaps occasionally venturing into trees in search of food or a hiding place from predators.
- HyPtA B: “Reptilian Squirrel”
These proto-pterosaurs had limbs more adapted to life hopping amongst branches and between canyon walls than in HyPtA A. They also hunted more of their prey in high places. The fifth finger became vestigial and would be lost in later forms. The limbs were more mobile than in the previous stage, which would allow later pterosaurs to spread them out to create efficient airfoils.
- HyPtA C: “Early Glider”
As pictured above, these animals had evolved short but somewhat wing-shaped membranes between the now-elongated fourth finger and the body. This would have further improved their ability to jump long distances without losing height, unlike the membranes in passive gliders which would merely have served to slow falls.
- HyPtA D: “Poorly Flying Early Pterosaur”
The wings in these pterosaurs had become larger and more elaborate than in the previous stage, requiring them to be folded back when not in use. The claw on the wing finger had been lost since it no longer served a purpose [ETA: assuming they had a digit IV claw, which would have been unusual for archosaurs].
- HyPtA E: “The Real McCoy”
The common ancestor of the colorful variety of later pterosaurs arose during this stage. These pterosaurs’ proportionally larger wing membranes made them more readily capable of powered flight than in HyPtA D.