What do sea turtles and pterosaurs have in common?

Sea turtles can swim for long periods without resting. 
Pterosaurs could probably fly for long periods without resting.

Figure 1. Sea turtle and the pterosaur Arthurdactylus, both in dorsal view.

Figure 1. Sea turtle and the pterosaur Arthurdactylus, both in dorsal view, not to scale.

Sea turtles have forelimbs transformed into underwater wings.
Pterosaurs have forelimbs transformed into aerial wings.

Sea turtles have complex lungs (large surface area and volume). 
Pterosaurs probably had complex lungs, too, as air sacs penetrated the skeleton, as in birds.

Sea turtles have regional endothermy. Active tissues are warmer than surrounding waters.
Pterosaurs were probably endothermic overall, but it’s interesting to think about them being able to regionalize hotter and cooler areas by restricting bloodflow.

This post was inspired by the question:
Did pterosaur wings act like gills to supplement gas exchange and blood cooling? After all, the wings were quilted with blood vessels. They were thin and exposed to lots of oxygen.

In sea turtles cloacal sacs supplement gas exchange while underwater. So, it’s true, some turtles do breathe through their butts! The champion turtle in this regard is Rheodytes leukops, an Australian river turtle.

Now, I’m not saying the same for the pterosaur cloaca.

I’m just suggesting that a thin membrane festooned with blood vessels and exposed to a clear airstream, might have taken advantage of this for gas and heat exchange. Probably a question that can never be answered, or else is painfully obvious.

Nature always finds a way.

Certainly in pterosaurs the trachea, lungs and air sacs did most of the work, even in pterosaurs with no external nares. Oxygen was always surrounding them. Yes, in certain pterosaurs, the external nares did not merge with the antorbital fenestra, but became vestigial, as in plesiosaurs. Even so, simply opening the mouth even a tiny slit, would admit plenty of air to the pterosaur trachea.

Added August 16, 2013. Reprinted from earlier. 
Sam Johnson commented on July 28, 2012: “It seems more logical that the hollow bones transported oxygen from the membranes to the flight muscles than from the lungs to the membranes without muscles.

“Flying requires an enormous amount of energy. the bat Pallas has the fastest metabolic rate of any mammal (including shrews), requiring a lot of oxygen. Mammalian lungs simply cannot supply such large amounts of oxygen, being far less efficient than bird lungs. For example, the Mexican free-tailes bat can reach 60 mph for a few seconds and its lungs simply cannot provide the huge amount of oxygen required for that much power. Their low wing loading (relative to birds) may perhaps be due to respiration. During flight the underside is at a higher pressure, enhancing oxygen uptake and the top side is at a lower pressure, enhancing CO2 release.”

So maybe there’s something to this wing/gill idea. It works in bats (Makanya and Mortola 2007), who wrote: “We conclude that in [the fruit bat] Epomophorus wahlbergi, the wing web has structural modifications that permit a substantial contribution to the total gas exchange.”

References
Makanya AN and Mortola JP 2007. The structural design of the bat wing web and its possible role in gas exchange. J Anat. 2007 Dec;211(6):687-97. Epub 2007 Oct 26.

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