Is there any biological examples of freely rotating power systems? We have nice rotating joints with muscles to provide power, but I can't think of any joint that would allow the sort of free rotation while also producing torque, a wheeled animal would require.
Some microorganisms have cilia that rotate like a propeller. With complex molecular structures to provide a rotor effect
Something internal to some shellfish, I believe, a kind of mixing rod that rotates. Hold on, I'll check if it's powered. (Also rotifers but they're tiny.)
Hmm, no, it sounds like it's externally powered:
> The style consists of a transparent glycoprotein rod which is continuously formed in a cilia-lined sac and extends into the stomach. The cilia rotate the rod, so that it becomes wrapped in strands of mucus.
https://en.wikipedia.org/wiki/Rotating_locomotion_in_living_...
Or maybe the cilia ( = wiggly hairs) could be seen as a kind of motor. Depends how you count it and exactly what the set-up is, I can't tell from this.
I think I would count internal power created by the rotating component itself. I hadn't though of that possibility, since human made machinery usually has the power producing component located in the main body and transferring that power to a freely rotating component is quite hard. Biological systems wouldn't necessarily look like that, and could feasibly be powered by the wheels themselves deforming as if the wheels were a separate, but connected, biological system.
That's quite interesting.