[ncmncm]

Let me know when you spot an animal that drives around on wheels.

[dtech]

Physically you could have wheeled animals, but wheels just won't evolve. They are perfect for somewhat smooth surfaces where you want to travel in a somewhat straight line. Organisms just cannot confine themselves to that and survive.

Unless we find a planet composed entirely of solid smooth rock we won't find wheeled animals.

[jhgb]

How would wheels get fed with nutrition? Would they have tiny mouths?

[mod]

Gears have evolved, though, which is equally cool IMO.

[ncmncm]

Half-gears.

[pvaldes]

rotifers

[ncmncm]

Rotifers do not, in fact, have wheels.