2. The sloth is the only animal in the world to not have seven cervical (or neck) vertebrae (apart from manatees). BUT the Two-toed and Three-toed sloths have a DIFFERENT number of vertebrae. The Two-toed has six and the Three-toed has NINE.
3. This crazy vertebrae inconsistency is because the Two- and Three-toed sloths do not have a shared ancestor until you go back 40 MILLION YEARS. They are a brilliant example of convergent evolution, where critters look the same and do the same things, because they've been surviving next to each other in the same environment for a very long time.