Living sloths fall into two genera, Choloepus with two claws on the forelimb and Bradypus with three. Both hang beneath branches, both have long curved claws, both move slowly, and both grow algae in their hair. Yet they sit in separate families, Choloepodidae and Bradypodidae, in different branches of the sloth tree, and the common ancestor of the two genera is dated to roughly 28 million years ago.
The similarities are therefore convergence: two lineages arriving independently at a suspended, leaf-eating, low-energy canopy life. Differences remain. Two-toed sloths take a more varied diet that can include fruit and animal matter, they have caniniform front teeth, they have fewer neck vertebrae and so less neck rotation, and they are more willing to bite.
The naming is itself a common error. All sloths have three claws on each hind foot; the two-toed and three-toed labels refer only to the forelimbs, so the more accurate terms are two-fingered and three-fingered. Exactly where each living genus sits among the extinct ground sloths is still being revised as ancient protein and DNA data accumulate.