On the surface of things, a two-toed sloth doesn’t look much like its closest fossil kin. The tubby, pug-nosed mammal is not quite as imposing or majestic as Megalonyx – the “great claw” Thomas Jefferson discovered and mistakenly identified as an enormous lion over two centuries ago. But the two are relatively close relatives. In fact, today’s shaggy two-toed sloths are more closely related to Megalonyx than that other variety of upside-down mammal, the three-toed sloths. Together, the two- and three-toed flavors of sloth represent a remarkable case of convergent evolution.

The distant relationship between the two modern sloth genera – species of two-toed sloth are grouped under Choloepus, and three-toed species under Bradypus – isn’t immediately obvious. Both suspend their