Today there are seven living species of tree sloth split between two very different genera: the three-fingered (Bradypus) and the two-fingered (Choloepus) sloths.
Although often grouped together, research suggests that the two types of modern-day sloths are only very distant relatives separated by 30 million years of evolution, acquiring their unusual upside-down lifestyles separately.
As a result, two-fingered and three-fingered sloths are surprisingly different animals with very different lifestyles.
2.1 The Xenarthrans
The superorder Xenarthra contains sloths, anteaters, and armadillos.
The sloth’s closest relative is the anteater, followed by the armadillo. These three groups of animals combine to form the Xenarthra superorder which includes a grand total of 31 extant (still living) species.
The xenarthrans are an odd order not closely related to other mammals. Xenarthra translates from Greek to “strange joints”, and they have unusual skeletons compared to most mammals.
2.2 Two-Fingered and Three-Fingered sloths
Two-fingered and three-fingered sloths are very different animals with very different lifestyles. These discrepancies can be seen in the biology, ecology, and morphology of each genus.
There are five species of three-fingered sloth (Bradypus)
- Brown-throated sloth (Bradypus variegatus)
- Pygmy sloth (Bradypus pygmaeus)
- Pale throated sloth (Bradypus tridactylus)
- Northern Maned sloth (Bradypus torquatus)
- Southern Maned sloth (Bradypus crinitus)
Molecular research has shown that the maned sloths split from the brown-throated and the pale-throated sloths over 12 million years ago, while the brown-throated and pale-throated diverged only 5-6 million years ago.
There are two species of two-fingered sloth (Choloepus)
- Hoffmann’s two-fingered sloth (Choloepus hoffmanni)
- Linnaeus’s two-fingered sloth (Choloepus didactylus)
2.3 Giant Sloths
Both the three-fingered and two-fingered tree sloths that we see today evolved from giant ground sloths, of which there were thought to be over 80 different kinds!
The largest of these was Megatherium, which weighed as much as an elephant and stood over 6 meters (18 feet) high. Thousands of ancient sloth burrow networks have now been discovered, the largest of which measures over 2000 feet long and is thought to have been dug out by many generations of extinct sloths.
Some ancient marine sloths even lived in the ocean, feeding from seagrass and seaweed in shallow water. Other giant ground sloths were covered in bony, armored plates, much like the armadillos of today.
Extinction of the Mega Sloths
Megatherium, also known as M. Americanum, lived from the Early Pliocene through the end of the Pleistocene.
The only larger land mammals that existed during this time were the woolly mammoth and the giant rhinoceros.
These giants went extinct approximately 10,000 years ago, probably due to an expanding population of human hunters.
