[3] Megatheriids appeared during the Late Oligocene (Deseadan in the SALMA classification), some 29 million years ago, in South America.

An early genus that was originally considered a megatheriid, the more slightly built Hapalops, reached a length of about 1.2 metres (3.9 ft).

Their thick bones and even thicker joints (especially those on the hind legs) gave their appendages tremendous power that, combined with their size and fearsome claws, provided a formidable defense against predators.

The earliest megatheriid in North America was Sibotherium which arrived 5.3 million years ago, after crossing the recently formed Panamanian land bridge.

[8] Pseudoglyptodon Bradypus Nematherium Analcitherium Neonematherium Sibyllotherium Proscelidodon Catonyx Scelidotherium Valgipes Pseudoprepotherium Brievabradys Octodontotherium Paroctodontotherium Octomylodon Urumacotherium Baraguatherium Octodontobradys Mylodon Thinobadistes Lestodon Lestobradys Bolivartherium Simomylodon Pleurolestodon Glossotherium Paramylodon Schismotherium Pelecyodon Hapalops Hyperleptus Eucholoeops Pseudortotherium Megalonychotherium Pliometanastes Megalonyx Australonyx Megistonyx Ahytherium Pliomorphus Megalocnus Parocnus Acratocnus Neocnus Choloepus Analcimorphus Thalassocnus Lakukullus Mionothropus Nothrotherium Nothrotheriops Pronothrotherium Aymaratherium Xyophorus Prepoplanops Planops Prepotherium Diabolotherium Megathericulus Anisodontherium Pyramiodontherium Proeremotherium Eremotherium Megatherium