The family Mylodontidae contains large ground sloths and belongs to the order Xenarthra, which also includes armadillos, glyptodonts, anteaters, tree-sloths and ground-sloths. Species in Xenarthra are known for low body temperatures and massive limb bones with well-developed claws. They have a second set of ribs that connects their regular ribs to their breastbone (sternum), and their hip and tail vertebrate are fused to form a solid structure called a synsacrum. All Xenarthrans lack an appendix.
Species in the Mylodontidae were the most successful during the Plio-Pleistocene (about 2.5 million to 10,000 years ago). They were found throughout South America and some (such as Paramylodon) reached as far north as North America. They were about 1.8m long and weighed about 1.5 tons. Like other sloths mylodonts were herbivores (only ate plants), and their lobe-like teeth with flat chewing surfaces suggests they were grazers that fed on grasses. Fossil dung of Mylodon darwinii found in a cave in Chile contained crushed blades and stems of grasses and sedges.
Scelidotherium and Paramylodon represent two different subfamilies of the Mylodontidae. Scelidotherium and its close relatives have long narrow skulls while Paramylodon and its relatives have wide broad skulls. The difference in skull shape suggests they fed differently. Scelidotherium was more selective in the parts of plants it ate while Paramylodon was more of a generalist and consumed all of the plant.
Some members of the Mylodontidae family, like the North American Paramylodon harlani, had small pebble-like bones in their skin called dermal ossicles. These were thought to help protect against predators.
