Mylodon

Mylodon is an extinct genus of large ground sloths that belonged to the family Mylodontidae. It inhabited southern South America from the Lower Pleistocene to the end of the Late Pleistocene–early Holocene transition. Well known from abundant fossil remains—including exceptionally preserved skin, hair, and dung—the genus is represented primarily by the species Mylodon darwini, first described in 1840 from specimens collected by Charles Darwin during the second voyage of HMS Beagle. Mylodon was widely distributed across the Pampas and Patagonia and survived until roughly 12,000–10,000 years ago, overlapping with the earliest human populations in the region.

Geographic and Temporal Distribution

Fossils of Mylodon have been recovered from southern Bolivia, through the Argentine Pampas, and southwards to the extreme tip of Patagonia, marking it as the southernmost-ranging of the Pleistocene ground sloths. Although the earliest known remains may date to the Lower Pleistocene, the majority are Late Pleistocene in age. The genus is especially famous for specimens found in Cueva del Milodón, a cave in southern Chile where hide, hair, and dung were preserved under stable cold conditions. These unusually fresh-looking remains once fuelled speculation that the species survived into historical times.
Recent debate has proposed distinguishing a second species, Mylodon lista, for Patagonian material, but the traditional classification retains M. darwini as the sole recognised species.

Discovery and Preservation

The genus was named by Richard Owen following the discovery of a nearly complete lower jaw and associated teeth collected by Darwin near Bahía Blanca. Subsequent cave discoveries in Chile and Argentina during the late nineteenth century yielded remarkably preserved samples, including hide fragments thickened with ossicles and coprolites demonstrating the animal’s diet. Early twentieth-century explorers, misled by the fresh appearance of these remains, mounted expeditions in search of living representatives.
Material from Cueva del Milodón, excavated in 1896, is among the best preserved Pleistocene mammal remains known. Associated fauna included other extinct Patagonian species. Items such as dung samples were later displayed in major museums and remain valuable for reconstructing palaeoecology.

Physical Characteristics

Mylodon was a heavily built ground sloth measuring approximately 3–4 metres in length and weighing between one and two tonnes, though some estimates suggest a maximum mass approaching 1.6 tonnes. Its body proportions resembled those of related genera such as Glossotherium and Paramylodon, though it was smaller than the giant Lestodon. The preserved skin suggests the presence of embedded dermal ossicles, contributing to a defensive covering.
The skull was notably elongated and narrow compared with other mylodontids. Lengths ranged from about 59 to over 70 centimetres. The rostrum was long and tapered, with the nasal bones extending far forward and forming a fully closed nasal arch in adults—a distinctive feature within the family. The zygomatic arch was slender, not forming a completely fused structure, and the occipital region showed a curvature of roughly 120 degrees relative to the skull roof. Numerous foramina were present in the palate, and the glenoid pit permitted wide mandibular movement.
The mandible was elongated, with a forward-sloping symphysis positioned above the occlusal plane. The ascending ramus rose at a shallow angle, and the coronoid process reached up to 20 centimetres in height. The angular process projected distinctly but did not extend above the occlusal surface.

Dentition and Feeding Adaptations

Mylodon possessed a simplified dental formula compared with other mylodontids. Whereas many relatives retained five upper and four lower teeth per side, this genus lacked the upper caniniform tooth entirely. The remaining teeth were molariform, producing a total of sixteen teeth. The lower caniniform was transformed into a molariform tooth, unlike the more pointed condition seen in Paramylodon, and more reduced dentitions of Glossotherium and Lestodon.
The teeth were high-crowned (hypsodont) and had flattened, weakly indented surfaces—adaptations consistent with intensive grinding of vegetation. Coprolite evidence demonstrates a predominantly grazing diet consisting of grasses and sedges, indicating ecological reliance on open habitats such as Patagonian steppe.

Behaviour and Ecology

As a large herbivore, Mylodon likely played a significant role in shaping Pleistocene grassland ecosystems. Its robust limbs and skeletal structure suggest a terrestrial lifestyle similar to that of other ground sloths. The presence of dermal ossicles in the hide implies defensive adaptations against predators.
The discovery of dung and hide in caves suggests that the animals may have used such shelters, though whether they resided there or merely entered occasionally remains uncertain.

Interaction with Humans and Extinction

Mylodon survived into the terminal Pleistocene, coexisting with early Paleoindian populations. Archaeological and palaeontological evidence from several South American sites suggests possible human predation or exploitation. The extinction of Mylodon around 12,000–10,000 years ago coincides with widespread megafaunal losses in the Americas.
Climatic fluctuations associated with the end of the last glacial period likely altered vegetation and habitat structure. Combined with increasing human presence, these changes may have contributed to the species’ decline. The exact weight of each factor remains the subject of ongoing research.