Limb bone proportions, strength and digging in some Lujanian (Late Pleistocene-Early Holocene) mylodontid ground sloths (Mammalia, Xenarthra)

The mylodontid ground sloths (Xenarthra, Tardigrada) are among the most int riguing components of the Lujanian (Late Pleistocene-Early Holocene) fauna of the Pampean region in South America. Limb proportions and resistance to bending forces were studied in Scelidotherium, Glossotherium and Lestodon t o infer their locomotory abilities. Body masses were estimated using scale and computer-generated geometric models. Allometric equations were calculat ed from humeri, ulnae, radii, femora, tibiae, and the pes, and were used to predict linear dimensions from body mass. Slopes and intercepts were obtai ned using Model I and II regressions. An indicator of strength was calculat ed for humeri and femora. Body masses of approximately 850, 1,500 and 4,100 kg were estimated from scale models for Scelidotherium. Glossotherium and Lestodon, respectively. The proportions and capacity to resist bending forc es of the limb bones of Scelidotherium and Glossotherium indicate that they were well adapted for strenuous activities in which force is enhanced over velocity, such as digging. We consider these taxa as possible builders of the large Late Cenozoic burrows present in the Pampean region. Although lim b proportions of Lestodon are comparable to those of the others, its low st rength indicator suggests that its limbs were not as well designed to perfo rm such strenuous activity.