During the early Pleistocene the South American landscape was filled with giant animals. Out birds from last week were (specifically Phorusrhacos) not a significant feature of the landscape anymore. Not that they were waiting specifically for the terror birds to become less important fauna of this landscape, but mammals were becoming more dominant and larger during this time. This coincided with stabilization of the animal movements across the isthmus of Panama, but the animal we will look at this week was not necessarily interested in that migration; though some of the descendants of the migration into South America likely impacted the evolution of this animal from its ancestors over the years since the Great American Biotic Interchange had started. The image below, by Dmitry Bogdanov, shows three of Argentina's large land animals of the early Pleistocene. From left to right they are Argentavis, a large flying bird, Glyptodon, a relative of the extant armadillos, and Megatherium, otherwise known as the Megathere, or Giant Ground Sloth. As a genus, Megatherium has been divided into two recognized subgenera (more on this later) and 6 recognized species. Ultimately, Megatherium and the Glyptodon are related, as both are in the Superorder Xenarthra, which also includes modern armadillos, tree sloths, and South American anteaters (other animals called anteaters, like the aardvark actually belong to different groups of animals). One of the largest ever known land mammals, Megatherium was approximately 6 m (20 ft) from nose to tail and 4 tons. These estimations make Megatherium as large as modern elephants and second, in history, to very few mammals; a few species of contemporaneous mammoths were larger. Possibly at least partially bipedal, Megatherium was capable of reaching vegetation that other herbivorous mammals, like Glyptodon, would not have been able to reach. Their shear size would have made them vulnerable to very few predators as full grown adults.