Giant ground sloths, wooly rhinoceros, glyptodonts and short-faced kangaroos*: All have gone the way of the dodo, vanished from the face of the Earth. That's just a sampling of the large mammals that are no longer with us. Of the 57 species of megaherbivores that are known to have existed 50,000 years ago, only 11 of them survive. That's a grim 81 percent extinction rate. Defined as large-bodied terrestrial mammals with a mean adult body mass of 2,200 pounds or more, today's remaining megaherbivores comprise the likes of elephants, rhinoceroses, giraffes, and hippopotamuses. These animals play critical roles in their ecosystems, from seed dispersal to landscape management.