Isolated from the influences of migration from other continents, the titanosaur sauropods of South America evolved in their own way to cope with such changing conditions as the spread of the grasslands. The lumber is a grassland-living sauropod that has evolved from a tree-browser into a grass-grazer. During the Paleogene and Neogene periods many strange grassland-dwelling titanosaurs developed to live in the newly evolved Pampas, including a number of long-legged, running forms. These were very vulnerable to the swift-footed carnivorous dinosaurs that spread across the continent about 4 million years ago, when the present land bridge of Central America was established to the North American continent to the north, resulting in the Great American Interchange. The long-legged sauropods were fast runners, but they were nowhere near as fast as the meat-eaters and were eventually made extinct.The sauropods that survived did so because they evolved defensive strategies. The turtosaur, for instance, evolved armor. Others, such as the lumber, took refuge in mere size. The lumber is the largest land animal alive in the Quaternary today, with a length of 25 meters (80 feet) and a weight of 70 tons. It cannot raise its neck far above its shoulders, but it can sweep round and reach vast areas of grass without moving its feet. Its teeth are confined to the front of its mouth and are hard-wearing, adapted for cropping grass. The trunk helps to pull bunches of grass towards the mouth, where they are torn off and swallowed. The skin is thick and leathery, quite resistant to most meat-eaters' claws and teeth, and most carnivores leave the huge creature well alone. One exception, however, is the cutlasstooth.