Alamosaurus is perhaps the biggest animal that ever walked North America. These giants averaged 50-70 feet long and 15-35 tons. However, some isolated bones suggest that it could reach over 100 feet long and 60-80 tons or more! This puts it in the same size range as giants like Brachiosaurus and Argentinosaurus. The only animal that could take on this giant would have been the king himself, Tyrannosaurus rex. Alamosaurus lived roughly 65 million years ago.