Mammoth femur. Palaeontologist Pascal Tassy, director of the palaeontology laboratory at the National Museum of Natural history, Paris, France, holding a fossilised femur (thigh bone) from an American mastodon (Mastodon americanum). The American mastodon (Mastodon americanum) is an extinct North American proboscid mammal that lived from about 3.7 million years ago until about 10,000 BC. It was the last surviving member of the mastodon family.