Mastodon was revealed to have been a member of Team X (along with Wolverine, Sabretooth, Maverick, Kestrel and Silver Fox), and later a test subject of Weapon X. He was a well-built man who was well-trained in the use of firearms. He appeared in the flesh for the first time in Wolverine #61 as an old man, although he was supposed to be aging much slower than other humans due to an aging-suppression factor given to him at Weapon X. As the aging-suppression had somehow failed, he ultimately died of old age in Wolverine #62.