Rex Reynolds was a promising S.H.I.E.L.D. scientist assigned to assist Vincent Stegron while he was working with Dr. Curt Connors studying dinosaur DNA from the Savage Land. Rex was later assigned as a liaison to a strike team assigned to take down Stegron. Rex was seriously injured in the encounter and although he recovered, damage to his spine had left him paralyzed. Rex was determined to get his revenge against Stegron, and using his S.H.I.E.L.D. connections, managed to uncover Connors' original research as well as several of the dinosaur DNA samples. Using the DNA from a Tyrannosaurus Rex, Reynolds underwent a transformation similar to that of Connors and Stegron, but improvements he made to the process left his sanity intact. Calling himself T-Rex, Reynolds then tracked down Stegron and attacked him, but during the encounter several innocent bystanders were seriously injured. Reynolds and Stegron were captured by S.H.I.E.L.D. after their battle and placed in The Vault. When Nick Fury learned of the identity of T-Rex, and approached him with an opportunity to rejoin S.H.I.E.L.D. T-Rex worked on an elite S.H.I.E.L.D. black ops team for several years before taking an assignment in the Savage Land. Upon arriving in the Savage Land, T-Rex felt as if he found a place he truly belonged and Fury allowed him to stay there, acting as S.H.I.E.L.D.'s eyes and ears in the often tumultuous land. T-Rex has since joined forces with Ka-Zar and Shanna, acting as one of the guardians of the Savage Land.