Saurification is a physiological mutation process.
Saurification typically occurs within Parasites from the moment they start receiving doses of yellow blood cells and as they begin to pilot a FRANXX. The more frequent they pilot, the more the process increases and they develop accelerated ageing conditions, such as gray hair and other physical and emotional effects. If one of the pilots possesses a genetic makeup that is the slightest from that of a normal human, a genetic mutation can occur. Saurification is normally kept under control due to the yellow blood cell injections but that is never the case if two pilots’ physiology does not match.
Hiro’s yellow blood cell count began to increase at an abnormally fast rate due to him riding in Strelizia with Zero Two, who is a klaxosaur-human hybrid and thus her physiology was incompatible with his. After his first two rides and looking at his medical report, Nana and Hachi surmised it was a matter of time before he died due to the process’ effects. After surviving his third ride with her, they discovered his saurification process has increased and, due to Hiro ingesting Zero Two’s blood when he was a child, he would undergo a genetic mutation because his yellow blood cell count has risen to a level no human could survive.
As a result of this, Hiro develops horns and fangs that are characteristics of a klaxosaur like Zero Two. Other parasites suffer effects, such as Miku's greying hair; Mitsuru and Goro's child fevers and Ikuno's rapid ageing after overusing her strength in Episode 21. This process is eventually kept under control after years of Ikuno's extensive research enables her to develop a cure for the parasites' rapid ageing.