Yeah, I guess the idea is that when Crono, Glenn and Magus arrive in Prehistory, they say that something should be done with the Dragon Tooth but they panic and don't know what to do exactly. Then Sorin (who thinks faster thanks to being a cyborg) decides to simply jump in the Dragon Tooth portal and stop the Tooth or the resurrected Reptites on the other side of the portal, in 3 million B.C.