On the game spore do I have to have the omnivore mouth to keep being an omnivore on the creature stage?

I like being an omnivore so I normally try to get the mouth by killing the creatures that have them but I couldn't this time. So I got to the end of the stage with a herbivore mouth and a carnivore mouth, it says that I am an omnivore on the history thing. There is a creature bigger then me who has an omnivore mouth I can't kill it so I am trying to get another creature who is as big as that one is to kill it. So will I need to get an omnivore mouth or can I just advance to the creature stage with both mouths?