They grew from one common ancestor into a genus over fifty thousand species strong. Ammonites filled every ecological niche in the early oceans where they dwelled. Some ammonites were passive plankton feeders that relied on the currents for propulsion. And some were aggressive predators that hunted in packs, jetting along via hydro-propulsion with their powerful, razor-sharp beaks at the ready. These creatures were believed to hold one of the largest brains in its time, and the largest specimen on record was over ten meters long.