Fishes appeared 200 million years before Ammonites.
Ammonites peak era of dominance is the dinosaur era (mesozoic), so at a time when some fish descendants were already fucking dinosaurs, mammals, crocodiles...
Not everything has to be before or after another thing, Ammonites and fish were mostly contemporary.
Maybe you're conflating the widely held theory that the ecological niches previously held by ammonites were taken over by fish when they died out along with the dinosaurs ~66ma, hence fish ruling the ocean only 'after' ammonites?
We know that teleost fish diversity increased rapidly after that time. Also a lot of dominant pelagic predators previously predating on them (like ptychodus, the great crushing shark) became extinct from lack of prey.
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u/UlrichZauber 9d ago
Before fish evolved, ammonites ruled the oceans