So what did it eat? While the standard answer is "anything it wanted," new research covered in this article by David Moscato shows that the animal's jaws got heavier and its forward "fangs" sturdier as it got older. That implies a feeding strategy, seen today in the largest non-mammal predator in the world, the great white shark, of switching to bigger, slower prey as it gets older. For the Dunkleosteus, that may have included the largest fish available - other Dunks. Cannibalism is not definite, as the bites found on the armor of Dunks may have come from intraspecific competition for mates (the arthodires pretty much invented modern male-on-female sex).
The Dunk remains one of the most awesome marine predators ever. The largest mosasaurs and megalodons may have been bigger, but they couldn't copy the Dunk's best feeding strategy - opening those jaws and letting the prey faint dead away from sheer terror.
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Dunk figures from (top) Wild Safari and (bottom) Jeff Johnson.