The marine animals of the Devonian period, over 100 million years before the first dinosaurs, tended to be small and meek, but Dunkleosteus was the exception that proved the rule.
This huge (about 30 feet long and three or four tons), armor-covered prehistoric fish was probably the largest vertebrate of its day, and almost certainly the largest fish of the Devonian seas.
Reconstructions can be a bit fanciful, but Dunkleosteus likely resembled a large, underwater tank, with a thick body, bulging head, and massive, toothless jaws.
Dunkleosteus wouldn’t have had to be a particularly good swimmer, since its bony armor would have been sufficient defense against the smaller, predatory sharks and fish of its briny habitat, such as Cladoselache.
Because so many fossils of Dunkleosteus have been discovered, paleontologists know a good deal about the behavior and physiology of this prehistoric fish.
For example, there’s some evidence that individuals of this genus occasionally cannibalized each other when prey fish ran low, and an analysis of Dunkleosteus jawbones has demonstrated that this vertebrate could bite with a force of about 8,000 pounds per square inch,putting it in a league with both the much later Tyrannosaurus Rex and the much later giant shark Megalodon.
Dunkleosteus is known by about 10 species, which have been excavated in North America, Western Europe, and northern Africa.