Name: Dunkleosteus (Greek for "Dunkle's bone"); pronounced dun-kul-OSS-tee-us
Habitat: Shallow seas worldwide
Historical Period: Late Devonian (380-360 million years ago)
Size and Weight: About 30 feet long and 3-4 tons
Diet: Marine animals
Distinguishing Characteristics: Large size; lack of teeth; thick armor plating
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. (By the way, if the name Dunkleosteus sounds funny, that's because it was named in 1958 after David Dunkle, a curator at the Cleveland Museum of Natural History.)
Dunkleosteus is known by about 10 species, which have been excavated in North America, western Europe, and northern Africa. The "type species," D. terrelli, has been discovered in various U.S. states, including Texas, California, Pennsylvania and Ohio. D. belgicus hails from Belgium, D. marsaisi from Morocco (though this species may one day be synonymized with another genus of armored fish, Eastmanosteus), and D. amblyodoratus was discovered in Canada; other, smaller species were native to states as far afield as New York and Missouri. (As you might have guessed, we can attribute the profusion of Dunkleosteus remains to the fact that heavily armored skin tends to persist unusually well in the fossil record!)
Given the near-worldwide success of Dunklesteus 360 million years ago, the obvious question presents itself: why did this armored fish go extinct by the start of the Carboniferous period, along with its "placoderm" cousins? The most likely explanation is that these vertebrates succumbed to changes in ocean conditions during the so-called "Hangenberg Event," which caused marine oxygen levels to plunge--an event that definitely would not have favored multi-ton fish like Dunkleosteus. Secondarily, Dunkleosteus and its fellow placoderms may have been out-competed by smaller, sleeker bony fish and sharks, which went on to dominate the world's oceans for tens of millions of years thereafter, until the advent of the marine reptiles of the Mesozoic Era.