Long before enormous sharks like the great white, long before even the giant marine reptiles of the Age of Dinosaurs, in a world where animal life had only just begun to leave the seas, the biggest, meanest, scariest predator around was an armoured sea monster named Dunkleosteus.
Living around 360 million years ago, Dunkleosteus was one of the largest – and one of the last – of a group of fish called the arthrodires. These fish had thick, bony plates covering their skulls, and with a full body length of up to six metres, the armoured head gear of the largest Dunkleosteus fossils are positively nightmarish.
New fossil research presented at this year's meeting of the Society of Vertebrate Palaeontology shows that, as full-grown adults, these top predators had jaws strong enough to take down just about anything in their habitat – even each other.
Dunkleosteus did not have true teeth; instead, the skull's bony plates extended into sharpened "fangs" in front of the mouth. These fangs scraped together, continuously sharpening each other as the fish opened and closed its jaws. "You can almost see yourself in the polished surface of the fangs," remarked Michael Ryan of the Cleveland Museum of Natural History.
Ryan and colleagues examined the jaws of Dunkleosteus fossils from the famous Cleveland Shale in Ohio. They found that as these monstrous fish grew up, their mouths changed. The jaws gradually lengthened, while the fangs up front grew sturdier. This meant the jaws of an adult closed more slowly, but with a lot more power. While the younger fish were eating smaller, softer prey, the adults were capable of punching through even other heavily armoured arthrodires.
This sort of change in diet is common in large marine predators. As Eric Snively of the University of Wisconsin explained to me, modern-day great white sharks also exhibit a change in their jaws – and how they use them – as they get older, switching from dining on smaller fish to taking on larger prey such as marine mammals. The diet shift in Dunkleosteus even looks like it took place around a similar life stage as in great whites, at around two-fifths of their maximum size.
While Snively described the self-sharpening fangs of Dunkleosteus as "the best paper-cutter you can imagine", Cleveland Museum's Lee Hall opted for a more dramatic analogy. "Imagine a big guy with an axe swinging down as hard as he can, while another guy swings upward," he told me.
While showing me the fossils he has been examining – skull bones from a medium-sized Dunkleosteus (perhaps three metres long) with huge gouges across them, left by enormous fangs – Hall explained that the only creatures with jaws big and strong enough to inflict that damage were other Dunkleosteus.
What's more, these bites don't seem to be randomly placed. In some fossils, there are several gouges across the bone ending in a big fracture – the result of an attacker biting numerous times until the victim's armour finally cracked. And these marks were most often found near the joints and gaps in the armour, particularly on weak points toward the back of the skull. "If you’re going to disable this thing, the back of the head, the gills, are a good place to go for," Hall explained.
Why would one monster fish attack another? The obvious first answer is for food. Perhaps these marine terrors were cannibals? On the other hand, the bite marks could be the result of competition: one big fish fighting another for control of resources.
Growing up in Devonian seas teeming with early sharks and armoured leviathans could not have been easy. But if a young Dunkleosteus managed to survive to adulthood, its powerful jaws would have developed into some of the nastiest chompers the oceans have ever seen, and nothing in the sea would mess with that … except a bigger Dunkleosteus.
Boyle, J. T., Ryan, M., Snively, E., and Hlavin, W. J. 2016. Jaw ontogeny of the late Devonian “T. rex” with implications for feeding strategies and life history of the arthrodire Dunkleosteus terrelli. Journal of Vertebrate Paleontology, Programs and Abstracts, 2016, 103.
Hall, L., Ryan, M., and E. Scott. 2016. Possible evidence for cannibalism in the giant arthrodire Dunkleosteus, the apex predator of the Cleveland Shale Member (Fammenian) of the Ohio Shale. Journal of Vertebrate Paleontology, Programs and Abstracts, 2016, 148.