Move over, megalodon, there’s a new contender for most terrifying marine predator of all time. An analysis of fossil jaws belonging to octopuses that lived between 100 million and 72 million years ago, during the Late Cretaceous period, shows that these creatures may have been the largest animals in the sea and smart to boot, rivaling sharks and marine reptiles in their powers of predation. The findings challenge received wisdom about the role of soft-bodied organisms in marine ecosystems and hint at how large body size and intelligence evolved in the denizens of the ocean realm.

Large vertebrates (animals with a backbone) are known to have occupied the top of the marine food chain for the past 370 million or so years—think sharks and ichthyosaurs, plesiosaurs and whales. In contrast, soft-bodied animals, or invertebrates, seemed to have served as prey. In the Late Cretaceous, while dagger-toothed dinosaurs such as Tyrannosaurus rex dominated the land, giant marine reptiles called mosasaurs were thought to have ruled the seas unchallenged.

The new study suggests that mosasaurs had competition from invertebrates, however. Shin Ikegami of Hokkaido University in Japan and his colleagues examined fossils of giant octopuses dubbed “krakens” after the sea monsters of Norwegian lore. Like modern-day octopuses and squids, these Cretaceous krakens had a “beak” composed of two jaws made of a tough material called chitin. The team studied fossilized jaws and the wear patterns on them to reconstruct the sizes and feeding behaviors of the ancient octopuses.

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The researchers determined that the fossils, which came from Japan and Vancouver Island, off Canada’s Pacific Coast, represent two species in a group known as the finned octopuses, Nanaimoteuthis jeletzkyi and Nanaimoteuthis haggarti. Their calculations indicate that the larger of the two species, N. haggarti, reached a total body length of up to 18.6 meters—comparable to an articulated bus. Today’s giant squid is around 13 meters long at most. Longer than the large vertebrates known from its time, N. haggarti may have been the biggest creature in the Cretaceous seas.

These ancient octopuses weren’t just huge. They were also formidable predators. Scientists have known for a while that large-bodied octopuses existed in prehistory. But researchers didn’t know what these animals ate—and thus their position in the food chain—because their stomach contents were not preserved. Ikegami’s team got at the question of diet a different way: studying damage to the beak that was caused by eating. The wear on the edges and tips of the fossil jaws suggest that N. jeletzkyi and N. haggarti were carnivores that crushed the shells and hard bones of their prey. Although the octopuses lacked the long jaws that mosasaurs and other top vertebrate predators used to grab prey, they could have used their long, flexible arms to seize large quarry while dismantling the prey with their powerful beaks. The asymmetry of the wear on the krakens’ jaws, meanwhile, hints that different parts of the animals’ brain were specialized for different tasks—a sign of advanced cognition in these invertebrates.

The authors of the new study observe that large marine vertebrates and cephalopods (octopuses and their kin) independently evolved similar adaptations. Both evolved jaws, which improved feeding efficiency. Vertebrates ditched their body armor and scales for smooth skin; cephalopods lost their external shell. These changes boosted swimming performance, body size, and cognition and ultimately produced predators in both groups that could claim pride of place at the top of the ocean’s food chain.