The ancient cephalopod, Nanaimoteuthis haggarti, appears to have been an apex predator that rivaled mosasaurs to rule prehistoric seas.

Mesozoic seas were full of marine monsters. There were snaggle-toothed fish, shell-crushing sharks, and, of course, enormous mosasaurs.

Now, researchers have revealed another dangerous denizen of the ancient deep, one that wielded eight arms, grew to mythic proportions, and likely bit prey with a bone-breaking beak.

Enter the "Cretaceous kraken": Nanaimoteuthis haggarti.

Paleontologists recently examined a trove of fossilized beaks from octopuses that lived between 100 and 72 million years ago. Using the jaws, they estimated the size of the creatures. They found that N. haggarti stretched to about 60 feet long, longer than a city bus and surpassing the largest known giant squid by nearly 20 feet. That makes these ancient octopuses among the largest invertebrates to have ever lived.

The study, which was published Thursday in Science, also suggests that prehistoric vertebrate predators — such as sharks, plesiosaurs, and mosasaurs — may have met their match in these spineless cephalopods.

“It challenges the common view of an ‘age of vertebrates’ in marine ecosystems,” says Yasuhiro Iba, a paleontologist at Hokkaido University in Japan and an author of the new paper. He thinks these octopuses used their massive size, flexible arms, and powerful bites to achieve apex predator status in the ancient ocean.