Imagine an octopus as long as a school bus, lurking in the oceans when dinosaurs still walked the Earth. That image might not be as far-fetched as it sounds, according to a team of scientists in Japan.
Researchers from Hokkaido University have uncovered evidence that ancient octopus relatives were actually gigantic predators, not the small deep-sea creatures we see today. Their findings, published in April 2026, reveal that these massive mollusks ruled the seas between 100 million and 72 million years ago.
The scientists examined fossilized jaws found in ancient seabeds near Hokkaido, Japan and on Vancouver Island in Canada. Though these remains had been known for years, nobody was sure what kind of animal they belonged to or how they lived.
"It was still unclear whether they were truly octopuses, how large they were, what they ate, and what role they played in the marine ecosystem," said Yasuhiro Iba, a paleontologist at Hokkaido University who co-authored the study.
Now those questions have answers. The largest species identified, called Nanaimoteuthis haggarti, may have reached lengths of 6.6 to 18.6 meters — or 22 to 61 feet. At the bigger end, that's roughly two-thirds the size of a blue whale, the largest animal ever known to exist.
"That is an enormous size for an octopus," Iba noted. "It suggests that it may have been among the largest invertebrates ever known."
The fossil jaws showed signs of wear, scratches, and chips — marks suggesting these giants snacked on creatures with hard shells, placing them among the top predators of their time. Modern relatives like the dumbo octopus, by contrast, eat only slow-moving invertebrates and grow no bigger than a few feet.
The discovery also challenges what scientists thought they knew about ancient ocean life. Until now, researchers believed that the top levels of the Cretaceous marine food web were occupied mainly by swimming reptiles, sharks, and large fish. The new evidence suggests giant octopuses belong in that elite group too.
"It changes how we think about Cretaceous marine ecosystems," Iba said.
The researchers point out that soft-bodied animals like octopuses rarely become fossils because they lack bones or shells. This means the fossil record skews toward hard-bodied creatures, and experts may have been missing huge parts of the picture — literally.
The findings remind us that the deep past still holds surprises, even about animals we thought we understood.