Giant Octopuses Up to 19 Meters Long Once Inhabited the Oceans of the Dinosaur Era

The oceans of the dinosaur era harbored inhabitants of unimaginable proportions: octopuses with an estimated total length of 7 to 19 meters. At the upper end of that estimate, its length approached that of an adult sperm whale. The animal is Nanaimoteuthis haggarti, a finned octopus that inhabited the ancient North Pacific Ocean during the Late Cretaceous period. Its size makes this species a candidate for one of the largest non-colonial invertebrates in Earth's history. Interestingly, the depiction of this giant was reconstructed from jaw fossils, without the discovery of an intact body that would directly reveal the length of its arms. Research led by Shin Ikegami, along with a team from Hokkaido University and the Natural History Museum in London, analyzed 27 jaw fossils from Japan and Canada. The results were published in the journal Science on April 23, 2026. To estimate body size, researchers compared the proportions of the jaw to the bodies of modern octopuses. Therefore, the 19-meter figure should be understood as the upper limit of the estimation. Zoe Hughes, a cephalopod fossil curator at the Natural History Museum in London, described the difference in size compared to the findings she typically encounters. “Most Cretaceous octopuses I know are the size of a palm,” she said. “This is far, far larger.” Beyond size, the jaws hold clues about eating habits. Heavy wear suggests that this octopus likely frequently crushed hard-shelled prey. The surfaces of the right and left jaws were also not worn evenly, indicating a tendency to use one side when processing food. This pattern resembles lateral dominance in several modern animals, although it is not yet enough to confirm its intelligence. Its massive body and ability to crush prey support the suspicion that N. haggarti occupied a high position in the food chain, likely competing with large marine reptiles such as mosasaurs. The research team also examined Nanaimoteuthis jeletzkyi, a relative estimated to be 3 to 8 meters long that lived approximately 100 to 72 million years ago. Both species were previously linked to the vampire squid group before being re-identified as finned octopuses, or Cirrata. This group still has living members today, including the much smaller dumbo octopus. This comparison demonstrates the diversity of finned octopus sizes throughout its evolutionary history. However, the relationship of the group does not in itself prove that the dumbo octopus is a direct descendant of these two ancient species. The history of octopuses is difficult to trace because their soft bodies rarely fossilize. Their hard, beak-like jaws are more likely to survive, but they only reveal a portion of their lives. Both Nanaimoteuthis species disappeared from the fossil record about 72 million years ago, approximately six million years before the mass extinction that ended the Cretaceous period. Whether they truly went extinct at that time, or if subsequent traces of their life have simply not yet been found, remains an open question. Jaw fossils have revealed the existence of giant octopuses, while the end of their story remains hidden within the incomplete geological record. Main photo: The largest living octopus today, the giant Pacific octopus, is far smaller than Nanaimoteuthis haggarti. Photo by Abby Wood CC BY-NC-ND 2.0.