Ancient Giant Octopuses: Whale-Sized Predators from 72 Million Years Ago May Have Been the Largest Invertebrates Ever

A new analysis of fossilized cephalopod jaws reveals that giant octopuses from the Late Cretaceous period, living between 100 and 72 million years ago, may have reached lengths of up to 19 meters (62 feet), rivaling the size of whales and potentially making them the largest invertebrates ever known to science.

The findings, published in the journal Science on April 23, 2026, are based on the examination of more than two dozen exceptionally well-preserved fossil jaws discovered in Japan and Canada’s Vancouver Island. These fossils, some of which show extensive wear patterns, indicate that these ancient octopuses were active predators capable of crushing hard-shelled prey such as marine reptiles and large fish.

According to researchers led by Yasuhiro Iba of Hokkaido University in Japan, the size and wear on the jaws suggest these creatures were not passive scavengers but apex predators in Cretaceous oceans, occupying a ecological niche similar to that of modern killer whales or ancient marine reptiles like mosasaurs.

The study estimates body lengths ranging from 1.5 to 4.5 meters, with the addition of long arms bringing total length to between 7 and 19 meters. Even at the lower end of this range, these animals would have been enormous by today’s standards, far exceeding the size of the largest living octopus, the giant Pacific octopus, which rarely exceeds 5 meters in total length.

One particularly notable observation is the uneven wear on the fossilized jaws from left to right, which may indicate a preference for using one side of the body during feeding. In modern animals, such lateralization is often associated with advanced brain function, suggesting these prehistoric octopuses may have possessed complex neurological capabilities.

Because octopuses lack hard bones, their soft tissues rarely fossilize. Paleontologists must rely on durable parts like beak-like jaws to infer size, behavior, and ecology. In this case, the jaws provided critical evidence not only of the animals’ massive scale but also of their active role as hunters in ancient marine ecosystems.

The research team emphasized that these findings challenge long-held assumptions about the evolutionary role of invertebrates in prehistoric oceans. For decades, scientists believed that the largest marine predators were vertebrates with backbones, while cephalopods like octopuses and squid played only minor roles. This discovery suggests that, under certain conditions, invertebrates could evolve to dominate as top predators.

While the exact appearance of these creatures remains unknown due to the absence of complete fossils, artists’ reconstructions based on the jaw data depict them as large, kraken-like animals gliding through Cretaceous seas, using powerful arms to capture prey and strong jaws to break through shells and bones.

As with all paleontological inferences, uncertainties remain. Scientists continue to debate the precise classification of these fossils and whether they represent true octopuses or closely related cephalopod lineages. However, the consistency of jaw morphology, wear patterns, and geographic distribution across multiple specimens supports the interpretation of large, predatory cephalopods in Late Cretaceous oceans.

This study adds to a growing body of evidence that prehistoric oceans hosted a far greater diversity of giant invertebrates than previously recognized. It also underscores the importance of rare hard-part fossils in reconstructing the biology of soft-bodied organisms that leave little trace in the geological record.