Octopuses' earliest relatives that lived 100 million years ago may have been "gigantic" predators that hunted alongside dinosaurs, according to new research.
Although scientists previously believed that the earliest finned octopuses lived around 15 million years ago, researchers with Hokkaido University found fossilized jaws inside Late Cretaceous rock samples, according to a study published in the journal Science Thursday.
Because octopuses are soft-bodied animals, they don't fossilize well except for the jaw bones, making their evolutionary history difficult to trace, the researchers explained in a news release.
The researchers used high-resolution grinding tomography and an artificial intelligence model to find the fossils, which were located in rock samples that had been preserved in seafloor sediments found in Japan and Vancouver Island from 100 to 72 million years ago.
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The Late Cretaceous was the final epoch of the Mesozoic Era, which was dominated by the dinosaurs, including tyrannosaurus rex and triceratops.
The fossils belonged to a group of extinct finned octopuses known as Cirrata that researchers believed crushed their prey with powerful jaws.
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"Our findings suggest that the earliest octopuses were gigantic predators that occupied the top of the marine food chain in the Cretaceous," Professor Yasuhiro Iba of Hokkaido University said.
"Based on exceptionally well-preserved fossil jaws, we show that these animals reached total lengths of up to nearly 20 meters, which may have surpassed the size of large marine reptiles of the same age."
Iba added that the most surprising finding was the "extent of wear on the jaws," which showed extensive chipping, scratching and cracking.
"In well-grown specimens, up to 10% of the jaw tip relative to the total jaw length had been worn away, which is larger than that seen in modern cephalopods that feed on hard-shelled prey," he explained. "This indicates repeated, forceful interactions with their prey, revealing an unexpectedly aggressive feeding strategy."
These findings change the way scientists view predators during the Late Cretaceous period, which they previously believed was dominated by vertebrate predators, with invertebrates at the bottom of the food chain.
"This study provides the first direct evidence that invertebrates could evolve into giant, intelligent apex predators in ecosystems that have been dominated by vertebrates for about 400 million years," Iba added.
"Our findings show that powerful jaws and the loss of superficial skeletons, common characteristics of octopuses and marine vertebrates, were essential to becoming huge, intelligent marine predators."
