The 'world's oldest octopus' held that title for 25 years. It's not really an octopus.

A team cracked the case by examining the fossil using a synchrotron, which produces beams of light by accelerating electrons to high speeds.

The World’s Oldest ‘Octopus’ Fossil Isn’t an Octopus After All—Here’s What Led to the Discovery

For 25 years, a hand-sized sea creature embedded in Illinois rock held the title of the world’s oldest octopus. Now, thanks to a machine that shoots extremely powerful beams of light through stone,
New Scientist

Largest-ever octopus was great white shark of invertebrate predators

During the Cretaceous, 19-metre-long predatory octopuses swam the seas, and evidence from their fossilised remains suggest they may have been highly intelligent hunters

Inky the octopus pulled off the most daring aquarium escape you've probably ever heard

"Octopuses are smart, highly exploratory, very strong and their body is completely compressible because they have no fixed bones."

300-Million-Year ‘World’s Oldest Octopus’ Fossil Exposed as Nautilus Cousin

A fossil once believed to show the world’s oldest imprint of an octopus – some 300 million years old – is actually another ancient sea monster instead.

Inky the Octopus Squeezed Through a Drainpipe to Freedom - and Was Never Found

I don't think he was unhappy with us, or lonely, as octopus are solitary creatures. But he is such a curious boy."