Imagine walking along a beach and stumbling across a piece of rock — only to discover that hidden inside was the fossil of the largest-known invertebrate ever to live on Earth.
That is exactly what happened at Howick Bay in Northumberland, England, in January 2018.
A chunk of sandstone had fallen from a nearby cliff and cracked open on the beach. Inside was part of the fossilized remains of Arthropleura, a gigantic millipede-like creature that crawled across Earth roughly 326 million years ago — more than 100 million years before dinosaurs appeared.
Scientists estimate that the creature could have reached an astonishing 8.6 feet (2.63 meters) in length, measured around 22 inches (55 centimeters) wide, and weighed as much as 110 pounds (50 kilograms).
That made Arthropleura larger than the ancient sea scorpions that previously held the record for the biggest known invertebrate.
And the discovery itself was almost pure luck.
Paleontologist Neil Davies described it as a “complete fluke,” while noting that the enormous fossil was so heavy that four people were needed to carry it up the cliff.
Researchers believe the fossil may actually be a section of an exoskeleton that the animal shed as it grew. Its body likely separated after death, making complete fossils extremely rare.
Only two other Arthropleura fossils had previously been discovered — and both were much smaller.
The creature ruled ancient ecosystems for around 45 million years, but exactly why these enormous giants disappeared remains a mystery.
For now, one question still haunts scientists: what did this monster really look like? They have yet to find a fossilized head.