An extinct millipede the length of a car once roamed northern England

Scientists stumbled upon the fossilized remains of an ancient millipede the length of a small car dating from before the time of the dinosaurs. The animal is believed to have broken the record for the largest-known arthropod, a phylum of segmented invertebrates that includes insects, lobsters and spiders.

The nearly three-foot-long Arthropleura fossil, discovered in England, about 40 miles north of Newcastle, is described in the Journal of the Geological Society on Tuesday. It represents just one section of a creature thought to be three times as long — measuring 2.7 meters long (nearly 9 feet) and weighing about 50 kilograms (approximately 11o pounds).

Lead author Neil Davies, an Earth scientist at the University of Cambridge, tells NPR that he and a group of Ph.D. students almost literally stumbled upon the fossil in January 2018.

The group was on a "social trip" to Northumberland, near England's border with Scotland. They'd stopped in an area that Davies says he knew well from previous holiday visits.

"Just as it was getting dark, we saw that there was a boulder that had fallen from the cliff," he says, then they noticed "a big crack ... and then saw this really large fossil inside."

Davies says he and his colleagues weren't sure what they had found, and only convinced themselves of what it was once they'd returned the next morning for a closer look.

This type of animal lived during the Carboniferous Period between 346 million and 295 million years ago, when what is now England was much closer to the equator.

While a number of preserved trackways believed to be from the creature have been uncovered over the years, only two other relatively intact fossilized specimens have ever been found. Neither of them were as big as the one Davies and his colleagues found, however.

One of the reasons that not many fossils have been uncovered is probably because the bodies of these giant millipedes tended to come apart after they died, Davies says. For this reason, he thinks what they found is actually a fossilized carapace — the casing that's discarded after the animal molts.

"People have found fragments of skeletons, individual legs or individual bits of skeleton, but not something [like this] that's kind of articulated," he explains.

Furthermore, he says, the area of Northumberland where the fossil was found is mostly sandstone, which "is normally not brilliant for preserving fossils."

"So the fact that this has been preserved is, on the one hand, surprising. But it just suggests that actually there might be a lot more and similar things in places where people haven't really looked for fossils before," Davies says.

The fossil appears to knock another ancient species known as a sea scorpion out of the top spot for largest-ever arthropod. The sea scorpions lived roughly at the same time as the Arthropleuras, but most lived in brackish and fresh water shallows rather than the open sea.

Davies' big find is not only a new and rare example of an extinct giant millipede, but because of the fossil's dated age, it might help shed light on why these creatures were so big. High levels of atmospheric oxygen during the late Carboniferous and Permian periods have been thought to account for it. But, the fossil that Davies and his colleagues found predates that peak, as do some the fossilized trackways found previously.

"The oxygen really doesn't take off until after these things have evolved, and it doesn't really peak until after they apparently go extinct," he says. "They don't quite match up."

Instead, he says, the verdant landscape that existed at the time and the lack of much competition might have given Arthropleura the chance to get bigger.

"They might be interacting with predators, they might be growing larger to develop ... predatory attributes," Davies says. "But yeah, it doesn't seem to be oxygen, and it seems to be something much more environmental."

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