Chances are good that you would be startled to find a rhinoceros beetle perched over your bed one night. This South American insect is one of the largest in the world. It measures 6 inches long and 2 inches wide. That’s almost as big as your hand.

But that’s nothing compared with invertebrates from the Carboniferous Period. About 326 million years ago, before there were dinosaurs, invertebrates could grow to the length of today’s cars. (Invertebrates are animals without backbones. Many, such as crabs, spiders and insects, have exoskeletons, which offer protection and support on the outside of their bodies.)

A fossil discovered in 2018 in England set a new world record for largest land-based invertebrate. It is a species of Arthropleura. Neil Davies, a biologist at the University of Cambridge who studied the fossil, says it’s sort of like a second cousin of today’s millipedes. It would have measured eight feet long and weighed 110 pounds.

Other Arthropleurafossils have been found before. But none is as big. None is as old. And almost none of them has been complete.

“The evidence is fragments,” Davies says. Scientists have found pieces of leg or an animal’s back, and this latest, largest invertebrate fossil is missing its head and other parts. “One very small Arthropleura specimen from France appears to be complete, but is only [two ], but that gives the impression of what the shape of the animals were like,” Davies says.

The Arthropleura and its behaviors are partly a mystery to scientists. For example, they don’t know what it ate. No heads for any of the large specimens have been discovered. That means scientists don’t know what kind of teeth they had and whether they were suited to eating plants or meat or both. One Scottish specimen was found near seeds. But, says Davies, no one has “found any plant material that’s been nibbled” by Arthropleura. “When we find an actual head, we’ll have a better idea of how these things were living.”

We do know, thanks to this new fossil discovery, that Arthropleura lived in open woodland. Scientists previously thought that it lived in coal swamps. And we know it must have had plenty of nutritious food to allow it to grow to its massive size. Davies says it’s unlikely that there were huge swarms of Arthropleura during the Carboniferous Period; there were fewer of all types of animals at that time. But it’s possible that invertebrates were the dominant animal group on land, just as they are the most plentiful group of animals on Earth today.

What caused these animals to disappear? They were probably outcompeted by creatures with backbones, such as reptiles, “which were more able to take advantage of resources on land,” says Davies.

Davies is excited about the discovery. “This was found in a pretty well-populated area, in a quite popular tourist area known for castles and scenery rather than fossils,” he says. “That tells you to go out and look at things, because you can find unexpected discoveries waiting to be made even in well-trodden places,” he said.

correction

An earlier version of this story incorrectly stated that invertebrates are animals with exoskeletons instead of backbones. Invertebrates are animals without backbones, but not all invertebrates -- including worms and jellyfish -- have exoskeletons. This story has been updated.