The world’s largest arthropod lived 300 million years ago. Now fossils show what it really looked like

LONDON (CNN) — For nearly two centuries, scientists have been trying to solve an enduring mystery surrounding a giant millipede-like animal called Arthropleura, which used its many legs to roam the Earth more than 300 million years ago.

Now two well-preserved fossils of the creature unearthed in France have finally revealed what Arthropleuras’ head looked like and provided insight into the giant arthropod’s lifestyle.

Today, arthropods are a group that includes insects, crustaceans, arachnids like spiders, and their relatives – and the extinct Arthropleura remains the largest known arthropod to ever live on the planet.

Scientists in Britain first found fossils of Arthropleura in 1854, with some adult specimens reaching 8.5 feet (2.6 meters) in length. But none of the fossils contained a head, which would help researchers determine important details about the creature, such as whether it was a predator similar to the centipede or an animal that, like the centipede, simply fed on decaying organic material .

In search of the first complete head, researchers conducted an analysis of Arthropleura fossils belonging to two young individuals discovered in France in the 1970s. The results were published Oct. 9 in the journal Science Advances.

The strange story of Arthropleura took a new twist when the study team scanned the fossils still trapped in the stone.

The head of each animal has features that belong to both millipedes and millipedes, suggesting that the two types of arthropods are more closely related than previously thought, according to the study authors.

“By combining the best available data from hundreds of genes from living species in this study with the physical properties that allow us to place fossils like Arthropleura on evolutionary trees, we have managed to square this circle. “Cenipedes and millipedes are actually each other’s closest relatives,” said study co-author and paleontologist Dr. Greg Edgecombe, an expert on ancient invertebrates at London’s Natural History Museum, in a statement.

A time for giants

Using the fossils and step-shaped tracks left behind by Arthropleura, scientists determined that the giant creature lived in what is now North America and Europe between 290 and 346 million years ago – and that it was just one of many giants that roamed the planet.

An abundance of atmospheric oxygen caused creatures such as scorpions and now-extinct dragonfly-like insects called prehensile flies to reach enormous sizes that dwarf their modern counterparts, the study authors said. But Arthropleura still stood out, reaching about the same length as modern alligators, said lead study author Mickaël Lhéritier.

Lhéritier is doing his doctorate on ancient myriapods, An arthropod group that includes millipedes and centipedes at France’s Claude Bernard University of Lyon 1 to understand how arthropods adapted to life on land millions of years ago.

After the animals died over time and were buried in layers of sediment, some of them became buried in a mineral called siderite, which solidified and formed a nodule around the remains. The stone enclosure helped preserve even the most delicate aspects of the fossilized creatures.

Such nodules were first discovered in the 1970s in a coal mine in Montceau-les-Mines, France, and then transferred to French museum collections.

“Traditionally, we split the nodules and took impressions of the samples,” Edgecombe said. “Nowadays we can examine them with scans. We used a combination of micro-CT (micro-computed tomography) and synchrotron images to examine the arthropleura inside and reveal the fine details of its anatomy.”

A fascinating arthropod ancestor

The 3D scans revealed two nearly complete specimens of Arthropleura that lived 300 million years ago. Both fossilized animals still had most of their legs, and one of them had a complete head, including antennae, eyes, mandibles and its feeding apparatus – the first Arthropleura head ever documented, Lhéritier said.

The team was surprised to find that Arthrorpleura had body features found in modern millipedes, such as two pairs of legs per body segment, as well as head features of early millipedes, such as the positioning of its mandibles and the shape of its feeding apparatus. The creature also had stalked eyes, like crustaceans, Lhéritier said.

The discovery not only helps researchers better understand what Arthropleura looked like, but also provides a closer evolutionary connection between modern millipedes and centipedes.

Scientists once thought the two arthropods were more distantly related, but in recent years genetic studies have shown that millipedes and centipedes are more closely related.

“This new scenario was (criticized) due to the fact that there was no ‘fossil’ or anatomical argument to defend this grouping, but our new findings on Arthropleura, combining features of both groups, tend to confirm this new scenario,” Lhéritier wrote in an email.

Researchers believe the two Arthropleura fossils belonged to juveniles because they are only 25 millimeters (0.9 inches) and 40 millimeters (1.5 inches) long.

Studies of Arthropleura specimens have shown that the animals vary in the number of body segments they have, similar to most millipedes, which add body segments until they reach a set maximum. However, according to the study authors, all body segments in millipedes are present at birth.

This finding suggests that Arthropleura reached its maximum segmentation in adulthood rather than at birth. However, the researchers are curious to know whether they have found real juveniles or a previously unknown smaller species and what the growth rate of such an animal is over time.

“Traces found elsewhere in Montceau-les-Mines indicate that these Arthropleura were probably about 40 centimeters (1.3 feet) long at their longest point,” Edgecombe said. “While there is no evidence to suggest that they could not be larger, we have no evidence of this at this time.”

What Arthropleura ate and other secrets

Now that researchers have discovered a complete Arthropleura head, they hope the discovery can help them solve more mysteries about the giant animal, including what it ate and how it breathed. However, other fossils have yet to be found that preserve additional aspects of the arthropod’s body, including the adult head.

“While clear gut contents have not yet been found, other details from these fossils add to the debate about Arthropleura’s diet,” Edgecombe said. “They do not have fangs or legs (specialized) for catching prey, suggesting they were probably not a predator. Because its legs are better suited for slow movement, they were probably more similar to the detritus-feeding centipedes that live today.”

Lhéritier, who is studying another group of ancient myriapods that may have been amphibious, said he was curious about Arthropleura’s stalk eyes.

“Nowadays, stalked eyes are a typical feature of aquatic arthropods such as crabs or shrimps,” he said. “Could it mean that Arthropleura could have been amphibious? To answer this question, we need to find the respiratory system of Arthropleura. Finding these organs can help us (understand) the connection of Arthropleura with water. Gills like crustaceans would imply an aquatic/amphibious lifestyle, while tracheas (like insects or other myriapods) or lungs (like spiders) would imply a terrestrial lifestyle.”

But deciphering what Arthropleura’s head looks like solves a central mystery, said James C. Lamsdell, an associate professor of geology at West Virginia University, in a related paper that appeared in Science Advances. Lamsdell was not involved in the new study.

“(These) remarkable findings, based on two nearly complete juveniles, provide a new perspective on this enigmatic arthropod,” Lamsdell wrote.

“The most exciting discovery comes from the heads of the specimens, which display a mosaic of millipede and millipede features. … While the mystery of the relationships of the largest known arthropod has been revealed, the reconstruction of the life history of this extraordinary creature can finally begin.”