Fossils Reveal the Face of an Extinct Nine-Foot-Long ‘Millipede,’ the Largest Arthropod to Ever Live

In the oxygen-rich air of the Carboniferous period, between roughly 300 million and 360 million years ago, some animals swelled to monstrous sizes. Dragonfly relatives with two-foot-long wingspans flew through lush forests, but they weren’t the biggest. Scurrying across the earth, a 110-pound, nine-foot-long creature called Arthropleura ranks as the largest arthropod to ever live.

For years, however, the face of the gigantic crawler eluded scientists—Arthropleura fossils discovered since the 1800s were often only remnants of headless exoskeletons left behind during molting. Now, in a new study published this month in Science Advances, researchers have finally been able to piece together the head of Arthropleura and shine a light on the face of this many-legged beast.

The breakthrough came from a discovery of two well-preserved juvenile Arthropleura fossils at Montceau-les-Mines, France, according to Live Science’s Sierra Bouchér. These specimens, which were each less than two inches long, might not represent a species that grows to a full nine feet in adulthood, but they provided the first-ever glimpse of Arthropleura’s head.

The mockup of the insect’s head shows a rough circle adorned with two antennae, a small mandible hidden underneath and eyestalks protruding from the sides.

“We have been wanting to see what the head of this animal looked like for a really long time,” James Lamsdell, a paleobiologist at West Virginia University who was not involved in the study, tells Christina Larson of the Associated Press.

Arthropleura is an enormous arthropod—one of a group of invertebrates that includes crustaceans, spiders, insects, centipedes and millipedes. However, confusion surrounding the animal’s body has driven fierce controversy around where exactly it lies on the arthropod family tree, ever since its discovery in 1854. Scientists hadn’t been sure whether to place it among the millipedes or the centipedes.

But when examining the new fossils, they found a strange combination of features. “We discovered that it had the body of a millipede, but the head of a centipede,” study co-author Mickaël Lheritier, a paleobiologist at Claude Bernard University Lyon 1 in France, tells the AP.

In fact, Arthropleura displays various millipede and centipede characteristics—like millipedes, it has two pairs of legs per body segment, whereas centipedes only have one pair per segment. (Arthropleura had 24 body segments and 44 pairs of legs.) And while its jaw position resembles that of a centipede, its shape and antennae are most similar to those of a millipede.

One of its features, however, was a surprise that is not seen in any living members of the millipede or centipede families: eyestalks. The stalks on the juvenile Arthropleura resemble those of a crab, which could point to the creature having an amphibious youth before becoming terrestrial in adulthood.

Lhéritier tells Reuters’ Will Dunham that the fearsome-looking beast may have actually been the “cow of the Carboniferous.” Researchers concluded Arthropleura probably chowed down on decaying plants like millipedes do today, instead of hunting down prey like a centipede.

“Even if it had some centipede mouthparts, its trunk anatomy seems to indicate that it was not carnivorous like modern centipedes, as it did not have forcipules—centipede ‘fangs’—or any appendages built for hunting,” Lhéritier says to Reuters. “Having two pairs of legs by segments, like millipedes, affected its locomotion and implies it was a rather slow arthropod.”

After piling together all these lines of evidence, the team suggests they’ve solved the mystery of where Arthropleura belongs on the arthropod lineage. They say the gigantic, leggy animal is most closely related to millipedes.

Arthropleura lived between 290 million and 346 million years ago, skittering around the Earth’s tropical equator alongside other massive arthropods like the two-foot-long scorpion Pulmonoscorpius. The leading theory for this gigantism comes down to oxygen concentration. During the Carboniferous era, oxygen concentration was estimated to be at 30 percent compared to the 21 percent of our modern era.

For insects, oxygen enters their bodies through tiny openings called spiracles, traveling through internal networks of tubes called tracheae. Per Arizona State University’s School of Life Sciences, a higher concentration of oxygen in the air will push more oxygen into the deepest reaches of the insect’s trachea, which could have helped them balloon in size during the Carboniferous.

While finding the head of Arthropleura has been pivotal, future fossil discoveries could enable even more breakthroughs. Identifying the creature’s breathing system could reveal whether Arthropleura used tracheae like insects or lungs like spiders, Lhéritier tells CNN’s Ashley Strickland. And finding the head of an adult specimen could answer additional questions.

“As our knowledge of this iconic invertebrate continues to evolve, we’ll need to keep our minds open to new possibilities,” says study co-author Greg Edgecombe, an expert on ancient invertebrates at the Natural History Museum in London, in a statement.

Olatunji Osho-Williams | READ MORE

Olatunji Osho-Williams is an intern with Smithsonian magazine.