Arthropleura: The Giant Millipede of the Carboniferous Period

Arthropleura was a giant myriapod, a relative of millipedes, that lived in the forests of Europe and North America from about 345 to 290 million years ago. Reaching an estimated 2.6 metres in length, it is the largest known arthropod ever to have lived on land. Its fossils, from a famous 2021 find on a beach in Northumberland to tiny juveniles in French coal mines, have helped scientists understand how life on land flourished in the Carboniferous Period.

Background: life in the Carboniferous forests

During the Carboniferous Period, between about 359 and 299 million years ago, the lowlands of Euramerica lay close to the equator. Wet Carboniferous coal forests of giant clubmoss trees, horsetails and ferns spread across what is now Britain, Germany, France and eastern North America.

These forests were home to many arthropods, the group of animals with jointed legs and external skeletons that includes insects, spiders and crustaceans. Some grew remarkably large, including the dragonfly-like Meganeura with a wingspan of around 70 centimetres. On the ground, the largest of all was Arthropleura.

Discovery and naming

Arthropleura was first named in 1854 by the German naturalists Hermann Jordan and Hermann von Meyer, based on fragments from coal-bearing rocks in Germany. The name means “jointed ribs”, referring to the segmented plates of its body. The best-known species is Arthropleura armata.

For more than a century its fossils were rare and fragmentary. Arthropleura had a body made of many segments, each protected by a broad plate, and after death these plates usually fell apart. Most finds were isolated pieces, and early scientists were unsure what kind of animal they came from.

Trackways

Some of the best evidence for Arthropleura comes from its footprints. Trackways known as Diplichnites show two parallel rows of many small footprints, sometimes up to about half a metre wide. A famous example from the Isle of Arran in Scotland was described in 1979 by Derek Briggs, Ian Rolfe and Jane Brannan. Others have been found in Nova Scotia, the United States and elsewhere.

These tracks show that the animal walked on land, not in water, and that it could move across open ground between areas of vegetation.

Trackways are especially valuable because they record living behaviour. Their width and the spacing of the footprints allow scientists to estimate the size of the animal that made them, and their presence in sediments laid down on river plains and dry ground shows that Arthropleura was not confined to the wettest parts of the swamps. Trackways have also been found in rocks with no body fossils, suggesting the animal was more widespread than its rare skeletal remains alone would indicate.

The 2021 Northumberland find

In January 2018 a large sandstone block fell from a cliff onto the beach at Howick Bay in Northumberland, north-east England. It split open to reveal a fossil section of Arthropleura’s body about 76 centimetres long. The block was spotted by chance by a former research student from the University of Cambridge, and the fossil was recovered later that year.

A team led by Neil Davies of the University of Cambridge described the specimen in the Journal of the Geological Society in 2021. The rocks date to about 326 million years ago, in the early Carboniferous. By comparing the fossil with more complete specimens, the team estimated that the animal was about 2.6 metres long and weighed around 50 kilograms.

The find attracted worldwide attention as the largest known arthropod in Earth’s history. It was also important because large Arthropleura fossils from this earlier part of the Carboniferous are rare. The specimen appears to be a shed exoskeleton or a body part that was carried into a river channel, where it was quickly buried in sand.

The head: a long mystery solved

Until recently no Arthropleura fossil showed a well-preserved head, which left its classification uncertain. In 2024 a team led by Mickaël Lhéritier published a study in Science Advances of small juvenile Arthropleura from the Montceau-les-Mines fossil site in France, around 300 million years old.

Using CT scanning, the researchers examined the heads of these young animals for the first time. They found features typical of millipedes, such as the arrangement of the mouthparts, combined with some traits seen in centipedes. Combined with genetic data from living myriapods, the study concluded that Arthropleura belonged with the millipedes, within a broader group that also includes centipedes.

Why did it grow so big?

The most familiar explanation links Arthropleura’s size to high oxygen. Arthropods breathe through systems of tubes or gills that rely on diffusion, and higher oxygen levels may allow larger bodies. Atmospheric oxygen is thought to have risen well above present levels in the late Carboniferous.

However, the Northumberland find dates from a time when oxygen was probably not especially high, suggesting that oxygen alone does not explain its size. Davies and colleagues suggested that a nutritious diet and the absence of large land predators were also important. In the Carboniferous, the largest land vertebrates were still modest amphibians and early reptiles.

What it ate

Arthropleura’s diet is still debated. Fragments of plant tissue have been reported from fossil gut contents, supporting the view that it ate plants or decaying vegetation, as most millipedes do today. Some researchers have suggested it may also have eaten seeds or small animals, but there is no direct evidence of predation.

Extinction

Arthropleura disappears from the fossil record in the early Permian, roughly 290 million years ago. By then the great coal forests of Euramerica had broken up as the climate became drier, an episode known as the Carboniferous rainforest collapse. The spread of reptiles and other amniotes, some of which became large plant-eaters and predators, may also have played a part. The exact cause remains unknown.

Myths and facts

Arthropleura is sometimes shown as a dangerous predator chasing other animals. There is no evidence for this; most scientists see it as a plant-eater or scavenger.

It is also often described as an insect. It was not: it was a myriapod, closer to millipedes and centipedes, and it lived on land rather than in water, as its trackways show. Finally, its size is sometimes exaggerated beyond 3 metres. The best current estimates for the largest known individuals are about 2.6 metres.

By the numbers

Timeline

• c. 345 million years ago: Arthropleura appears in the fossil record in the early Carboniferous.
• c. 326 million years ago: The giant Northumberland specimen is buried in a river channel.
• c. 305 million years ago: The tropical coal forests break up as the climate dries.
• c. 290 million years ago: Arthropleura disappears in the early Permian.
• 1854: Jordan and von Meyer name Arthropleura from German fossils.
• 1979: Briggs, Rolfe and Brannan describe a giant myriapod trackway from Arran, Scotland.
• 2018, January: The Northumberland fossil is found in a fallen block at Howick Bay.
• 2021, December: Davies and colleagues publish the Northumberland find.
• 2024: CT scans of juvenile fossils from France reveal Arthropleura’s head.

Why it matters

Arthropleura shows how large invertebrates could become on land before big vertebrate predators existed. Its fossils, footprints and recently revealed head anatomy help scientists reconstruct the Carboniferous forests and test ideas about oxygen and body size. It remains one of the most striking animals in the history of life on land.