Create a list of articles to read later. You will be able to access your list from any article in Discover.
You don't have any saved articles.
Largest ever millipede’s head revealed by 300-million-year-old fossils
Well-preserved fossils uncovered in France have revealed new insights into one of the biggest invertebrates to ever walk on Earth.
Arthropleura was a millipede-like animal which lived more than 300 million years ago during the Carboniferous Period, with some individuals reaching more than two metres long.
The head of one of history’s biggest arthropods has been revealed in detail for the first time.
Arthropleura is an arthropod, the group containing insects, crustaceans, arachnids and their relatives. For many years, only fossils of its body survived, which saw it placed among the earliest millipedes. Now, the discovery of the first complete head has revealed a surprising twist.
While the new fossils are not from fully grown Arthropleura, some of which reached 2.6 metres long, they reveal important characteristics. Most notably, the head has some features of early centipedes, suggesting millipedes and centipedes might be more closely related than previously accepted.
Dr Greg Edgecombe, an expert in ancient invertebrates at the Natural History Museum, says that the new fossils refine our understanding of how these animals evolved.
“Until recently, we’ve only had well-preserved fossils of the body of Arthropleura,” Greg explains. “These had two pairs of legs on each segment, a key trait of millipedes, so it was often seen as an enormous millipede.”
“Now we’ve found the head, we know that it had features that are also characteristic of centipedes, like enclosed mandibles with two pairs of head limbs behind them. Based on traditional views of how millipedes and centipedes are related, this just doesn’t make sense.”
“By combining the best available data from hundreds of genes from living species in this study, alongside the physical characteristics that allow us to place fossils like Arthropleura on evolutionary trees, we’ve managed to square this circle. Millipedes and centipedes are actually each other’s closest relative.”
While many details of Arthropleura’s life remain mysterious, including how it breathed and what it ate, ongoing research is gradually building a fuller picture of this invertebrate. The findings of the study were published in the journal Science Advances.
Previously, fossils found in siderite nodules were studied using casts such as this. © Lhéritier et al., Sci. Adv. 10, eadp6362 (2024)
How was Arthropleura preserved?
Living between 346 and 290 million years ago, Arthropleura was the largest invertebrate on land during an era of enormous arthropods. Everything from scorpions like Pulmonoscorpius to the dragonfly-like griffinflies grew to enormous sizes, with many living in swampy forests stretching around the world’s equator.
When these animals died, they were rapidly buried by sediment. Some became surrounded by a mineral called siderite, which solidified around them, forming a nodule. The resulting fossils have preserved even the most delicate parts of their anatomy, making them especially useful for researchers.
After hundreds of millions of years of continental drift, some of these siderite nodules eventually came to light in a coal mine in Montceau-les-Mines, France in the 1970s. They were later placed in the collections of the Museum of Autun, where CT scans are finally revealing their hidden contents.
“Traditionally, we’d split open the nodules and take casts of the specimens,” Greg says. “These days, we can investigate them with scans. We used a combination of microCT and synchrotron imagery to examine the Arthropleura inside, revealing the fine details of its anatomy.”
Newer approaches, such as CT scans and synchrotron imagery, allow Arthropleura to be seen in greater detail than ever before. © Lhéritier et al., Sci. Adv. 10, eadp6362 (2024)
What was Arthropleura ’s life like?
From the many siderite nodules in the Museum of Autun, two particular specimens stood out. Even though the two fossils measure just 25 and 40 millimetres long, these juveniles of an unidentified Arthropleura species reveal how the group achieved its enormous size.
Arthropleura specimens have different amounts of body segments, which suggests the invertebrates added them until they reached a fixed maximum number. This is like most millipedes but differs from many centipedes, which are born with all of their segments already in place.
This means that Arthropleura’s maximum size would have been reached by, or after, sexual maturity, rather than at birth. The maximum size of the species used in the study is an open question, but it may not have been as massive as some of the biggest species, such as Arthropleura mammata.
“Tracks found elsewhere in Montceau-les-Mines suggest that these Arthropleura were probably around 40 centimetres at their longest,” Greg explains. “While there’s nothing to say that they couldn’t be bigger, we don’t currently have any evidence of this.”
The latest reconstructions of Arthropleura show it has both characteristics of millipedes and centipedes, reflecting its position in the tree of life. © Lhéritier et al., Sci. Adv. 10, eadp6362 (2024)
Other aspects of their lifestyle are similarly ambiguous. While the researchers found that Arthropleura has club-shaped eyes, their structure has not survived. The team believe they were probably compound eyes, based on the animal’s relatives, but can’t be certain.
What the invertebrate ate is also something of a mystery. While some researchers believe they have found plant fragments preserved in its guts, this finding is not universally accepted by the scientific community.
“While definite gut contents are yet to be found, other details of these fossils contribute to the debate over Arthropleura’s diet,” Greg adds.
“They don’t have any venom fangs or legs specialised for catching prey, suggesting it probably wasn’t a predator. As its legs are better suited for slow movement, they were probably more like the detritus-eating millipedes alive today.”
To resolve these outstanding questions, more well-preserved Arthropleura specimens will need to be found. While this study has helped to firm up its place in the tree of life, it’s possible that more twists remain in store for the scientists studying Arthropleura.
“I overturned 20 years of my own work with the molecular trees we used to place Arthropleura in this paper, but I was willing to fall on my own sword because of the evidence we uncovered,” Greg says. “As our knowledge of this iconic invertebrate continues to evolve, we’ll need to keep our minds open to new possibilities.”
