The world's largest arthropod was a car-sized millipede

The largest-ever land-dwelling arthropod has been discovered on a beach in northern England after a chance find by a researcher.

The giant millipede Arthropleura would have been one of the largest invertebrates ever, living over 300 million years ago. A fossilised fragment of the huge creature found in Northumberland has enabled scientists to estimate its length at around 2.6 metres, similar to that of a small car.

Dr Greg Edgecombe, a specialist in the evolution of arthropods at the Museum and who reviewed the paper describing the discovery, says, 'Fossils of Arthropleura have been known for nearly 170 years, with the most significant finds coming from the UK, continental Europe and north America.

'The biggest articulated specimens we had previously known about, which were found in Germany, suggested that Arthropleura could reach a length of two metres and was the largest land-living arthropod of all time. This new specimen raises the bar.'

The findings of the researchers, led by the University of Cambridge, were published in the Journal of the Geological Society.

The behemoth bugs

Arthropleura existed during the Carboniferous Period, which lasted from around 359 to 299 million years ago. At the time, Earth was much more oxygen-rich than it is now as much of the carbon was locked up in woody plants which could not easily be broken down.

While the planet's atmosphere today contains around 21% oxygen, the atmosphere at the time had levels of over 26%, and for periods greater than 30%.

The Carboniferous is also known to have been the time when giant arthropods dominated. Insects like Meganeura, which would have looked like bird-sized dragonflies, were among the many large bugs found across the world.

It has been suggested that the higher level of oxygen at the time allowed for the insects to grow much larger. Insects breathe using trachea, tubes which deliver oxygen directly into the tissues of the body, whereas many other animal species use blood to deliver the gas.

With this system, oxygen moves relatively slowly into the bodies of insects. This means that tissues at the centre of larger arthropods wouldn't receive enough oxygen to survive under current conditions, but if there was more oxygen in the atmosphere during the Carboniferous, then these animals may have been able to grow larger.

This concept remains strongly debated, with other theories suggesting that insects grow large to avoid the toxic effects of oxygen at high concentrations, or that a lack of flying predators allowed for them to develop larger bodies.

In any event, most Arthropleura specimens come from the latter part of the period, leading scientists to suggest that their size was related to the large amount of oxygen in the atmosphere.

However, the most recent specimen was found in rocks dating to a time when proportions of oxygen in the atmosphere was only slightly higher than today, calling this idea into question.

Woodland wanderer

The specimen was found among a rock fall from a cliff in Howick Bay in Northumberland in 2018. In the Carboniferous, this area would have been located near to the Equator, with the rocks surrounding the fossil thought to have been part of an ancient river channel.

The paper's lead author, Dr Neil Davies, says, 'It was a complete fluke of a discovery. The way the boulder had fallen had perfectly exposed the fossil which one of our former PhD students happened to spot when walking by.'

After receiving permission to extract the fossil, the researchers found that it is one of the most complete specimens of Arthropleura ever discovered, and by far the largest. It is believed to represent the upper half of the exoskeleton of the giant millipede, which was filled in by river sediment after being cast off.

This would have helped preserve the shape of the animal, with the remains of Arthropleura's body often falling apart after death.

'Most specimens are disarticulated body parts,' says Greg, 'often just pieces of the dorsal exoskeleton that we can recognise from their distinctive tuberculate ornament. However, a few largely complete juveniles have been described which gave a solid clue as to how many body segments the animal had.'

Using this knowledge and evidence from preserved tracks, the scientists estimated that it would have weighed up to 50 kilogrammes, which is about the same as a large dog.

In addition to its larger size, the specimen has also revealed more about how the giant millipede lived.

'Arthropleura has often been thought of as preferentially living in coal swamps, which were widespread at the time from which most of its fossils are found,' Greg explains. 'The geology of the new find suggests that it also lived in open woodland.'

The fossil was also found close to the preserved tracks of a large tetrapod, suggesting that they lived together in the same environment. The woodland would have provided a wealth of food for the giant millipede, which may help explain its large size.

'While we can't know for sure what they ate, there were plenty of nutritious nuts and seeds available in the leaf litter at the time,' Neil says, 'and they may even have been predators that fed off other invertebrates and small vertebrates such as amphibians.'

After around 45 million years on Earth, Arthropleura is believed to have gone extinct over 250 million years ago. While the reasons remain uncertain, some have suggested that reptiles outcompeted them for food then replaced them, leaving the world clear for the rise of the dinosaurs.