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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Arthropleura may have reached about 2.63 metres (8.6 feet) in length and roughly 50 kilograms (110 pounds), making it the largest known land arthropod. That figure is a reconstruction from an incomplete fossil, not a direct measurement of a complete animal. The “as long as an alligator” description is a vivid size analogy, not a comparison with a particular alligator species.
The best-known specimen was found in 2018 at Howick Bay in Northumberland, England. It preserves part of a shed exoskeleton from the early Carboniferous, when the animal lived on a lower delta plain with open woody vegetation. Fossils described in 2024 added a complete juvenile head with some centipede-like features, complicating the familiar label of Arthropleura as simply a gigantic millipede.
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What was Arthropleura?
Arthropleura was a giant myriapod, the arthropod group that includes modern millipedes and centipedes. It lived during the Carboniferous, more than 300 million years ago, and is regarded as the largest terrestrial arthropod known from the fossil record. Its body was built from repeated segments protected by hard plates, and its legs were adapted for walking rather than for grasping prey.
For many years, scientists called it an enormous millipede because fossils of its trunk show two pairs of legs on each segment, the defining arrangement seen in millipedes. As Natural History Museum invertebrate expert Greg Edgecombe explained in 2024, “Until recently, we’ve only had well-preserved fossils of the body of Arthropleura. These had two pairs of legs on each segment, a key trait of millipedes, so it was often seen as an enormous millipede.” New juvenile fossils now show that its head had traits associated with centipedes, so its exact position within the myriapod family tree remains an active scientific question.
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How large was the fossil animal?
The Howick Bay specimen itself measures 76 by 36 centimetres. It does not include a complete body: the rock contains 12 to 14 anterior tergites, or dorsal body plates, from the front part of the trunk. Researchers used those plates, proportions from other Arthropleura fossils and evidence from fossil trackways to reconstruct the missing sections.
| What is being described | Value | How certain is it? |
|---|---|---|
| Preserved Howick fossil | 76 × 36 cm | Measured directly from the partial specimen |
| Estimated whole-animal length | 1.90–2.63 m | Reconstructed from incomplete remains, comparative fossils and trackways |
| Likely upper-end length | Up to 2.63 m | Authors considered the upper end more probable because compression made the fossil appear relatively short |
| Estimated maximum width | About 55 cm | Reconstructed for the largest inferred individual |
| Estimated mass | About 50 kg | Calculated estimate for the reconstructed animal |
The primary study therefore gives a range of 1.90 to 2.63 metres, with the authors arguing that the animal was probably near the upper end. The estimate of approximately 50 kilograms applies to that reconstructed, roughly 2.63-metre individual. It is not the weight of the rock or of the preserved plates, and it should not be treated as a measurement obtained from a complete skeleton. The University of Cambridge paper reports the upper estimate as “up to 2.63 m in length” and “c. 50 kg”; a Natural History Museum account rounds the length to “around 2.6 metres” and compares it with a small car.
Body compression is an important reason for the uncertainty. The fossil was flattened in sediment, and only part of the trunk survived. Scientists had to infer the proportions of the missing front and rear sections from other evidence. A complete Arthropleura skeleton has never been found, so the exact maximum length and weight remain estimates rather than fixed species measurements.
The Howick Bay discovery
A chance find in Northumberland
The fossil was discovered in January 2018 at Howick Bay, approximately 10 kilometres north-east of Alnwick in Northumberland, England. It lay in a large sandstone block that had fallen from the coastal cliff. Geological information indicates that the block probably detached sometime between April 2017 and January 2018, placing the fossil on the beach only briefly before it was noticed.
Its exposure was unusually fortunate. Dr Neil Davies, lead author of the study, described it as “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.” The specimen was collected with permission and accessioned by the Sedgwick Museum of Earth Sciences under the number CAMSM X.50355.
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What the rock preserves
The block comes from the early Carboniferous Stainmore Formation of the Northumberland Basin. Inside it is a partially sand-filled dorsal exoskeleton. The preserved plates are the tergites that covered the animal’s back, and their arrangement records only the anterior portion of the trunk.
Researchers interpret the remains as an exuvium—a shed outer skeleton left behind when the animal moulted—rather than the body of an animal buried intact. Arthropods grow by periodically shedding their exoskeletons. A shed shell can be transported, folded or broken before burial, which helps explain why this spectacular specimen does not preserve the entire animal or its soft tissues.
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How scientists reconstructed the giant’s size
Measured evidence versus inferred dimensions
The distinction between measurement and reconstruction matters. The 76-centimetre fossil is an observable object. The 1.90-to-2.63-metre animal is a model built by extending preserved body proportions across segments that are absent. Trackways provide an independent check: the spacing and form of footprints indicate how a long, many-legged animal moved, but they do not reveal every dimension of the body that made them.
Researchers compared the Howick plates with other Arthropleura material and used those comparative proportions to estimate the missing segments. Because the fossil was compressed, the authors considered a reconstruction close to 2.63 metres more plausible than the shortest value in the range. Even so, the result should be read as “could have reached,” not “every adult measured exactly this length.”
Why the alligator comparison has limits
An alligator comparison communicates scale to readers who have never seen a two-and-a-half-metre arthropod. It does not identify an alligator species, imply that Arthropleura had a similar mass distribution, or show that the two animals occupied the same environment. Arthropleura was a broad, many-legged terrestrial arthropod; an alligator is a vertebrate with a very different skeleton, posture and body density. The scientifically useful numbers are the reconstructed length, width and mass, together with their uncertainty.
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Where did Arthropleura live?
A lower delta plain, not necessarily a coal swamp
Sedimentological evidence from the Stainmore Formation indicates a lower delta plain depositional environment. The fossil was associated with deposits interpreted as an ancient river channel, and the surrounding landscape is described as open woodland or open woody habitat. This setting differs from the common assumption that every giant Carboniferous arthropod lived deep inside a permanently waterlogged coal swamp.
The geological interpretation suggests that Arthropleura could move through relatively open vegetation on land and share those habitats with tetrapods, the early four-limbed vertebrates living at the time. A delta plain could include channels, flood-prone surfaces and vegetated ground, so “open woodland” does not mean a dry modern forest. It means the evidence does not require a closed, swamp-only habitat.
What caused its gigantism?
Atmospheric oxygen is often offered as a simple explanation for enormous Carboniferous arthropods. The Howick study cautions against that conclusion: Arthropleura achieved its great size at a time when oxygen was not significantly above modern levels. The authors discuss favorable habitat, abundant food and limited competition as possible contributors, but these are proposed explanations rather than causes demonstrated directly by the fossil.
Because the specimen is a shed exoskeleton, it cannot reveal the animal’s metabolism, growth rate or daily activity. Habitat evidence can show where the animal was buried and what landscapes existed nearby; it cannot by itself prove how far an individual travelled, whether it lived permanently on floodplains or how often it entered water.
Was it a millipede or something between a millipede and a centipede?
The traditional millipede interpretation
Trunk fossils are the reason Arthropleura became known as a giant millipede. Two pairs of legs on each body segment are a key millipede trait, while centipedes generally have one pair per segment. The broad, overlapping tergites also resemble the body armour of millipedes. Those features provide direct anatomical evidence from the trunk, even though the head and many soft parts are missing in the Howick specimen.
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What the 2024 juvenile fossils add
A 2024 study examined two juvenile Arthropleura fossils from siderite nodules at Montceau-les-Mines in France. The specimens are only 25 and 40 millimetres long, vastly smaller than the reconstructed Howick animal. Micro-computed tomography and synchrotron imaging revealed a complete head, including structures associated with centipedes. The same fossils retain the two-pairs-of-legs-per-segment pattern associated with millipedes.
The researchers combined anatomical comparisons with molecular analysis to reassess relationships among Arthropleura, millipedes and centipedes. The result is a more complicated evolutionary picture: the animal cannot be classified confidently from trunk plates alone, yet the juvenile discoveries do not overturn the clear millipede-like leg arrangement. These French fossils are juveniles, so they do not establish how large that species became as an adult and cannot by themselves set a new maximum-size record.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What did the giant millipede eat?
No definitive gut contents have been identified, so Arthropleura’s diet remains unresolved. Plant fragments reported from some fossil guts have not been universally accepted as evidence of what the animal consumed. The absence of confirmed food remains means that a specific menu—leaves, roots, fungi or other material—cannot be stated as fact.
There are, however, anatomical clues about what it probably did not do. Greg Edgecombe noted that Arthropleura had no venom fangs or legs specialised for catching prey. Its legs were better suited to slow movement than to pursuing or subduing animals. On that basis, he said it was “probably” more like the detritus-eating millipedes alive today. That is an expert interpretation of anatomy, not proof from preserved stomach contents. Edgecombe also emphasised: “While definite gut contents are yet to be found, other details of these fossils contribute to the debate over Arthropleura’s diet.”
A detritus-based lifestyle would fit an animal moving through wooded or delta-plain habitats, where fallen plant material and decomposing organic matter accumulated. It remains possible that Arthropleura supplemented such food with living plant tissue, fungi or other resources, but the fossils currently do not distinguish among those possibilities.
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Other questions scientists cannot yet answer
How did it breathe?
The available fossils do not settle Arthropleura’s respiratory system. Millipedes and centipedes have different arrangements of air openings and internal breathing structures, and soft tissues rarely fossilise. Without a well-preserved head and body showing those features in an adult, scientists cannot confidently describe how the giant species exchanged gases or whether its respiratory system limited its size.
How many species were giant?
Arthropleura fossils come from different places and ages, and their sizes vary. The Howick specimen demonstrates that at least one animal could reach the reconstructed upper range, but incomplete remains make it difficult to determine the full size range of every species. The 25- and 40-millimetre French fossils are particularly important for anatomy, not for estimating adult body length.
Was every individual as large as the record estimate?
No. The 2.63-metre figure is an inferred maximum for the Howick reconstruction. Younger animals, smaller species and individuals that lived under different conditions could have been much shorter. Fossil plates also record a single moulting event, so they cannot show how long that individual lived or whether it had reached full maturity.
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The discovery combines several kinds of evidence that must be kept separate. The rock and plates provide direct measurements and anatomy. Comparative fossils and trackways support a reconstruction of body length. Sediments identify a delta-plain setting. The 2024 head fossils refine evolutionary relationships. Diet, breathing and the causes of gigantism remain hypotheses because the relevant soft tissues, gut contents and ecological observations are missing.
That combination is why Arthropleura is scientifically important beyond its spectacular size. It shows that a land arthropod could attain a length comparable to a small car without requiring an oxygen-rich coal swamp, and it demonstrates how a partial fossil can still reveal body proportions, habitat and evolutionary history when interpreted alongside other evidence. The animal was probably a slow-moving, non-predatory detritus feeder, but the strongest conclusion is narrower: Arthropleura was an exceptionally large myriapod whose exact millipede–centipede affinities and ecology are still being worked out.
Primary details of the Howick specimen and its reconstruction are reported in the University of Cambridge paper, The largest arthropod in Earth history: insights from newly discovered Arthropleura remains. The later head discovery and diet discussion are covered by the Natural History Museum’s 2024 report, Largest ever millipede’s head revealed by 300-million-year-old fossils. The museum’s account of the original find is available at The world’s largest arthropod was a car-sized millipede.
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