The fossil of Arthopleura, an invertebrate that could tower over a modern human, shows how the giant bug thrived for 45 million years
A Truly Curious correspondent
A sandstone block, buffeted by strong winds during winter storms, tumbled off a cliff in Northumberland, England, and cracked at the bottom, to expose a fossil.
That was how a specimen of Arthopleura, a giant ancestor of the modern millipede, was discovered on a cold January day in 2018 by a team from the Department of Earth Sciences at Cambridge University.
“Just as it was getting dark, we saw that there was a boulder that had fallen from the cliff,” lead author Neil Davies told NPR. The team noticed “a big crack … and then saw this really large fossil inside.”
Arthopleura fossils have been found in England, Europe, and the Americas. It didn’t matter where they were found, since then they were all together, part of the supercontinent Pangea. This fossil comes from the cast-off bits of the millipede representing 12-14 of the plates that form the hard exoskeleton of the animal.
It is the biggest arthropod (organisms with jointed feet that include insects, spiders, and more) ever discovered – dead, alive or fossilized.
It was preserved in three dimensions, sitting awhile in plant detritus in a riverbed near the coast, the sand pouring in through the cracks in the exoskeleton and replacing tissue, until the bank collapsed, shutting it out from the sun.
A few better-preserved fossils of complete juvenile Arthopleura suggest that the animal had about 30 plates in all. This fossil itself is only 76 cm (30 inches) long, but the whole animal, whose myriad feet trod the earth about 326 million years ago, would have been the size of a decent car – about 55 cm (2’) wide and 2.63 m (8.4’). Though Arthopleura weighed about 50 kg (about 110 lbs), they were also highly maneuverable. Not quite what you’d like to see on your bathroom floor.
Humans have shared history only with the fossil, not Arthopleura itself. It lived back in the Carboniferous period, between 360 million years ago and 300 million years ago, which was when amphibians first made their mark on land. They seemed to thrive in damp equatorial conditions. The evidence of surrounding sediments – with fossils of fern-like plants, and seeds – suggests that it preferred woody areas to swamps. Yes, that was what England was like at the time.
They managed to keep surviving in drier conditions, until the climate became really arid around the time the Carboniferous gave way to the Permian period about 300 million years ago.
Millipedes breathe through spiracles, the air going to its internal cells through tubes called tracheae and a pale liquid called haemolymph. The less the oxygen, the more quickly it is used up.
Unlike modern arthropods, Arthopleura lived in times of more oxygen – between 26 percent and 30 percent of the atmosphere – compared to 21 percent today. So, it was theorized, they could grow much larger.
One problem: This specimen was walking around long before oxygen levels peaked. When they were proliferating, it was only at about 23 percent, not much higher than what is seen today. So scientists now believe it could grow larger, bulking up on the detritus of plants and animals (and perhaps some prey), because it dealt with neither competition nor predators.
The Arthopleura lived large, thriving for about 45 million years, before they finally faded away, perhaps under intense pressure from newly ascendant reptiles. But if these giant millipedes could feel pride, they could rest easy on the legacy that as invertebrates go, there were none bigger than them.
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