At first glance, the stone looks a bit inconspicuous, and it was only found by chance. But a paleontological sensation is petrified in it: the largest known invertebrate of all time. It is the fossil of a segment that once belonged to a gigantic millipede. Overall that was as Arthropleura identified animal around 2.7 meters long and probably up to 50 kilograms, write Neil Davies from the University of Cambridge and his team in the “Journal of the Geological Society”: It thus surpasses the prehistoric sea scorpions, the previous record holders.
The fossil was discovered in 2018 when a large sandstone boulder broke off a cliff in Howick Bay in Northumberland and fell onto the beach. The chunk broke so perfectly that it released the fossil inside undamaged. A former doctoral student in the group found it while walking. It took four people to transport it away for examination.
© Neil Davies (detail)
Reconstruction of the giant | This millipede was up to 2.7 meters long and weighed about 50 kilograms.
In the laboratory, Davies and Co identified it as the remains of a millipede that lived in the area during the Carboniferous 326 million years ago. At that time the area was still near the equator and thus in tropical climes. Many of the invertebrates and early amphibians of that time only existed in the vicinity of lakes and rivers; there they fed on the vegetation or hunted each other. Also the segment of Arthropleura ended in an arm of the river where it was covered by sediment and fossilized over time. The working group assumes that the millipede did not die at the time, but rather shed its skin and petrified the exoskeleton.
So far only two other fossils are known of Arthropleura. Both were found in Germany, but are much smaller. “We haven’t found a petrified head yet. So it’s hard to know everything about her, ”says Davies. For example, it is a mystery why these animals became so big. Up until now, it was believed that there were mainly giant invertebrates when the oxygen levels in the atmosphere were highest. But Arthropleura lived before that time. He also needed a very nutritious diet. “We can’t say for sure what they ate, but there were probably plenty of nutritious nuts and seeds in the leaf litter back then. And maybe they were even predators that fed on other invertebrates and possibly small amphibians, ”says Davies.