Arthropods, a phylum of organisms that includes insects, arachnids and diplopoda (a class characterized by two pairs of jointed legs per body segment), have inhabited our world far longer than most other species on Earth. Nowadays, these creatures are rarely bigger than a dime, and never bigger than a dinner plate. Three hundred million years ago, the insect world was vastly different; bugs were bigger in the past. A millipede fossil recently found in England gives us an idea just how big they could get.
‘Mega’ millipede fossil
England seems to have a rich history in fossil records. Over 21 different dinosaurs have been discovered in the UK, and many other fossilized species can be discovered by the average explorer. In fact, it was here that the first dinosaur ever described in modern science was found in the 1800’s.
In January 2018, a large block of sandstone fell from a cliff on the beach at Howick Bay, located in Northumberland, England. When it was eventually found, it had broken up and revealed what appeared to be the remains of a prehistoric creature.
“It was a complete fluke of a discovery,” lead author Neil Davies of the University of Cambridge said. “The way the boulder had fallen, it had cracked open and perfectly exposed the fossil, which one of our former Ph.D. students happened to spot when walking by.”
“It was an incredibly exciting find, but the fossil is so large it took four of us to carry it up the cliff face.” Mr. Davies added.
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After the fossil was extracted in May 2018, it was brought to the University of Cambridge to be further studied. Eventually, on Dec. 21, 2021, the fossil was publicly revealed to be the segment of a giant millipede known as Arthropleura.
According to scientists at Cambridge, the fossil segment is around 75 centimeters long, and they estimated that the creature overall was probably 2.7 meters (nine feet) long and weighed about 50 kilograms (110 pounds).
Scientists further believe that the discovery may not be a fossil of the actual creature, but rather the fossilized exoskeleton shed upon the creature’s growth.
“Finding these giant millipede fossils is rare, because once they died, their bodies tend[ed] to disarticulate, so it’s likely that the fossil is a molted carapace that the animal shed as it grew,” Mr. Davies said. “We have not yet found a fossilized head, so it’s difficult to know everything about them.”
Arthropluera was one of the largest-known invertebrates ever identified, growing to the lengths of today’s cars. While today’s millipedes can have anywhere from 24 to 750 legs, most have fewer than 100. This giant is believed to have had at least 32, but not more than 64 legs.
This Northumberland specimen is only the third such fossil ever found, with the other two – much smaller and younger than the third – having been discovered in Germany.
According to a report filed in the Journal of the Geological Society, the location where the fossil was found was once a small tropical river, when England was located on the Equator millions of years ago.
The fossil will be put on display for the public to see at Cambridge’s Sedgwick Museum.
The Carboniferous Period
The Carboniferous Period began around 360 million years ago and ended around 299 million years ago – more than 100 million years before the first dinosaurs are believed to have appeared.
Coal was widespread in the Carboniferous period, produced by “bark-bearing” trees that grew in the swamp forests, with mosses, tree ferns and horsetails painting the picture of an entirely tropical Earth.
While Arthropleura may have crawled along the undergrowth of coal forests, it shared its home with other gigantic arthropods. One of the most common giant arthropods of the Carboniferous included the Meganeura, a hawk-sized dragonfly.
Giant sea scorpions called Eurypterids also inhabited the Carboniferous seas.
Unlocking the mystery behind prehistoric millipedes’ massive size
So how did the arthropods of the Carboniferous Period become so large, and why have their descendants remained at their tiniest state?
For a long time, scientists believed it had something to do with the air.
Insects do not breathe the same way most lifeforms do. They take the oxygen in and let it out through holes in their bodies called spiracles, which are connected by a network of tubes called tracheae. As insects grow, the tubes get longer and wider so that more oxygen can be carried.
A study in 2006 on beetles showed that when exposed to larger quantities of oxygen, the tracheal systems of these beetles actually grew to be much larger than those with lower levels of oxygen, prompting the insects themselves to grow. The growth, however, was limited, and the insects could grow no larger than 15 centimeters.
Because there was so much vegetation in the Carboniferous period, there was a lot more oxygen than there is today – the earth’s atmosphere contained almost 35 percent oxygen compared to today’s 21 percent. As such, diplopoda and other arthropods could have needed to grow to extraordinary sizes to accommodate the available oxygen.
However, the recent Arthropleura discovery has challenged that super-growth theory. According to the scientists at Cambridge, the new fossil came from rocks of a time period before the oxygen levels peaked, suggesting that oxygen could have not been the main factor.
Scientists now believe that Arthropleura may have had a “high-nutrient diet,” though they remain unsure what that diet consisted of.
“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, and they may even have been predators that [feed] off other invertebrates and even small vertebrates such as amphibians,” said Mr. Davies.
No one knows how these giant arthropods became extinct, but it is theorized that global warming could have dried out the forests, thus depleting the oxygen levels needed for large bodies. Another theory suggests that reptiles evolved to become larger and began to take over the lands, leaving us with the lesser bugs we know and love today.