How we discovered a rare giant millipede fossil on a beach, and why it matters

Ever stumbled upon a huge fossil on your holiday? That's what happened to me and two friends in January 2018, on a beach in Howick in the north of England,while on a geological road-trip across England and Wales. It was pure fluke, but it turned out that we had discovered the 326 million-year-old remains of a millipede-like animal of huge proportions.

Our research suggests that the living creature would have been around 50cm in width and 2.5 meters in length—about as long as an alligator—so we could safely conclude that we had found a fossil of an Arthropleura, the largest invertebrate to have ever lived.

It is a creature often found in art and museum displays depicting Earth's Carboniferous period. That was around 360 to 298 million years ago and is associated with abundant coal-producing rainforests. To put that into context, it's over 100 million years before dinosaurs roamed the planet.

Until now, large body fossils of this ancient monster were rare. Our knowledge had relied on piecing together snippets of clues from several sites; isolated fragments of legs and exoskeletons that clearly belonged to giant arthropods, or rare near-complete specimens of juvenile Arthropleura that shed light on the animal's body plan, despite being miniature examples no more than 5cm in length.

Perhaps most significantly, the most common evidence that Carboniferous Earth was crawling with giant millipede-like animals comes from trace fossils. They are the patterns in sedimentary rocks that were left by animals moving through or over the sediment. In the case of Arthropleura trace fossils, Carboniferous sandstones from Utah to Ukraine are frequently criss-crossed by staccato tramlines that attest to huge arthropods, with many, many legs, leaving their footprints behind as they traversed long lost beaches and riverbeds. Trackways over 50cm wide are relatively common, despite body fossils approaching these dimensions never having been discovered.

Stumbling upon the giant fossil

The story of our discovery began when my colleagues and I rolled out the geological map of England and Wales and plotted a route that would take us through millions of years of Earth's history over two weeks, going back to 560 million years ago.

It meant a back and forth journey, looping 3,000km across the country, with the intent of stopping at coastal cliffs, roadcuts, abandoned quarries and mountainsides. The trip was primarily social, but we were keeping our eyes open for geological stories to explore further—we don't accept the sometimes whispered myth that the geology of Britain is "done" and there is no more left to discover.

One of our stops took us to a beach at Howick in Northumberland, about 40km south of the Scottish border.

After wandering south along the shore during the afternoon, we were heading back for the evening because dusk was settling in. Just before we clambered back up the cliff, one of my friends spotted a recently fallen block of sandstone. It had split down the middle, by chance revealing an enigmatic fossil on both sides of the broken rock—a 76cm long collection of 12-14 segments. We took as many photos and notes as possible and contacted a number of Carboniferous arthropod experts worldwide. They all confirmed it had to be Arthropleura.

So, with permission from Natural England and the landowners, the Howick Estate, we returned to the site to collect the fossil in May 2018.

The collection of the fossil involved taking both a sledgehammer and pneumatic chisel to the fallen block, and we were able to excavate one slab of the fossil completely. The second slab unfortunately broke apart as we removed it, meaning that the slab is now a jigsaw puzzle of about ten pieces. But even this proved serendipitous as it let us see a cross-section through the fossil.

Why it's important

The fossil, while incomplete, is 76cm by 36cm and weighs over 80Kg, so it takes the crown of the largest arthropod fossil from a slightly smaller Ordovician trilobite specimen (an extinct group of marine arthropods from Canada).

It's also the largest Arthropleura fossil ever found—prior to our discovery, the largest semi-complete fossil, on display in the Senckenberg Museum in Frankfurt, only reaches a modest 23cm in width.

More importantly, using estimates of width to length ratio obtained from smaller and more complete specimens of Arthropleura, we could estimate that in life it again pinches the record for the largest arthropod to have ever lived, this time off a Devonian sea scorpion from Germany.

The discovery adds to the global record of Arthropleura evidence from 60 sites in 13 different countries. Collectively, these specimens all occur in locations very close to the ancient Equator—which at that time ran through North America and across the UK and Europe to the Ukraine—suggesting that the genus had a very restricted geographic range.

Significantly, the Howick specimen is among the oldest evidence for giant Arthropleura and pre-dates any major rises in Carboniferous atmospheric oxygen—previously suggested to account for arthropod growing so large. This means that the organism may have been so big simply due to environmental factors such as an abundance of food (for example, woody plant debris) and a lack of competition from vertebrates.

The sedimentary geology of the fossil is also interesting—as with many known trace fossils, the Howick Arthropleura clearly wasn't living in a Carboniferous coal swamp, but rather in an open wooded habitat on a sandy coastline traversed by small rivers.

So, our discovery helps to refine the picture of these giant arthropods.

Yet the most significant aspect of all is probably the discovery itself. It shows that it is still possible that the world's largest arthropod fossil can be discovered just sitting on a beach, in a well-populated part of the world which has been trawled by geologists, tourists and miners for almost 200 years.

An ancient alligator-length millipede is wonderful in itself. But this discovery suggests that there are a lot more unexpected and spectacular finds still to be discovered from Earth's geological record, so long as people keep looking.