In January of 2018, a group of geologists were on vacation, taking a road trip across the UK.

One evening, while strolling along Howick Beach in northern England, one of them spotted something…strange.

A big sandstone boulder had fallen from a nearby cliff and split in two, revealing the fossilized body segments of a giant arthropod - some kind of enormous ancient millipede.

Once it was pieced together, it became clear that this accidental discovery was something really special.

Even though the fossil was far from complete, its size made it the single largest arthropod fossil ever found.

This huge millipede may have been more than 2.6 meters long, 55 cm wide, and 50 kilograms in weight.

…Which broke yet another record, making it the largest known invertebrate to ever live on land.

It dates to the Carboniferous Period - a time pretty renowned for its huge bugs - which we’ve talked about before in our episode “The Age of Giant Insects.” And, like we said in that episode, one major hypothesis about why bugs could get so huge during this period was that oxygen levels were much higher then.

But there's just one problem.

This new giant fossil millipede dates to tens of millions of years before the spike in oxygen.

In fact, at the time this alligator-sized millipede lived, oxygen levels weren't much higher than they are today.

So how’d this millipede get so big??

The fossil found on Howick Beach came from an ancient millipede called Arthropleura, which was first described in 1854.

They’ve left dozens of fossils behind across Europe and North America, which were close to the equator during the Carboniferous, so it’s thought that they thrived in warm, tropical environments.

And those fossils have been a mixture of both trace fossils, like trackways of them scuttling across the ground, and body fossils, like fragments of legs and back segments.

But well-articulated, even partially-complete body fossils of giant Arthropleura have been much harder to come by.

In fact, the Howick specimen is only the third one ever found – and it’s from the biggest individual, by far.

It also might actually represent a recently molted exoskeleton, rather than the animal itself – which means that this thing could’ve been getting even bigger at the time.

2.6 meters of millipede was apparently still not enough millipede.

*The more to admire, I guess* Aside from its incredible size, what was also interesting about this fossil was its age.

See, it wasn’t just the most extreme example of gigantism in Arthropleura, it was also the oldest partially-complete example.

It dates to 326 million years ago - much earlier in the Carboniferous than previous body fossils of giant Arthropleura.

Now, in the later part of the Carboniferous, the planet’s atmospheric oxygen levels surged, eventually peaking around 30 to 35%.

And this was thought to have been the main driver behind the rise of the notoriously big arthropods of this period.

Because, in arthropods, oxygen is diffused to tissues and cells through a network of hollow tubes.

So the idea was that higher oxygen levels in the Carboniferous allowed this respiratory system to support much bigger body sizes than was possible during times of lower oxygen.

But 326 million years ago when this Arthropleura lived, the rise in oxygen hadn't really begun yet.

Oxygen levels were only 23% back then, not a whole lot more than the 21% they are right now.

So if oxygen wasn't the key driver behind the evolution of such a ridiculously large arthropod, what was?

Well, maybe instead of the atmosphere, the big factor - literally - was simply being at the right ecological place at the right evolutionary time.

This giant Arthropleura would have pre-dated any large terrestrial vertebrates capable of rivaling it - meaning it had few, if any, competitors or predators.

Because about 326 million years ago, the vertebrate invasion of the land was still in its early stages.

Amphibians were around at this time, spending at least some of their time out of the water, but tetrapods hadn't yet established a permanent presence on dry land.

This giant Arthropleura lived in an environment without any reptiles, birds, or mammals - the vertebrates that would eventually fill the ecological niches open to medium and large terrestrial animals.

Arthropleura and other arthropods had simply beaten us vertebrates by colonizing the land much earlier than we did.

It's free real estate.

And they got to reap the rewards of having the terrestrial environment all to themselves - a chance to experiment with gigantism without any major challengers to keep them in check.

Now, it’s still entirely possible that later oxygen spikes also helped some arthropod groups get really big, especially in the late Carboniferous and Early Permian Period when it reached its 35% peak.

But at least in the case of Arthropleura - the biggest of the big bugs - oxygen levels weren’t the key to evolving huge proportions.

Food availability though?

That definitely would have been a major constraint on their size.

After all, to support such a large body, they would have needed to consume a lot of high-nutrient food.

Now we don't know much about their eating habits, because we haven’t found any fossils of their mouthparts, but it’s likely that they were primarily herbivorous, like modern millipedes.

And here again, the absence of any serious competition from herbivorous vertebrates would have been pretty ideal.

They would have had their pick of nutrient-rich seeds and other plant material that littered the coal swamps and open woodlands of the Carboniferous.

But their size might have also allowed them to be pretty capable predators too.

So it’s possible that they supplemented their plant-based diet with animal prey, including other, smaller invertebrates and maybe even small amphibious vertebrates - our ancestors at the time.

But the conditions that had allowed the giant millipedes to thrive wouldn’t last - and neither would they.

Both their trace and body fossils vanish from the record around 290 million years ago, in the Early Permian Period.

And their extinction was probably driven by two major changes.

For one, this was around the time that the supercontinent of Pangea formed, which led to drier conditions around the equator.

Arthropleura seems to have only lived in those latitudes and may not have been able to adjust to the increasing aridity.

And at the same time, they finally began to face serious ecological competition from vertebrates, due to the rise of reptiles in the Permian.

From this point on, vertebrates began to take center stage on land - becoming larger, more diverse, and better adapted to terrestrial niches.

So this double hit of a changing environment and the appearance of strange new reptilian rivals may have just been too much for Arthropleura.