The discovery of a ‘prehistoric Pompeii’ is poised to rewrite what we knew about animals colonizing the land.

Close-up of a looping millipede death-trail.
Image credits Anthony Shillito.

A duo of geoscientists from the University of Cambridge reports that Ordovician strata in Borrowdale, a site in the Lake District area of England, don’t actually hold the earliest known animal footprints on dry land. In fact, the side bore witness to volcanic-ash-induced mass dying.

Ashes to ashes

Our best estimates of when animals first moved in on dry land come, unsurprisingly, from fossils. However, these aren’t your usual run of the mill fossils — rather, paleontologists rely on a special kind, known as trace fossils, for the job. These include the fossilized tracks, prints, slither marks, and all other manners of impressions left by ancient animals moving over soft ground. Places with significant such trace fossils are known as trackway sites.

Ordovician-era (about 455 million-year-old) strata in Borrowdale were believed to hold the earliest such trace fossils on dry land. This was actually pretty problematic, as it didn’t fit with the rest of the evidence. In all other known trackway sites, the earliest evidence of animals moving onto dry land comes from the Silurian period (cca. 420 million years ago)

Geoscientists Anthony P. Shillito and Neil S. Davies of the University of Cambridge studied the site in England and have finally managed to explain why. Trackways in the Borrowdale area haven’t captured animals living on land, they say — it shows a massive dying of millipede-like arthropods at the hands (particles?) of volcanic ash.

The fossilized tracks formed from volcanic ash settling underwater, the team explains, not within freshwater lakes or sub-aerial sands as previously believed. It “is actually a remarkable example of a ‘prehistoric Pompeii’,” says Shillito. In the course of their study, the team found 121 new millipede trackways, all within volcanic ash, with evidence for underwater or shoreline deposition.

Volcanic ash is particularly deadly for such animals — even modern arthropod communities, and particularly in water. The ash itself is made up of very small particles, ranging from under 2 mm to 1μm (a micron, or 1-millionth of a meter). These particles are also very hard — they’re basically ground volcanic glass. Because they’re so tiny, they can get inside anthropoids’ exoskeletons/shells and clump around soft tissue, especially breathing and digestive organs. Because they’re so hard and generally sharp, they wreak havoc on these organs, killing the animals via, quite literally, a thousand cuts.

Shillito and Davies noticed that most of the trails were extremely tightly looping — a feature which is commonly associated with “death dances” in modern and ancient arthropods, which first made them suspect they were looking at a massive dying rather than traditional trace fossils. This study, the authors say, overturns what is known about the earliest life on land and casts new light onto a key evolutionary event in the history of life on Earth.

“It reveals how even surprising events can be preserved in the ancient rock record, but — by removing the ‘earliest’ outlier of evidence — suggests that the invasion of the continents happened globally at the same time,” Shillito notes.

The paper “Death near the shoreline, not life on land: Ordovician arthropod trackways in the Borrowdale Volcanic Group, UK” has been published in the journal Geology.