Urokodia aequalis Fossil Reveals Earliest Known Evidence of Spider Fangs

Afraid of spiders? If you ever travel back through time, here's one creature you might want to squash.

It's called Urokodia aequalis, and finding it would mean setting your time machine for China in the early Cambrian period, around 518 million years ago.

What did Urokodia aequalis look like?

It doesn't look much like a spider just yet. Instead, it has a long, segmented thorax carried on far too many tiny feet, with a shrimp-like tail at one end and an armoured head bearing creepy eyes on stalks at the other.

The entire animal was only about 2 to 3 centimetres long, so crushing it underfoot should be straightforward.

But why might Urokodia attract the attention of a time-travelling arachnophobe? Scientists have discovered that its fossils hold the earliest known evidence of structures that would eventually become spider fangs. Their findings appear in Nature.

These off-putting appendages are properly called chelicerae. They occur in spiders, scorpions, mites, ticks, horseshoe crabs and other arthropods belonging to a group known as chelicerates.

Urokodia now appears to be a founding member of that group, having pioneered the small mouthparts that have served its descendants for hundreds of millions of years.

How X-rays revealed Urokodia's proto-fangs

The little creature had kept those features hidden, though. Fossils of Urokodia have been known for decades, but its tiny proto-fangs had escaped notice until now.

They were finally identified by scientists from Yunnan University in China and the University of Leicester in the UK.

The team used X-ray analysis to examine several fossil specimens and the rock around them, uncovering soft body parts that were surprisingly well preserved.

The resulting images revealed a small pair of pincer-like appendages immediately behind Urokodia's eyes. The researchers suggest these may represent an early form of chelicerae.

"We were using X-ray tomography analysis of these fossils to reveal their soft anatomy buried in the rocks for hundreds of millions of years, when suddenly we noticed the pincer-like limbs at the front of the animal," says Yu Liu, a palaeobiologist at Yunnan University.

"We knew immediately that this was a very exciting fossil and indeed a distant ancestor of living chelicerates like scorpions and spiders."

Before this find, the oldest known example of chelicerae came from a similar-looking arthropod, Megachelicerax cousteaui. Urokodia, however, was roaming the oceans some 14 million years earlier.

From Urokodia's mouthparts to spider fangs

According to the researchers, Urokodia "provides an unusually direct blueprint" for the evolution of small appendages near the mouth into the fangs and pincers found in later chelicerates.

Those appendages had several segments in earlier species, whereas the mouthparts of Urokodia had begun to take on a pincer-like shape.

That basic form survived over immense spans of time. Today, its descendants use specialised versions to feed in a variety of ways.

Horseshoe crabs and scorpions have small, pincer-like chelicerae that act like little hands when they feed. Ticks use theirs to pierce a host's skin and drink blood.

Spiders, meanwhile, have evolved chelicerae with fangs, which help them subdue prey or deter larger threats.

With only rudimentary versions of such weapons, Urokodia will have to rely on the integrity of the spacetime continuum to protect it from arachnophobes.

The research was published in the journal Nature.

Jess Cockerill fact-checked this article, and Clare Watson edited it. We take pride in our process, but we're only human. Please let us know if you notice a mistake.