An ancient marine predator has helped explain where spiders’ fangs come from

Scientists from the University of Leicester and Yunnan University have studied a fossil of the ancient arthropod Urokodia and discovered structures in it that may explain the origin of spider fangs.

These were not true fangs, as found in modern spiders, but early claw-like appendages known as chelicerae. In spiders, it was precisely these chelicerae that eventually evolved into the organs they use to pierce their prey and inject venom.

The study has been published in *Nature*.

What is Urokodia?

Urokodia is a small marine arthropod that lived during the Cambrian period. Its fossils were found at the famous Chengjiang site in Yunnan Province in southern China. This is one of the world’s most important sites for the preservation of ancient Cambrian soft-bodied animals.

The animal was very small — about 2–3 centimetres long. It had an elongated, segmented body, jointed limbs and large eyes on stalks. It bears almost no resemblance to a modern spider. But it was in its front section that the scientists found what makes the discovery particularly significant.

What the scientists found

The researchers used X-ray tomography to look inside the rock where the fossil was preserved. This method allows them to see details that cannot be observed from the outside.

Analysis revealed that Urokodia had preserved not only its external form but also part of its soft anatomy. In the front part of the body, just behind the eyes, the scientists noticed a pair of claw-like appendages. It is these that resemble an early form of chelicerae — specialised appendages characteristic of the group that includes spiders, scorpions, ticks, opiliates and sea spiders.

In modern spiders, chelicerae have evolved into fangs. In scorpions, they perform different functions, whilst in opiliones they are used as small grasping appendages near the mouth. Therefore, the discovery is significant not only for the history of spiders, but for the entire large group of chelicerates.

In simple terms: what’s the connection with spider fangs?

In spiders, fangs are not teeth. They are modified front appendages called chelicerae.

In the ancient Urokodia, scientists have found an early version of such appendages. They were not yet spider fangs, but they already pointed to the direction of evolution: the front appendages of some ancient arthropods began to transform into specialised tools for grasping and piercing prey.

This is precisely why we can say that this fossil helps us understand where spiders’ fangs came from.

Why this discovery is important

Spiders, scorpions and ticks belong to the Chelicerata — one of the most successful groups of arthropods. It currently comprises over 100,000 described species. Their common feature is specialised front appendages, the chelicerae, which can function as claws, mouthparts or fangs.

But the question of how and when such appendages first appeared has long remained a mystery. Soft tissues and small appendages are rarely well-preserved in fossils. In the case of Urokodia, the scientists were in luck: X-ray analysis revealed details that had lain buried in the rock for hundreds of millions of years.

The new discovery shows that the origins of the spider-like ‘bite’ can be traced back to marine animals of the Cambrian period — a time when many major groups of animals were rapidly emerging on Earth.

It also had ‘gills’

The study is significant not only because of the chelicerae. Signs of structures on Urokodia’s limbs were found that may have functioned as book gills. These are respiratory organs similar to those found in modern aquatic chelicerates, such as swordtails.

This strengthens the link between the discovery and the early history of the entire group. Urokodia helps us understand not only the origin of grasping appendages, but also how ancient chelicerates were able to breathe and live in a marine environment.

Why this is not the ‘first spider’

It is important not to be mistaken here. Urokodia was not a spider. Spiders appeared much later and on land. The animal found lived in the sea and looked completely different.

However, it illustrates an early stage in the evolution of features that later became important for spiders and their relatives. It is therefore more accurate to say that scientists have discovered an ancient marine arthropod with an early form of chelicerae – the structures from which spiders’ fangs evolved.

This is not the direct ‘grandfather of all spiders’ in the colloquial sense, but an ancient representative of a lineage closely related to the origin of the chelicerates.

Why are spider fangs so important in the first place?

Fangs have made spiders effective predators. They use them to seize their prey, pierce it and inject venom. But this mechanism did not emerge fully formed straight away.

Evolution often works gradually. What may have been small, claw-like appendages in an ancient marine animal eventually evolved into different tools in its distant descendants and relatives: in spiders — into fangs; in scorpions — into mouthparts; and in swordtails — into structures for capturing food.

The discovery of Urokodia reveals one of the early stages in this long history.

Background

The Cambrian Period is often referred to as a time of dramatic complexity in animal life. Various groups of animals appeared in the seas, featuring new body plans, limbs, eyes and hunting methods.

Urokodia lived in an ancient ecosystem that was home to over 200 types of animals. According to the researchers, such well-preserved fossils allow us to see how life experimented with new body forms at the very earliest stage of animal evolution.

This is precisely why a small fossil, just a few centimetres long, can be of great significance: it reveals how one of the most successful lines of predators in the history of arthropods began.

Source

Study: Yu Liu et al., “Urokodia sheds light on the origin of chelicerae and book gills of Chelicerata”, Nature, 2026.