518-million-year-old creature reveals the origins of spider fangs

Scientists have discovered the earliest known evidence of the structures that eventually became spider fangs in a fossil dating back 518 million years. The finding was made by researchers from Yunnan University and the University of Leicester.

Whether associated with the deadly reputation of the Black Widow or the fictional bite that transformed Spider-Man, fangs have become one of the most recognizable features of spiders. Their evolutionary story, however, began long before modern spiders appeared.

A new study published in Nature traces the origins of this powerful hunting tool to Urokodia, a small marine creature that lived during the early Cambrian Period.

The Ancient Origins of Spider Fangs

Spiders belong to a large group of invertebrates called chelicerates, which also includes scorpions and ticks. More than 100,000 chelicerate species have been described.

These animals have jointed limbs and hard external skeletons. Their defining feature is a pair of specialized appendages called chelicerae, located near the front of the body. Depending on the species, chelicerae can function as pincers or fangs that grip, pierce, or stab prey.

Urokodia fossils were discovered at the renowned Chengjiang fossil site in Yunnan Province in southern China. The study was published on the 42nd anniversary of the site's discovery.

The creature measured only around 2-3 cm in length. It had large eyes extending from stalks at the front of its body, along with a segmented skeleton and jointed limbs attached beneath its narrow frame. Its appearance bears little obvious resemblance to the spiders and scorpions that descended from its broader evolutionary lineage.

X-Rays Reveal Preserved Soft Anatomy

Researchers from Yunnan University, China, and the University of Leicester used X-ray technology to examine the rock surrounding the fossil. The scans revealed that much of the animal's soft anatomy had remained preserved in a mummified state for hundreds of millions of years.

Most importantly, the researchers identified two pincer-like appendages positioned just behind the creature's eyes. These structures represent an early form of chelicerae and provide evidence of the evolutionary beginnings of the pincers and fangs seen in chelicerates today.

The fossil also preserves features on Urokodia's legs that may have served as book gills, allowing the animal to breathe underwater. Similar respiratory structures are still found in aquatic chelicerates such as horseshoe crabs.

A Successful Lineage of Hunters

Chelicerates have become one of the most successful animal groups in both marine and terrestrial environments. Those that moved onto land developed into highly effective predators, and fossils show that their ancestors had already been hunting for hundreds of millions of years.

Despite the frightening image of spiders presented in movies such as Arachnophobia, most species pose no danger to people. Their venom and bites evolved to subdue prey that is far smaller than a human.

The research was led by Professor Yu Liu of Yunnan University, who is also a Visiting Professor at the University of Leicester.

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

A Window Into the Dawn of Animal Life

Urokodia lived within a rich marine ecosystem during a crucial period in the history of animal evolution. The Chengjiang fossils preserve evidence of more than 200 types of animals that inhabited the oceans over 500 million years ago.

Co-author Professor Mark Williams from the University of Leicester School of Geography, Geology and the Environment said: "Urokodia was part of an ancient ecosystem of over 200 different types of animals living in the seas over 500 million years ago. These spectacularly preserved fossils provide real insights into how life was evolving on our planet at the very dawn of animals."

This study was supported by a grant from the Department of Science and Technology of Yunnan Province (202401BC070012) to Professor Yu Liu, who is further funded by the Yunnan Revitalization Talent Support Program.

Reporting from ScienceDaily contributed to this story.