518-million-year-old fossil reveals earliest spider fang ancestors

A fossilized marine creature from 518 million years ago has yielded the earliest known evidence of the anatomical structures that eventually evolved into spider fangs. Researchers from Yunnan University and the University of Leicester identified primitive chelicerae—pincer-like appendages—in a specimen of Urokodia, a small Cambrian Period animal unearthed at the Chengjiang fossil site in Yunnan Province, China.

The discovery affects the scientific understanding of chelicerates, a vast group of invertebrates that includes spiders, scorpions, and ticks, with over 100,000 described species. By revealing the early form of chelicerae, the study provides a direct evolutionary link between ancient marine life and the specialized mouthparts that modern chelicerates use to grip, pierce, or stab prey.

This finding matters because it pinpoints the origin of a key predatory adaptation that helped chelicerates become one of the most successful animal groups in both marine and terrestrial environments. The fossil also preserves potential book gills on Urokodia’s legs, suggesting it breathed underwater like today’s horseshoe crabs, and underscores the importance of the Chengjiang site, which holds over 200 types of animals from the dawn of animal life.

The study was published in Nature on August 4, 2026, coinciding with the 42nd anniversary of the Chengjiang site’s discovery. Urokodia measured only around 2–3 cm in length and had stalked eyes, a segmented skeleton, and jointed limbs. Using X-ray tomography, the team scanned the rock encasing the fossil and found that soft tissues were preserved in a mummified state, revealing the pincer-like limbs just behind the eyes.

The research was led by Professor Yu Liu of Yunnan University, also a Visiting Professor at the University of Leicester, with co-author Professor Mark Williams from the University of Leicester School of Geography, Geology and the Environment. Professor Liu said the team noticed the pincer-like limbs during analysis and immediately recognized the fossil as a distant ancestor of living chelicerates. The work was supported by a grant from the Department of Science and Technology of Yunnan Province (202401BC070012) and the Yunnan Revitalization Talent Support Program.

According to the researchers, Urokodia lived in a rich marine ecosystem during a critical period of animal evolution, and the Chengjiang fossils offer real insights into how life was evolving at the very dawn of animals. The study highlights that while modern spiders often evoke fear, their venom and bites evolved to subdue much smaller prey, and most species pose no danger to humans.