A 518-million-year-old fossil discovered in Yunnan Province, southern China, has shed new light on the evolutionary origins of spider fangs. The specimen belongs to a prehistoric creature known as urokodia and reveals some of the earliest known evidence of chelicerae—specialized pincer-like limbs that modern spiders and their relatives use for capturing and injecting prey.

Researchers identified the fossil at the Chengjiang site, a well-known location for exceptionally preserved Cambrian-era fossils. Although the urokodia does not closely resemble contemporary spiders or scorpions at first glance, detailed examination uncovered two distinctive pincer limbs positioned just behind its eyes. These limbs represent the earliest known form of chelicerae, a defining feature of the arachnid lineage.

The study’s lead author, Professor Yu Liu, explained that the discovery came through X-ray tomography, a technique that allowed scientists to visualize soft tissues embedded within rock. “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,” said Liu. “We knew immediately that this was a very exciting fossil and indeed a distant ancestor of living chelicerates like scorpions and spiders.”

Chelicerae are a hallmark of the chelicerate group of arthropods, which includes spiders, scorpions, and related species. These appendages serve as fangs or pincers, enabling the animals to grasp and immobilize prey, often delivering venom. Until now, evidence of early chelicerae has been scarce, limiting understanding of their evolutionary development.

The presence of these structures in such an ancient fossil provides valuable insights into the early stages of arachnid evolution during the Cambrian period, a time when many major animal groups first appeared. This finding helps to clarify how complex predatory adaptations like spider fangs emerged over half a billion years ago.

Further research into the fossil’s anatomy may illuminate additional details about the evolutionary relationship between urokodia and modern chelicerates, refining scientists’ grasp of how these iconic arthropods originated and diversified.