518-million-year-old Urokodia fossil reveals early spider fangs

A 518-million-year-old fossil from southern China preserves the earliest known trace of the fanged pincers found today in spiders and scorpions. The marine animal, named Urokodia, measured barely 2.5 centimetres long. Just behind its eyes was a pair of small, pincer-like limbs that marked the beginnings of what would become spider fangs.

The limbs help to fill a long-standing gap in the account of how one of Earth’s most successful animal groups emerged. Spiders, scorpions, ticks and horseshoe crabs are members of a lineage containing more than 100,000 living species. The fossil records one of its defining tools at an intermediate stage of evolution.

A closer look at the Urokodia fossil

The fossils were found in the Chengjiang biota, a site in China’s Yunnan Province renowned for its detailed preservation of soft-bodied Cambrian animals. Its rock can retain the outlines of gills, guts and limbs that almost never survive elsewhere.

Professor Yu Liu of Yunnan University led the work alongside colleagues from the University of Leicester. Instead of interpreting the fossil from its surface, the researchers used X-ray scanning to create a three-dimensional model of the anatomy still enclosed in the stone.

The scans revealed a much older evolutionary story to the team. Liu, who led the study, told Earth.com, “Spider fangs, scorpion pincers, and horseshoe crab chelicerae all belong to the same great evolutionary story: the origin of the chelicera, the first pair of feeding appendages in chelicerates.”

Urokodia itself bears little resemblance to its living relatives. Roughly 2.5 centimetres long, it had large stalked eyes at the front and dozens of jointed limbs hanging beneath its long, narrow body.

Within the rock, the researchers discovered soft tissue that had effectively been mummified. Delicate limb branches, gill flaps and the outline of the gut had survived in three dimensions, revealing far more than a flattened impression could have shown.

The animal’s position in the arthropod family tree had been uncertain for years. An earlier study of the same fossil, based chiefly on its back plates, interpreted it as a distant relative of trilobites. Its pincers remained unnoticed until the scans revealed them.

The first pincers

The scans showed a pair of limbs close to the mouth, each made up of four short joints. The outer two joints were larger than the inner pair. This arrangement allowed the limb to close in a pinching action, a detail that drew the researchers’ attention.

The head, apart from the pincers, was made up of seven segments, with a hardened plate around the mouth. Against an otherwise fairly ordinary body, the newly identified limbs stood out as the one major novelty.

In living chelicerates, those front limbs are known as chelicerae: pincers or fangs used to seize and stab prey. Earlier Cambrian arthropods instead had long grasping limbs with many joints. How the former developed from the latter had remained unclear.

Urokodia sits between the two. Its pincers are less complex than a true chelicera, but more compact than the long grasping arms of earlier Cambrian hunters, making them an intermediate step in the transition.

Compared with older arthropods, the pincer offers a clue to the origin of the chelicera. In an email to Earth.com, Liu said, “It shows that the chelicera may have evolved from a more complex, grasping appendage, rather than appearing abruptly in its modern form.”

A separate paper published earlier this year reported the oldest confirmed chelicera in a larger predator from Utah, dating to around 500 million years ago. Urokodia is older still, and its partly formed pincers suggest what that completed weapon developed from.

Breathing through the legs

The scans revealed more than the pincers. Along the trunk of Urokodia, rows of thin, overlapping flaps were stacked on the back of each leg.

The flaps correspond to book gills, the stacked breathing plates that living horseshoe crabs still use to extract oxygen from seawater. Their presence on the legs of Urokodia suggests that these breathing structures developed from ordinary walking limbs rather than appearing from nothing.

The idea that book gills originated as branches of legs is not new. A study of another Cambrian arthropod a few years ago identified early, partial versions. Urokodia extends that evidence further back and connects the breathing plates to the ordinary limbs from which they most likely developed.

Liu links the flaps to the earliest chelicerate breathing. Speaking with Earth.com, Liu explained, “In Urokodia, we see stacked structures associated with the trunk appendages that may represent an early stage in the evolution of these respiratory organs.”

A quiet beginning

Together, the pincers and breathing flaps place Urokodia as the earliest known branch of the chelicerate lineage, just below the point at which true fangs and complete book gills appear. It is the oldest fossil showing both features partway through their formation.

Urokodia lived alongside a remarkable variety of other animals. “Urokodia was part of an ancient ecosystem of over 200 different types of animals living in the seas over 500 million years ago,” said Professor Mark Williams, a co-author on the study.

The pincers that began on an animal the size of a fingernail now equip some of the most widespread hunters on land and in water. For palaeontologists, Urokodia provides a rare fixed point from which to trace how that body plan developed, one limb at a time.