蜘蛛牙是它们最可怕的特征之一——新化石揭示了其起源

The publicity posters for the 1955 cult monster movie Tarantula! displayed a giant spider rampaging across the Arizona desert and clutching a poor human victim in its viciously long fangs. The film captured our fear of spiders, their creepy crawly motion and the hideous way they stab their prey to death, injecting a lethal venom.

Spiders, like their cousins the scorpions, are some of nature’s most accomplished hunters. Our new research looks at remarkable fossils excavated from Yunnan, southern China. We have found evidence that helps reveal the earliest origins of fangs.

Spiders and scorpions are chelicerates, part of a large group of animals called arthropods. Arthropods are characterised by their jointed limbs and skeletons that grow on the outside of their bodies. It’s that skeleton you hear scrunching if you accidentally step on one.

Chelicerates also include mites, ticks, horseshoe crabs and sea spiders, all with specialised limbs at the front of the animal called chelicerae. It’s the chelicerae that are used as pincers or fangs for eating or stabbing prey. If you’ve ever been bitten by a tick, it’s the saw-like chelicerae that drew blood and got stuck in your skin.

Chelicerates are among the most successful animals to live on land and in the sea, numbering well over 100,000 species. Fossils show that their ancestors have been hunting on land for more than 400 million years.

However, the earliest fossil evidence of chelicerates is not found on the land, but in the sea, in creatures that lived more than 500 million years ago during the Cambrian period. The Cambrian was a time of rapid evolution for animals that would one day lead to the amazing diversity of life in the oceans and on land. One of these Cambrian sea-creatures is called Urokodia, its name derived from the ancient Greek words for its shielded tail. It lived in the tropical seas that covered southern China about 518 million years ago.

The beginnings of spider fangs

At first sight Urokodia – pictured at the top of this article – doesn’t seem like a contender for a distant relative of spiders or scorpions. It was a small animal 2-3 cm long with large eyes protruding on stalks from the front, and an elongate shrimp-like body, with jointed limbs strung from its underside. So, while Urokodia displays the tell-tale signs of arthropods, with its skeleton on the outside and jointed limbs, it doesn’t look much like a spider or a scorpion.

But when we used X-rays to peer a little closer at the front end of Urokodia, we saw two pincer-like limbs emerging next to its eyes that are an ancient type of chelicerae. Some of the other legs of Urokodia bear finely layered structures that were probably used as gills for breathing in the sea. Modern aquatic chelicerates, like the horseshoe crab, have similar structures called “book gills” that literally look like the pages of a book.

Urokodia is an important link in the evolutionary chain, connecting forward to the incredible diversity of modern chelicerates like spiders, and also to other important Cambrian fossils, like Megachelicerax. That animal lived just a few million years after Urokodia in the tropical seas of ancient North America and its name means “big claw” honouring the awesome pair of chelicerae on its head.

Urokodia, like Megachelicerax, was almost certainly a specialised predator. These fossils also hint at links to another group of ferocious Cambrian hunters, including the half-metre-long shrimp-like Anomalocaris, one of the planet’s first apex predators. Those animals bore formidable frontal limbs for grasping and catching prey and a circular mouth with plates that could tear apart a victim’s body.

In Urokodia we see the beginnings of the specialised limbs that would one-day evolve into the deadly fangs of spiders. We also see the beginnings of the lineage that would eventually produce some of the most formidable predators that have ever lived – though mercifully none as large as the spider in Tarantula.

Yu Liu receives funding from the Department of Science and Technology of Yunnan Province (202401BC070012) , and is further funded by the Yunnan Revitalization Talent Support Program and the Chinese Scholarship Council to support his 1-year academic sabbatical at the University of Leicester. He works at the Yunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming, China.

Lorenzo Lustri, Mark Williams, and Tom Harvey do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.