Artist’s impression of Urokodia.
Credit: Xiaodong Wang
The World Spider Catalog currently recognises more than 54,000 living species of spider, and tens of thousands more may remain undiscovered. Every one of them possesses a pair of specialised mouthparts called chelicerae, each of which terminates in a fang. In most spiders, these fangs inject venom, although a few lineages have lost their venom glands and subdue their prey by other means.
Chelicerae are not, however, unique to spiders. They are the defining appendages of the Chelicerata, the major arthropod group that also includes scorpions, mites, ticks, harvestmen and horseshoe crabs. Scorpions possess small, pincer-like chelicerae near the mouth; their conspicuous claws are modified pedipalps, while their venom is delivered through a sting at the end of the tail. In other chelicerates, the same basic pair of appendages has been modified into pincers, piercing mouthparts or other feeding structures.
This shared anatomy strongly suggests that chelicerae were inherited from a common ancestor and subsequently modified as the different chelicerate lineages evolved. But what did the ancestral appendage look like, and when did it first appear?
Such questions inevitably involve gaps in the fossil record—gaps that creationists routinely misrepresent as evidence against evolution. Their argument depends upon portraying evolutionary theory as little more than a story constructed from fossils and therefore requiring an unbroken succession of specimens linking every species to its ancestors. Evolutionary theory requires no such thing. Fossilisation is an exceptionally rare event, and evolutionary relationships are reconstructed from multiple independent lines of evidence, including comparative anatomy, embryology, genetics and the fossils that happen to have survived.
Nevertheless, creationists will be disappointed to learn that another important part of this particular gap has now been bridged. A paper published in Nature by a Chinese-British research team led by Yu Liu and Lorenzo Lustri of Yunnan University describes the remarkably preserved anatomy of Urokodia aequalis, a small marine arthropod recovered from the famous Chengjiang fossil site in Yunnan Province, southern China. The rock formation containing it dates from approximately 518 million years ago.
Urokodia was not a spider, nor can it be identified as the literal common ancestor of modern chelicerates. Instead, phylogenetic analysis places it on the chelicerate stem lineage—as an early evolutionary relative close to the line from which true chelicerates arose. Crucially, it possessed a pair of pincer-like “short great appendages” that appear intermediate between the multi-segmented grasping appendages of earlier Cambrian arthropods and the true chelicerae of later chelicerates.
Using X-ray microtomography, the researchers examined anatomical structures hidden inside the surrounding rock. They reconstructed much of the exceptionally preserved soft anatomy of this 2–3-centimetre-long animal, including its stalked eyes, segmented body, jointed limbs and the pair of pincer-like appendages immediately behind its eyes. Features of its trunk limbs also support the hypothesis that the book gills of aquatic chelicerates evolved from the limb structures of earlier arthropods.
Urokodia lived during the early Cambrian, a period creationists habitually misrepresent as one in which complex animals appeared suddenly and without evolutionary ancestry. In reality, the Cambrian radiation was an extended episode of evolutionary diversification in which increasing mobility, new feeding strategies and escalating predator–prey arms races helped produce an extraordinary range of defensive and offensive adaptations. Many of the fundamental body plans associated with the major animal groups were assembled and modified during this evolutionary experimentation.



































