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A worm-like creature preserved in ancient rock has just been identified as the oldest known relative of insects, spiders and crustaceans.

The ecdysozoans are a group of invertebrate animals with a tough outer coating, or cuticle, that they moult as they grow. It is clearly a successful evolutionary strategy, because today the group contains millions of species. 鈥淪ome estimates suggest that about half of all known animal species are ecdysozoans,鈥� says at Harvard University. All arthropods 鈥� including insects and spiders 鈥� belong to the group, and so do nematode worms and a few bizarre marine animals.

However, we know very little about the early evolution of this vast group, because the first animals typically lacked hard shells or skeletons that readily fossilise. By the time those hard parts became more common 鈥� in an event 539 million years ago known as the Cambrian explosion 鈥� the ecdysozoans were already abundant and diverse. This means they must have a much deeper evolutionary history.

Now, Hughes and his colleagues have thrown light on that earlier history by finding dozens of fossils of the oldest known ecdysozoan in rocks from South Australia that date back 550 million years. The 3-centimetre-long worm-like animal 鈥� which the researchers have named Uncus dzaugisi 鈥� lived on a soft, soupy microbial mat on the ancient seafloor. 鈥淚t was a little like pond scum, not necessarily the kind of thing you would want to step on,鈥� says Hughes.

Then, the microbial mat was rapidly smothered by a 10-centimetre-thick layer of sand, trapping and killing the tiny animals. Hughes says it is the fact that the worm-like animals held their 3D shape while they were buried that shows they were ecdysozoans: this demonstrates that U. dzaugisi had a tough outer cuticle, which is a key feature of the group.

However, Hughes and his colleagues don鈥檛 think U. dzaugisi represents the earliest stage of ecdysozoan evolution. They suspect it is more closely related to the nematodes than to arthropods, suggesting the group had already begun to split into its major lineages even 550 million years ago. 鈥淚f researchers keep looking, they may find fossils that may truly be that common ancestor of all ecdysozoans,鈥� he says.