Researchers working with 535 million-year-old rocks from southern China have discovered the fossils of an armored worm-like creature that’s just a few millimeters long with at least five pairs of large spikes along its body. They named it Eokinorhynchus rarus, and it’s closely related to – and possibly an ancestor of – marine invertebrates called kinorhynchs, also known as mud dragons. The findings are published in Scientific Reports this week.

These days, there are about 240 kinorhynch species, and their bodies are divided into three sections: head, neck, and trunk. The head includes a mouth cone with circlets of teeth, and the trunk is further split up into 11 segments. Previous molecular clock estimates suggest that they diverged just before the Cambrian, but no kinorhynch fossils have been uncovered until now. The new species E. rarus – Latin for “rare” – would be the first fossil kinorhynch unearthed from the rock record.

A team led by Huaqiao Zhang from the Chinese Academy of Sciences and Virginia Tech’s Shuhai Xiao examined several microfossils collected from the early Cambrian limestone deposits of Sichuan and Shaanxi provinces. Using scanning electron microscopy, microCT scans, and phylogenetic analyses, the team uncovered several similarities between the new extinct species and today’s kinorhynchs to suggest a close evolutionary relationship.

For example, the trunk of E. rarus and modern kinorhynchs are divided into segments each consisting of small, articulated plates, and they all have hollow spines called spinose sclerites. But compared to living kinorhynchs, E. rarus has about twice as many segments, and it’s armored with larger, more distinct spines. In addition to the five pairs of large spikes arranged bilaterally around the trunk, E. rarus also has a single large spine located in the middle of its body (visible in the image labeled “b” above) and two pairs of spines located near the anus.

Xiao thinks that the spines helped facilitate locomotion. "These tiny worms probably lived in the interstitial spaces within sandy sediments, so they likely used the spiky spines to anchor themselves when navigating among sand grains," he tells IFLScience. "They may have also functioned as defensive structures."



Scanning electron microscopic image of Eokinorhynchus rarus. Dinghua Yang

Many bilaterally symmetrical animals – or bilaterians, which includes people and bugs alike – have some version of repetition or iteration of anatomical structures. Examples include the body segments and legs of both vertebrates and arthropods (the group encompassing insects, spiders, and crustaceans). Since the 19th century, scientists have been debating whether body segmentation and appendages evolved only once in the last common ancestor of bilaterians, or if they evolved multiple times independently among different animal groups.

"The current consensus is that, although the genes to pattern body segments and limb development might have been present in the last common ancestor of bilaterians, body segmentation and appendages evolved independently in different groups that recruit the same or similar genes to do the job," Xiao explains.

But some dissenters think that the last common ancestor of kinorhynchs and arthropods had legs, which were lost later along the lineage toward mud dragons and their relatives. "These new primitive fossils tell us that early kinorhynchs diverged and evolved body segmentation more than 530 million years ago, independent of arthropods," Xiao adds, "and that they never had legs."