Newfound Bug Ancestor Had Legs Under Mouth

A 520-million-year-old fossil arthropod known as a fuxianhuiid literally shoveled food in its mouth. Photograph courtesy Jie Yang.

or most animals, the rules of anatomy are pretty straightforward: head and brain at the very top and legs at the bottom. But a 520 million-year-old fossil animal recently discovered in southern China breaks both of these rules.

Called a fuxianhuiid, this common ancestor of modern arthropods—insects, spiders, and sea creatures like lobsters—has some pretty bizarre traits—its legs are under its mouth, and its spine extends far above its brain.

The find, published earlier this week in the journal Nature, is one of the oldest animal fossils ever discovered, said lead researcher Javier Ortega-Hernandez, of the University of Cambridge in the U.K. (Also see “Biggest Fossil Spider Found.”)

“Fuxianhuiids have been crucial for understanding the origin of the arthropod head, which provides a lot of information that allows biologists to reconstruct the evolutionary history of arthropods,” Ortega-Hernandez said.

For instance, the head “allows us to reconstruct what the common ancestor looked like, which is of great importance for fleshing out the evolutionary relationships of the group.”

Scientists knew that fuxianhuiids (pronounced foo-see-an-who-eeds) existed from previous excavations in China’s Yunnan Province (map). However, the creatures’ hard outer shells, or carapaces, that hindered attacks from predators also prevented researchers from seeing the full array of their body parts. What’s more, their softer antennae and legs generally decay before they have a chance to fossilize, unlike the carapace.

The newfound fuxianhuiid specimens, however, were fossilized in an unusual position that preserved the arrangement of their soft bodies.

Stuffing Its Face

A closer examination of the fossils revealed that the fuxianhuiids had legs directly under their mouths that they used to literally shovel food into their mouths on their seafloor habitat.

Ortega-Hernandez said that these legs are quite possibly the earliest examples of limbs used for eating—though their existence has been controversial, since previous fuxianhuiid fossils did not clearly show these limbs. (Also see “New Genitalia-Headed Fish Is Evolutionary Mystery.”)

“These structures have been subject of a heated debate for almost a decade,” he said.

Another unusual find from the fuxianhuiid fossils is that a small piece of neural cord protruded from their brains—making the species the oldest example of a nervous system that stretches out above the head. Only a few types of modern animals still have this anatomy.

Getting a clear look at the fuxianhuiid head is important, since evolutionary biologists frequently use the head to determine the closest relative of an unknown arthropod species.

If you look at modern insects and crustaceans, it seems like fuxianhuiid features have disappeared over the course of 520 million years. Look more closely, however, and Ortega-Hernandez said that the forces of natural selection have simply altered these structures, not eliminated them.

Take spiders, for instance. The fuxianhuiids’ antennae have become the spider’s fangs, and the legs under their mouths have become pedipalps—a pair of specialized appendages that the spider uses to get food into its mouth.

A reconstruction of what scientists thought one type of fuxianhuiid looked like when alive. Courtesy Javier Ortega-Hernandez
A reconstruction of what one species of fuxianhuiid may have looked like when alive. Illustration courtesy Javier Ortega-Hernandez


“Very Strange Animals”

Fuxianhuiids flourished during the Cambrian period, from 540 to 485 million years ago, which was when life first became very complex—and very weird. (See a prehistoric time line.)

It was the planet’s experimental period, like a teenager flitting between fashion fads, trying to find himself. Some of the best Cambrian fossils have been found in the Yunnan Province and Canada‘s Burgess Shale. (Read about a 3-foot (0.9-meter) “shrimp” that dominated the Cambrian seas.)

That’s why fuxianhuiids, in all of their weirdness, provide valuable insights into the ancestors of not just of arthropods, but of all animals, Ortega-Hernandez said. He and his team, which includes scientists from Yunnan University in China, are currently analyzing other finds from the area, hoping to tease out more of what Cambrian life was like as a whole.

“Fuxianhuiids in general are very strange animals,” he said—so don’t be surprised if the other finds are equally bizarre.

Another fossil fuxianhuiid. Photo by Jie Yang.
Another species of fossil fuxianhuiid. Photograph courtesy Jie Yang.


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Carrie is a freelance science writer living in Virginia. When she's not writing about cool critters, she's spending time outside, drinking coffee, or knitting. You can visit her website at