A tiny fossil barely the width of a grown-up's thumb is rewriting the origin story of starfish—and overturning a nearly 150-year-old guess about how they first came to be.
Meet Yujingia glutenotunica, a soft-bodied animal that lived 518 million years ago in what is now China's Yunnan Province. At just 8 to 22 millimeters long, it is unassuming. But buried in its squishy body is a big surprise: it appears to have eaten with delicate, featherlike tentacles, not by passively filtering water through its gills as scientists long believed.
For decades, researchers assumed that the ancient ancestor of starfish and their relatives was a wormlike creature that lived in seabed burrows and strained food from the water. A new international team, including scientists from the University of Cambridge and Northwest University in China, says those worms may have had the story all along.
The fossils date from the Cambrian explosion, a brief window of rapid evolution roughly 500 million years ago when most major animal groups first burst onto the fossil record. Hard-shelled creatures dominate that record, but Yujingia was found in the Maotianshan Shales, one of only a handful of places on Earth where conditions preserved soft body parts before they decayed.
The specimens show two distinct sets of appendages. One set of featherlike tentacles captured organic particles drifting in the water—the animal's feeding system. A second set likely anchored it temporarily to the seabed, though Yujingia probably could move around a little.
Its name nods to its shape. The animal has an elongated, bottlelike form with tentacles spreading above a rounded lower body. Lead author Kaiyue He, from Northwest University, says its outline closely resembles the yujingping, a ritual vase in traditional Chinese culture—which inspired the genus name. The soft, gelatinous look earned it the species name glutenotunica.
Yujingia belongs to a group called ambulacrarians, which today includes starfish, sea urchins and acorn worms. Ambulacrarians are one of two major branches of a larger family, the deuterostomes, which also includes chordates—the branch that eventually led to humans. These closely related lineages evolved wildly different bodies: starfish with their fivefold radial symmetry, worms with their bilaterally symmetrical, wriggling forms, and us.
"The fivefold radial symmetry of starfish and their relatives, and the bilaterally symmetrical, wormlike bodies of acorn worms and their relatives have been a bit of a mystery until now," said co-author Dr. Giovanni Mussini of Cambridge.
To pin down the fossil's family position, Mussini and He built a huge data set of 505 anatomical features across 100 living and extinct species. The analysis places Yujingia surprisingly close to the last common ancestor of today's ambulacrarians, making it a key transitional form.
The finding, reported in the journal Current Biology, suggests that sophisticated tentacle-based feeding evolved early—while hard skeletons and other flashy features came much later. It also hints that the ancestor of starfish may have been more like a starfish than a worm all along.