A 518-million-year-old ambulacrarian fossil from China suggests that the common ancestor of starfish and their relatives was not a burrowing worm; it was probably a tentacled animal that fed from the water around it.
An artist’s impression of Yujingia glutenotunica. Image credit: He et al., doi: 10.1016/j.cub.2026.08.053.
Ambulacraria is one of the two major branches of deuterostome animals, and was first classified in the 1880s.
Ambulacarians include echinoderms (such as starfish) and hemichordates (such as acorn worms), two groups with strikingly different body forms.
However, fossil evidence of the origins of this group has remained scarce.
“The fivefold radial symmetry of starfish and their relatives, and the bilaterally symmetrical, worm-like bodies of acorn worms and their relatives has been a bit of a mystery until now,” said co-author Dr. Giovanni Mussini, a paleontologist at the University of Cambridge.
The researchers found several specimens of a previously unknown ambulacrarian in the Maotianshan Shales of China’s Yunnan province.
The site is known for preserving soft-bodied animals from the Cambrian explosion, the burst of evolution when most major animal groups first appear in the fossil record.
Named Yujingia glutenotunica, the new ambulacrarian species was small, between 8 and 22 mm long.
They were shaped like a bottle, with a soft, gelatinous body topped by feathery tentacles that caught food particles from the water.
A second set of appendages likely anchored them to the seafloor, though they may have been able to move a little.
An analysis of anatomical traits across 100 living and extinct species placed Yujingia glutenotunica close to the last common ancestor of the ambulacrarians.
“There has been a tendency to think of the last common ancestor of ambulacrarians as more closely resembling a worm living in its seafloor burrow, filtering food from the water,” Dr. Mussini said.
“This new fossil suggests instead that the common ancestor of the ambulacrarians might have actually fed with tentacles, and may have been more like starfish and their relatives.”
“It tells us how ecology can drive different lineages onto very different evolutionary paths, and very different morphological specializations,” he added.
“The chordates became reliant on movement for finding their food, so they streamlined their bodies and developed their brains, while members of the starfish lineage remained relatively anchored to the seafloor and specialized on this particle-feeding lifestyle.”
“How the major deuterostome groups diverged during the Cambrian explosion is one of the central questions in animal evolution,” said senior author Dr. Degan Shu, a paleontologist at Northwest University.
“Yujingia glutenotunica helps connect primitive, tentacle-bearing animals living on the seafloor with modern echinoderms and hemichordates.”
“It offers a new way to understand the profound transition from bilateral symmetry to the fivefold radial body plan of echinoderms, and adds an important fossil to a crucial gap in the early animal tree of life.”
The discovery is reported in a paper published in the October 19 issue of the journal Current Biology.
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Kaiyue He et al. 2026. Cambrian fossils illuminate the ambulacrarian common ancestor. Current Biology 36 (20): 5180-5190; doi: 10.1016/j.cub.2026.08.053
