Science1 publisher2 min readPublished
A 518-million-year-old Chinese fossil puts feeding tentacles on the ancestor of starfish
Yujingia glutenotunica preserves a crown of feeding tentacles in soft tissue. A phylogenetic analysis places it near the ambulacrarian ancestor, reassigning how that ancestor is thought to have eaten.
The Scientist · Science desk
What happened
- Several soft-bodied fossils from southwestern China, dated to 518 million years ago, have been named Yujingia glutenotunica by an international team that includes researchers at the University of Cambridge.
- Each animal carried featherlike feeding tentacles that captured organic particles from the water, plus a second set of appendages thought to have anchored it temporarily to the seabed.
- The phylogenetic analysis places Yujingia closer to the last common ancestor of living ambulacrarians than previously described fossils, linking early Cambrian tentacled animals with hemichordates and echinoderms.
- The specimens preserve body, gut and appendages. They are rare examples of ambulacrarians recorded without a mineralized skeleton.
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Why it matters
- constraint The tentacle-feeding ancestor is an inference from tree position, and for a 518-million-year-old animal that position is built from scored anatomy, so a revised matrix or a different model can revise the conclusion.
- capability Feeding apparatus can only be scored where soft tissue survived, so the handful of Cambrian sites with that preservation decide which ancestral questions are testable at all.
- precedent With tentacles now the better-supported ancestral state, the pharyngeal filtering of acorn worms becomes the condition that needs explaining, and that is a claim about hemichordates other fossils will be measured against.
Ancestral feeding modes are never observed, only inferred, and the claim here rests on where Yujingia falls relative to living ambulacrarians. Giovanni Mussini of Cambridge's Department of Earth Sciences and Kaiyue He of Northwest University in China built that position from a matrix of 505 morphological characters scored across 100 living and extinct taxa, analysed by Bayesian reconstruction [11]. The results are reported in Current Biology [4]. The account of the work published by phys.org does not report branch support values [21].
A long-standing hypothesis had the ambulacrarian ancestor as a tentacle-free worm that filtered food through its pharynx, resembling a modern acorn worm [10]. "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," Mussini said [8]. On the new material he 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" [9].
Most Cambrian animal fossils are of hard-shelled creatures, and only a handful of sites, among them the Maotianshan Shales of Yunnan Province where Yujingia was found, allow softer body parts to be preserved before they decay [5]. Preservation is why the question stayed open this long. Ambulacraria was first classified in the 1880s, roughly 140 years ago, and fossil evidence of the group's origins has remained scarce [15][19].
The team also links the fossil to a larger question, why the starfish lineage and the chordates, the phylum that includes humans, evolved such different body plans [16]. The reported evidence goes a shorter distance: it changes the feeding apparatus assigned to the ambulacrarian ancestor, and leaves the branch point between the two lineages where it was. Mussini said the fivefold radial symmetry of starfish and their relatives, and the wormlike bodies of acorn worms and theirs, "have been a bit of a mystery until now" [17]. If tentacle feeding is ancestral for the group, pharyngeal filtering in acorn worms becomes the derived condition, and other fossils will have to test that.
The specimens are small, 8 to 22 millimetres long, the largest nearly three times the length of the smallest [6][18]. He said: "The animal has an elongated, bottlelike form, with tentacles spreading above a rounded lower body" [13]. She said the outline resembles the yujingping ritual vase of traditional Chinese culture, which gave the genus its name, and that "All the specimens clearly preserve a pale, original membrane enclosing the visceral mass" [14].
What to watch
- Independent reanalyses of the 505-character matrix, and whether Yujingia keeps its position under different scoring or a different model.
- More specimens from the Maotianshan Shales that could test whether the second set of appendages anchored the animal as the authors propose.
- Rescoring of other Cambrian tentacle-bearing fossils against Yujingia. Their positions in the ambulacrarian tree would move too.