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Ancient comb jellies had skeletons, but they still lost the arms race

They weren't just a large mass of jelly

Dr. Qiang Ou and his colleagues

Despite going extinct over 400 million years ago, ancient comb jellies are still blowing scientists away. Long thought of as entirely soft-bodied creatures — like their modern counterparts — these predatory marine animals may have had hard, skeleton-like parts, according to a study published in Science Advances today. The finding uproots scientists' current understanding of comb jelly evolution — and opens a window into an ancient battle for survival.

Modern ctenophores are a lot "squishier"

Modern comb jellies — called "ctenophores," as per their scientific name — are already pretty weird looking. Their small bodies are basically just a mass of jelly surrounded by internal and external layers of cells. Many have tentacles, and they use their "combs," or cilia, to swim around. And despite their "squishiness," almost all ctenophores are predators that eat small crustaceans, larvae, and rotifers. But out of the 100 to 150 species that researchers have identified thus far, none have skeleton-like features. Comb jellies, as their name indicates, are soft — which is why this new discovery is so intriguing.

Modern comb jelly. Credit: Marsh Youngbluth/Wikimedia Commons

In the study, the researchers looked at six new fossils from the Cambrian Period, which ended about 485 million years ago. They found that the comb jellies had eight rigid plates surrounding their apical sensory organ, which they used as a gravitational receptor. They also lacked tentacles, sporting eight radiating spoke-like structures instead that probably supported the soft-bodied flaps of the main portion of their bodies.

"One of the species even had robust spines."

The spokes and plates may not have been used exclusively for mechanical support, however. The researchers think these hard parts might have served as a defense mechanism against predators, or as protection from difficult environmental conditions. "One of the species even had robust spines" that retained their structural integrity when separated at the joints, says Quiang Ou, a paleobiologist at the China University of Geosciences and a co-author of the study.

Ancient comb jelly fossils. Credit: Dr. Qiang Ou and his colleagues

These defense mechanisms didn't make it into modern comb jellies, however. This suggests that they may have been part of "unsuccessful evolutionary experiment" during the Cambrian explosion, Ou says. "This major [animal] branch might have struggled a strenuous life." And eventually, they went extinct.

Overall, the findings show that the earliest ctenophores were tentacle-less, despite DNA analyses that suggested that the opposite was true. But the study also hints at something larger. If ancient comb jellies had to develop these skeleton-like parts, it's likely that they faced a whole bunch of ecological challenges in their time — challenges that led them to develop a feature that wasn't conserved over evolutionary time. Ou calls this an "arms race" — one that ancient comb jellies apparently lost.