Ancient comb jellies might have had skeletons

Chinese fossils hint at rigid framework for filmy sea creatures

Comb jellies

SOFT LIFE  Comb jellies alive today, like this Mnemiopsis mccradyi common in the Gulf of Mexico, have soft bodies, but fossils hint that their ancient relatives had skeletons.

Gregory G. Dimijian/Science Source

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Comb jellies, just globes of shimmering film in today’s oceans, may have had rigid skeletons and hard plates millions of years ago.

Scientists analyzed fossils some 520 million years old from the Chengjiang site in China representing six species of comb jellies, or ctenophores. The fossils show signs of hard parts, such as rigid spokes and hardened plates, says Qiang Ou of China University of Geosciences in Beijing. He and his colleagues report the finding July 10 in Science Advances. Until now, biologists have thought of comb jellies as soft bodied, so the fossils reveal an “unexpected lost history,” Ou says.

“Exciting,” says paleontologist Stefan Bengtson of the Swedish Museum of Natural History in Stockholm. The notion of long-ago rigid parts “may help explain why these animals, now represented by forms with flimsy and rapidly degradable bodies, have a fossil record that is comparatively rich in the Ediacaran-Cambrian [periods] but quite depauperate after that.”

The skeletons weren’t necessarily bonelike and rich in minerals, says paper coauthor Shuhai Xiao, a geobiologist from Virginia Tech in Blacksburg. He suspects the hard parts were more like the exoskeletons of today’s insects, made of organic molecules.

Part of the evidence for a hardened framework comes from the shapes of the body parts preserved in the fossils. In the 37  specimens, the researchers don’t find the jellies’ flaps twisted or folded as they would expect from mere filmy structures. Instead, these parts lie flat, as they would if supported by rigid frames or if they were hard plates themselves.

What’s known about the genes of modern comb jellies makes the idea of a skeleton sound plausible, says Leonid Moroz, an evolutionary neuroscientist and geneticist at the University of Florida in St. Augustine. The jellies have genes for the calcium carbonate needed to make sensory structures called statoliths, as well as other genes for collagen. Using such genes to create rigid supports “is not a big stretch,” he says.

The extinct oddball comb jellies with skeletons fit with the idea that early life-forms may have been unusually diverse, Moroz says. In this interpretation of the story of life, the long-ago period when these comb jellies lived was a time of exploding novelty in shapes and lifestyles that has dwindled since. The number of species may have grown but the extent of bizarre differences within groups of organisms has shrunk.

FOSSIL COMB JELLIESFossils of several ancient comb jellies hint that these squishy creatures once had hard parts. The right-hand flap of one fossil (top row, left two images) lies flat, indicating that some rigid spoke kept it from crumpling. What might have been hardened, upright plates rise around the dark spot of another fossil (bottom row, left two images). The tall, skinny fossil (far right and adjacent lower image) appears to have radiating spikes. Ou et al/Science Advances 2015
JELLY STYLE Diagrams show basic forms (side views and slightly tilted) of four fossil comb jelly species. Brown indicates proposed rigid parts. Q. Ou and colleagues

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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