Not to be confused with jellyfish, ctenophores are soft, ethereal marine predators that propel themselves through water using their combs of hair-like cilia. For a long time, we thought that ancient comb jellies were also entirely soft-bodied. But according to a new Science Advances study, half-billion-year-old comb jellies sported hard, skeletonized structures that protected their gelatinous parts.

Researchers have long debated where members of the phylum Ctenophora should be positioned in the evolutionary family tree of animals. If ancient comb jellies were indeed soft and squishy like the 150 or so species we have today, it would be exceedingly hard to find them in the fossil record. However, 37 specimens were unearthed in lower Cambrian mudstones near the city of Kunming in the Yunnan Province of southwestern China. They had been deposited in a shallow marine environment.

A team led by Qiang Ou from China University of Geosciences analyzed six of the comb jelly fossils that came from the 520-million-year-old Chengjiang biota. These fossils belonged to three known species and three new ones. You can see the never-before-seen species above: from left to right, Gemmactena actinala, Batofasciculus ramificans, and Thaumactena ensis. 

These early Cambrian comb jellies were tentacle-less, and they had skeletons characterized by eight rigid, angular plates and eight struts, or spoke-like structures that radiated outward (colored white in the image above). The former surrounded their organs, while the latter surrounded the main body’s soft lobes. These hardened parts may have been made of chitin, like crab exoskeletons.

The team thinks that the skeletal framework – the armor-like plates and the spoke-like struts – may have provided support for soft tissues and defense against predators and hostile environments. 

The Cambrian Explosion was a wondrous time of diversity and evolutionary innovation. These findings suggest that marine organisms during this period maybe have been engaged in an evolutionary arms race to outcompete one another. These armored, partially skeletonized comb jellies, however, may have been part of an "unsuccessful evolutionary experiment," Ou tells The Verge. "This major branch might have struggled a strenuous life." They died out and likely left no descendants behind.

^{Thaumactena ensis (A to D), Galeactena hemispherica (E to I), and Batofasciculus ramificans (J to N). Q. Ou et al./2015 Science Advances.}