Over 500 million years ago, sea-dwelling invertebrates pioneered a new evolutionary experiment: skeletons. But while those durable, tube-like structures stood the test of time as fossils, the animals' soft bodies decayed and vanished, erasing all evidence of what these ancient animals may have looked like. Now, a recent re-examination of those ancient skeletal tubes has finally unveiled the identity of one of these mysterious organisms.
These early calcium-reenforced "skeleton" tubes date to a period known as the Cambrian explosion (541 million to 510 million years ago) and seem to have been an effective survival strategy, as they cropped up in multiple groups across a relatively short span of geologic time (about 50 million years). During this period, everything from the segmented ancestors of earthworms to the bizarre ancient relatives of tardigrades created tube-like protective structures.
However, tracing the evolutionary history of these early exoskeletons has proved tricky. "Soft tissues tend to decay away," Xiaoya Ma, an invertebrate paleontologist at Yunnan University in China and co-author of a study describing the findings, told Live Science. For this reason, identifying fossil Cambrian tubes has been a little like trying to guess the contents of an empty, unlabeled can based on the shape of the tin alone — most could just as easily have held chicken soup as creamed corn.
But scientists are shedding light on these enigmatic skeleton makers. In the new study, published Nov. 2 in the journal Proceedings of the Royal Society B, an international team of researchers described four incredibly well-preserved Cambrian specimens from China's Yunnan province. These 514 million-year-old fossils of the tube-dwelling creature Gangtoucunia aspera include soft tissue impressions left behind by the animals' bodies. By studying these impressions closely, the scientists determined that the tubes belonged to, of all things, an ancient skeleton-making jellyfish.
Soft-bodied invertebrates are hard to find in the fossil record, and jellyfish in particular are almost never preserved. "This fossil was a double whammy in terms of rarity," Luke Parry, a paleobiologist at the University of Oxford and co-author of the study, told Live Science in an email.
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Normally, when a marine organism dies, scavengers and bacteria make quick work of its soft tissues. But very occasionally, a wave of fine sediment covers the remains quickly enough to prevent aerobic bacteria from settling in. This is how the famous North American Burgess Shale fossil deposit formed, according to the Smithsonian National Museum of Natural History in Washington, D.C., and it is likely how the Yunnan site formed, as well.
The newfound fossils, which were discovered by lead study author Guangxu Zhang, Ma's graduate student at Yunnan University, were preserved in such detail that the paleontologists could even make out the animals' internal organs. The creatures' mouths were surrounded by a ring of tentacles, each measuring about 0.2 inch (5 millimeters) long. And they had a saclike gut with just one opening (unlike the separate mouth and anus that vertebrates are blessed with).
These characteristics led the team to conclude that G. aspera likely belonged to the phylum Cnidaria, which includes modern-day jellyfish, corals and sea anemones. It also laid to rest an older theory that the creature was an annelid worm, which is defined by its segmented body and gut with two openings.
G. aspera likely hung out in ancient oceans with one end of its tube anchored to other members of its species or to mobile creatures such as trilobites, retracting into its shell when predators swam by. It probably fed much like modern jellyfish polyps do, extending its stinging tentacles when prey was near.
Only the larvae of one jellyfish group, Scyphozoa, create exoskeletons today. Some other cnidarians, such as corals, retain their skeletons into adulthood. However, today's corals build their skeletons from calcium carbonate; in contrast, G. aspera crafted its tubes out of calcium phosphate, the same tough compound that makes up our tooth enamel and bones.
Why modern cnidarians switched from calcium phosphate to calcium carbonate exoskeletons remains a mystery. "One potential reason is that the environment before our current time was phosphorus rich," Ma said. But the answer could be found in cnidarian genetics as well. Ma and her team hope to answer this and other questions as their research continues. "Hopefully, we'll have more for everyone in the near future," she said.
