Secrets of the abyss: 505 million-yr-old jellyfish fossils may be oldest ever found

NEW YORK: Jellyfish have been floating through Earth’s oceans seemingly forever. But pinning down the exact origin of these squishy sea creatures, which are some of the earliest complex animals, is difficult. They rarely show up in the fossil record because jellyfish are 95 percent water and are prone to rapid decay.

“If you see a jellyfish outside of the water, a couple hours later it’s just a ball of goo,” said Jean-Bernard Caron, a paleontologist at the Royal Ontario Museum in Toronto. But Dr. Caron and other scientists recently described a cache of jellyfish fossils from the Cambrian period that found an improbable pathway to preservation. In a paper published on Wednesday in the journal Proceedings of the Royal Society B, the scientists posit that these 505-million-year-old animals are among the oldest swimming jellyfish known to science.

“These new fossils represent the most compelling evidence of Cambrian jellyfish to date,” said David Gold, a paleobiologist at the University of California, Davis, who was not involved in the new study.

The jellyfish specimens were found in the Burgess Shale, a fossil-rich site in the Canadian Rockies that provides a glimpse of life during Earth’s Cambrian explosion. Like other soft-bodied creatures found at the site, the gelatinous jellyfish are preserved in stunning detail. Most still possess upward of 90 finger-like tentacles, which stick out of the creature’s bell-shaped body like the strings at the end of a tassel rug. Some even retain their stomach contents and gonads.

Back in the 1990s, Royal Ontario Museum researchers unearthed more than 170 jellyfish fossils from the Raymond Quarry in British Columbia. When Dr. Caron and a Ph.D. student examined the specimens more recently, they realized the fossils represented a new species, which they named Burgessomedusa phasmiformis.

The species is part of a diverse group called medusozoans, which are thought to have originated at least 600 million years ago and are still swimming in the same seas we know today. But evidence of their rise is scarce. Most fossils from before the Cambrian period are either microscopic or little more than faint imprints, making it difficult to infer how these ancestral jellies lived. Over the past two decades, paleontologists have discovered several well-preserved jellyfish-like fossils from sites in Utah and China that are similar in ages to the Burgess Shale. However, the true identities of these creatures is still up for debate. In the new paper, Dr. Caron and his colleagues proposed that the fossils from Utah and China represent ancient ctenophores, or comb jellies, another group of gelatinous animals only distantly related to true jellyfish.

Not all researchers are sold by this reclassification. According to Bruce Lieberman, a paleontologist at the University of Kansas who studied the Utah fossils, the new paper lacks compelling evidence to connect the earlier fossils with comb jellies.