China opens a window on life before the dinosaurs.

Science News describes a dizzying array of unknown animals from “the Cambrian explosion,” when life took a sudden turn for the weird and wonderful. The 518 million-year-old site on the banks of China’s Danshui River has fossils of jellyfish, for goodness’ sake:

So far, researchers led by paleontologist Dongjing Fu of Northwest University in Xian, China, have collected 4,351 specimens at the new site, representing 101 different taxa, or groups of organisms. Of those taxa, about 53 percent have never before been observed, Fu and her colleagues report in the March 22 Science….

The new fossil trove, called the Qingjiang biota, was first spotted in 2007, says coauthor Xingliang Zhang, a paleontologist also at Northwest University. “I have been working on Burgess Shale–type fossils for many years, and know what kind of rocks preserve [them],” Zhang says.

During a field expedition that year, he and his students were investigating a different rock layer dating to the Cambrian. At lunchtime, he says, he happened to sit on the next lower layer of rocks as it was being lapped by the river’s water — and immediately recognized that the fine clay layer was the perfect preservation setting for fossils. “We split the clay stone and I found a Leanchoilia [a kind of segmented arthropod] quickly.” Many more discoveries soon followed.

The site is remarkable for the quality of the preservation of the animals, says Allison Daley, a paleontologist at the University of Lausanne in Switzerland who was not involved in the new study but wrote a commentary that accompanies it in Science. “There was very little metamorphism or weathering effect, which does affect some other [Cambrian fossil] sites, like Burgess or Chengjiang. We see almost pristine fossils at this site.” She mentions one startlingly clear image of a jellyfish. “I mean, if you were going to smack a jellyfish on a rock, that’s how it would look.”

With so many ctenophore fossils preserved so well, Daley says, studying their shapes may help to answer a long-standing debate: Whether comb jellies or sponges are the most primitive animal on their family tree. Scientists have thought that sponges appear closer to the base of the tree, based on their very simple shapes. But some molecular analyses have hinted that comb jellies may be at the base of the tree.

“It’s hard to disentangle the exact relationships of these [creatures],” Daley says. “These early branching groups diverged from each other such a long time ago…. So getting more info on [them] at this new site, where the preservation is really amazing, is really going to fill a gap.”