Paleontologists（古生物学者） have discovered a rich array of exceptionally preserved（保藏的，腌制的） fossils of marine¹ animals that lived between 480 million and 472 million years ago, during the early part of a period known as the Ordovician（奥陶纪） . The specimens² are the oldest yet discovered soft-bodied fossils from the Ordovician, a period marked by intense biodiversification. The findings, which appear in the May 13 issue of the journal Nature, greatly expand our understanding of the sea creatures and ecosystems⁴ that existed at a crucial point in evolutionary⁵ history, when most of the animal life on the planet was found in the oceans. The team—led by Peter Van Roy, a Yale postdoctoral associate, and Derek Briggs, the Frederick William Beinecke Professor of Geology & Geophysics and director of the Yale Peabody Museum of Natural History—uncovered more than 1,500 fossils of soft-bodied marine animals（海洋动物） in newly discovered sites in southeastern Morocco during a field expedition last year. Many are complete fossils, and include sponges（海绵） , annelid（环节动物） worms, mollusks（软体动物） and horseshoe crabs—in particular, a species similar to today's horseshoe crab⁶, which appeared some 30 million years earlier than previously⁷ known.

The Cambrian period, known for the "Cambrian Explosion" that saw the sudden appearance of all the major animal groups and the establishment of complex ecosystems, was followed by the "Great Ordovician Biodiversification Event," when the number of marine animal genera（种，类） increased exponentially over a period of 25 million years.

Because hard shells fossilize and are preserved more readily than soft tissue, scientists had an incomplete and biased⁸（有偏见的） view of the marine life that existed during the Ordovician period until now.

"The early Ordovician was a critical moment when massive diversification³（多样化，变化） takes off, but we were only seeing a small piece of the picture that was based almost exclusively（专有地，排外地） on the shelly fossil record," Briggs said. "Normal faunas⁹（动物群） are dominated by the soft-bodied organisms we knew were missing, so these exceptionally well-preserved fossils have filled in much of the missing picture."

The site in Morocco where the fossils were discovered was conducive¹⁰ to preserving even the soft tissues of the creatures that lived in its waters so long ago, thanks to generally calm waters, occasional（偶然的，临时的） rapid burial that protected the animals from scavengers（清道夫，食腐动物） , and favorable chemical conditions within the sediment¹¹ that allowed for the rapid mineralization of soft tissue as it decayed（腐烂，衰退） .

In addition to providing a more complete understanding of marine life at that time, the team's discovery upends a long-held belief that so-called Burgess Shale-type faunas, which are typical for the Early to Middle Cambrian, disappeared at the end of the Middle Cambrian epoch¹², some 499 million years ago.

"There was an anomaly（不规则，异常） in the fossil record. Most of these animals just seemed to disappear at the end of the Middle Cambrian," said Van Roy, first author of the paper.

The team found that these Burgess Shale-type species survived well into the Ordovician period, which would have had a major impact on those ecosystems and their evolution, Van Roy said.

The team expects to find even more fossils representing other species during future planned expeditions in Morocco. "We're only scratching the surface," Van Roy said. "I'm certain there will be more spectacular（壮观的，惊人的） fossils coming out of this site in the near future."