By Nicola Jones
Some of the unusual animals that lived in the sea 500 million years ago thrived tens of millions of years later than previously known, a treasure trove of fossils in Morocco has revealed. The fossils prove that the famously bizarre creatures of the Cambrian (542 million to 488 million years ago) didn't die out at the end of that period--something that fossil hunters had suspected, but could not back up with evidence until now.
The batch of more than 1,500 specimens, reported in the May 13 issue of Nature, sheds light on the Early Ordovician period, 488 million to 471 million years ago, at the start of a huge expansion in the diversity of living things. In most Ordovician fossils, only the hard parts of animals, such as shells, are preserved. "We're missing a huge chunk of the data," says Derek Briggs of Yale University in Connecticut, a co-author on the paper. The Moroccan fossils, which show soft-bodied animals, fill in some of the gaps.
The team has catalogued at least 50 types of soft-bodied animals, including a mix of creatures previously seen only in earlier or later rocks. At a species level they're all brand new, says co-author Peter Van Roy, also at Yale. But one step up the family tree, about two-thirds of them are the same as found in the earlier Cambrian.
The fossils were found at 40 sites in the Draa Valley, in the desert region of southern Morocco. The spread of locations means that the finds give a panorama of this crucial period in evolution, says paleontologist Graham Young of the Manitoba Museum in Winnipeg, Canada, who also works on Ordovician fossils. "It's like getting a photo album rather than just a snapshot."
The strange life of the Cambrian was first exposed in the Burgess Shale, an outcrop of the Rocky Mountains in British Columbia, Canada. This revealed more than a hundred unusual creatures, including halkieriids, which look like slugs wearing chain mail; Hallucigenia, which resemble long-legged centipedes with sea-urchin spines; and Opabina, which have five eyes and a long nose with a claw on the end.
More fossils revealing soft-body details have been found from the Early and Middle Cambrian, particularly in China. But until now, few had been discovered from more recent time periods.
The fossilization of soft-bodied parts requires rare conditions: for example, a landslide of silt that shuts out scavengers and oxygen-fed bacteria. Up till now, the only Ordovician fossils like this have come from "odd" locations, such as lagoons, which aren't necessarily representative of life at the time. As a result, no one knew how long the animals of the Cambrian survived.
The Moroccan trove is the first to show soft-bodied Cambrian beasties, such as halkieriids, living in the open ocean of the Ordovician.
"This puts a nail in the coffin of the suggestion that the Burgess things were a grand experiment that didn't last very long," says Young.
The Moroccan fossils also push back the earliest signs of horseshoe crabs, which survive to this day, by 30 million years. And they contain the first recorded signs of cheloniellids--extinct arthropods that look a little like woodlice and were previously known from the Devonian period, 416 million to 360 million years ago.
The Moroccan desert is strewn with fossils, and many farmers and goatherders make extra money from collecting them. "You'll find fossils from Morocco in every rock shop around the world," says Briggs.
The newly discovered fossils were found by a local collector, Mohammed Ben Said Ben Moula, whom Van Roy met during his PhD. When Van Roy saw the first soft-bodied fossil found by Ben Moula, he says, "I immediately knew it was something special."
Ben Moula, a professional fossil collector who lives in a small village, now works in partnership with Van Roy. Although Ben Moula has not been formally educated, says Van Roy, "he's an incredibly intelligent guy with a good eye for fossils. Without him this project wouldn't exist."
There is now much more fossil collection and analysis to do. "The detailed work on the individual fossils will be the part we'll really look forward to," says Young. Briggs says Morocco has the potential to do for the Early Ordovician what the Burgess Shale did for the Cambrian. In a few years, he says, the Moroccan finds might also have something important to say about the evolution of life.