Fossils widen range of biological burst.

Nicholas J. Butterfield's discovery started with a mistake. The paleontologist had planned on studying 1-billion-year-old rocks drilled recently from Canada's Northwest Territories. But when he sent away for the samples, Butterfield wrote down the wrong order number.

While analyzing the rocks under a high-powered microscope, Butterfield was surprised to find a tiny scale from a Wiwaxia, an odd creature from the Cambrian period (545 million to 510 million years ago) that he had previously studied.

"It happened utterly by chance, if only because I'm one of the few people in the world who would recognize a Wiwaxia sclerite when it floated by," says Butterfield, a researcher at the University of Cambridge in England.

From that initial foul-up, Butterfield found a range of exquisitely preserved fossils that filled the seas soon after the so-called Cambrian explosion -- when the first complex animals evolved. He reports his discovery in the June 9 NATURE.

Scientists had previously found well-preserved fossils from this period in only a handful of sites worldwide, most notably the Burgess Shale in southern Canada. Butterfield's happenstance now opens the door to new, high-quality finds.

"The type of preservation that we know from the Burgess Shale and thought was relatively limited might be much more widespread than we previously presumed," comments Stefan Bengtson of Uppsala University in Sweden.

The rocks Butterfield studied come from flat-lying deposits near Great Bear Lake that escaped the tremendous heat and pressures generated when mountains form. As a result, the fossils entombed there have held up remarkably well over the past half-billion years.

After dissolving away the stone with hydrofluoric acid, Butterfield found an array of different animals, including parts of small crustaceans, which would have measured about 1 centimeter long. Like many modern crustaceans, these Cambrian examples sported tiny bristles for filtering out pieces of plant material in the ocean.

In the past, researchers have suggested that Cambrian animals were jacks-of-all-trades, lacking the specialized features seen among animals today. But the new discovery refutes that idea, Butterfield says. Apparently, crustaceans evolved the apparatus for filter feeding as far back as the early Cambrian.

In today's oceans, filter-feeding crustaceans form a critical link in the food chain by consuming tiny yet abundant phytoplankton. According to Butterfield, the newly found Cambrian crustanceans played the same pivotal role, serving as prey for larger ocean creatures.

These animals may also help explain what sparked the Cambrian explosion. Perhaps the development of filter feeding, he speculates, lit the fuse by expanding the food chain. Butterfield hopes to address such issues this month when he prospects near Great Bear Lake for more of the Cambrian fossils.