Ever since their discovery in 1909, the spectacular Burgess Shale outcrops in the Canadian Rockies have presented scientists with a cornucopia of evidence for the “explosion” of complex, multicellular life beginning some 550 million years ago.

The fossils, all new to science, were at first seen as little more than amazing curiosities from a time when life, except for bacteria and algae, was confined to the sea — and what is now Canada was just south of the Equator. In the last half century, however, paleontologists recognized that the Burgess Shale exemplified the radiation of diverse life forms unlike anything in earlier time. Here was evolution in action, organisms over time responding to changing fortunes through natural experimentation in new body forms and different ecological niches.

But the fossil record then goes dark: the Cambrian-period innovations in life appeared to have few clear descendants. Many scientists thought that the likely explanation for this mysterious disappearance was that a major extinction had wiped out much of the distinctive Cambrian life. It seemed that the complex organisms emerging in the Cambrian had come to an abrupt demise, disappearing with few traces in the later fossil record.

Image SET IN STONE Among the 1,500 fossils found near the Atlas Mountains in Morocco by an international team of scientists was that of an extinct member of the arthropod family, above and in the drawing below, from about 480 million years ago. Its closest living relatives include horseshoe crabs, spiders and scorpions. Credit... Peter Van Roy

Not everyone was convinced, however, and now a trove of 480-million-year-old fossils in Morocco appears to strike a blow to the idea of a major extinction. The international team of scientists who discovered the 1,500 fossils said their find shows that the dark stretch in the fossil record more probably reflects an absence of preservation of fossils over the previous 25 million years.