New Discoveries Cast Doubt on Post-Cambrian Mass Extinction

by Sean Hinds, age 16

Discovered in 1909, the Burgess Shale Outcrops in the Canadian Rockies display a menagerie of prehistoric marine fossils. These rock formations offer scientists a glimpse at the incredible expansion of complex multi-cellular life that occurred 550 million years ago, during what is known as the Cambrian Era.

The abundance of fossils present in the Burgess Shale demonstrate the diversification and evolution of the Cambrian period. But until recently, one aspect of the Burgess Shale discoveries has puzzled scientists. The Cambrian life forms whose fossils are found there seem to have disappeared very abruptly. It is as though there was a mass extinction.

An international team of scientists recently made a new discovery that confronts this mystery. Near the city of Zagora in Morocco, researchers discovered around 1500 fossils. These fossils, mollusks, sponges, trilobites, worms, and others, are estimated to be around 480 million years old.

These creatures appear to have been similar to those of the Cambrian fossils found at the 550 million year old Burgess Shale site, and at other sites in China and Greenland. But most of these specimens are not known to have lived during the later Middle-Cambrian period; however, these findings prove that they did.

The fossil bed near Zagora was once a muddy ocean floor, where low oxygen levels prevented decomposition, and chemical reactions turned the animals into fossils. This discovery shows that the Cambrian life forms did not die out, they were simply poorly preserved. It took the ideal conditions of the Zagora site to fossilize these soft-bodied animals, as they lacked the bones and teeth that are more easily fossilized.

Charles Darwin once compared our fossil record to a book missing many pages and many words. Well-preserved fossils are hard to come by, and the ones we’ve discovered so far by no means paint a clear and complete picture of Earth’s history. It is especially surprising to find soft-bodied life forms so well-preserved. The discovery in Morocco makes it seem less likely that there was a great extinction at the end of the Cambrian Era. Rather, it now seems likely that these multi-cellular organisms lived on, although the fossil record is sparse.

These fossils may also shed light on the early stages of the Great Ordovician Biodiversification event, which, among other things, included the appearance of fish. This would have happened about 400 million years ago.

The discovery also makes it seem likely that there are other discoveries out there waiting to improve our so-far incomplete view of the fossil record.

[Sources: The New York Times; 100 Great Archaeological Discoveries]