A snapshot of an alien ocean
High in the Canadian Rocky Mountains of British Columbia lies a fossil deposit that records a bizarre chapter in the history of life. The Burgess Shale contains the 508-million-year-old remains of an entire marine ecosystem. It is important for the extraordinary preservation of its fossils. They were created when an underwater mudslide swept up a community of sea creatures and buried them in fine-grained sediment low in oxygen. This process prevented decay and scavenging, preserving soft tissues like gills, digestive tracts, and eyes in flattened carbon films.
The site was discovered on August 31, 1909, by paleontologist Charles Doolittle Walcott, then Secretary of the Smithsonian Institution. Between 1910 and 1924, Walcott excavated over 65,000 specimens from the area now known as the Walcott Quarry. This collection showed the Cambrian explosion, a period when most major animal body plans appeared with startling rapidity. The fossils reveal a world of evolutionary experiments, where many strange forms existed before vanishing completely.
The Cambrian menagerie
The creatures of the Burgess Shale are famous for their unusual anatomy. Opabinia regalis was a soft-bodied animal about 7 centimeters long with five eyes on stalks and a long, flexible proboscis ending in a grasping claw. It likely used this appendage to hunt for small, soft prey on the seafloor. When a reconstruction was first presented to a group of paleontologists in 1975, they laughed, believing it to be a joke.
Another animal, Hallucigenia, earned its name from its dreamlike appearance. Early reconstructions depicted it walking on seven pairs of sharp spines. Later fossil discoveries showed that the spines were a defense on its back, and it actually walked on pairs of soft, tentacle-like legs. The apex predator of this ecosystem was Anomalocaris canadensis, which translates to "abnormal shrimp." Reaching lengths up to 38 cm, it was a giant for its time. It propelled itself with side flaps and hunted with two large, spiny frontal appendages and a circular mouth made of sharp plates.
The shale also contains a distant relative of humans. Pikaia gracilens, a creature about 38 mm long, shows features of a primitive chordate, the group that includes all vertebrates. It had a flexible rod called a notochord and blocks of muscle tissue, both critical features in the evolution of backboned animals.