The Burgess Shale Fauna: Bizarre Animals That Changed Life

The Burgess Shale is a fossil deposit in the Canadian Rocky Mountains, discovered in 1909 by Charles Walcott. This site offers a window into life approximately 508 million years ago, during the Cambrian period. Its fame comes from the exceptional preservation of soft-bodied organisms, creatures that normally decay before becoming fossils. This unique record provides a detailed snapshot of a time of rapid evolutionary change.

The Cambrian Setting and Fossil Preservation

The Burgess Shale animals lived in a shallow, tropical sea at the base of a large underwater limestone cliff. Periodically, underwater mudflows would cascade down the cliff, sweeping away the creatures on the muddy seafloor. The mudslides buried the animals instantly in fine sediment devoid of oxygen.

This rapid, anoxic burial prevented both decay and scavenging. Because of this process, features not normally fossilized, such as internal organs, gills, and eyes, were preserved in stone. This level of detail provides a more complete picture of Cambrian life than skeletal remains alone.

Showcase of Bizarre Body Plans

Among the diverse collection of animals is Anomalocaris, which translates to “abnormal shrimp.” It was a formidable predator, reaching up to a meter in length, a giant for its time. It propelled itself through the water with flexible lobes and hunted using two large, spiny appendages at the front of its head. Its mouth was a circular arrangement of hard plates, likely used to crush the shells of trilobites.

Another inhabitant was Opabinia, a creature so strange its initial presentation at a scientific conference was met with laughter. It had five stalked eyes that provided a wide field of vision. A long, flexible proboscis extended from its head, ending in a claw-like structure used to grasp food and bring it to its mouth, which was located underneath its head.

One of the most perplexing fossils is Hallucigenia. Its initial reconstruction placed it walking on stilt-like spines with a row of tentacles on its back, a configuration that puzzled paleontologists for years. Later discoveries revealed this interpretation was upside down; the spines were for defense on its back, while the tentacles were actually pairs of legs for walking.

Wriggling along the muddy bottom was Wiwaxia, a slug-like animal that was well-defended. Its body was covered in small, scale-like plates called sclerites, with two rows of long, sharp spines running along its back. These spines would have made it a difficult meal for predators like Anomalocaris.

A small, ribbon-like animal, Pikaia swam by undulating its body from side to side. It is noteworthy because it possessed a primitive version of a notochord, a flexible rod that provides support. This feature is a hallmark of the phylum Chordata, making it a distant relative of all vertebrates.

Ancestors and Evolutionary Dead Ends

The fauna of the Burgess Shale represents a period of intense evolutionary experimentation. The vast array of body plans shows that while some designs led to the major animal groups we see today, many others vanished completely.

In contrast, many of the more bizarre animals of the Burgess Shale represent evolutionary paths that ultimately came to an end. Creatures like Opabinia and Anomalocaris belong to groups that have no living descendants. Their unique body plans, with features like five eyes or a circular jaw, were experiments that did not persist beyond the Cambrian period. Their extinction illustrates the contingent nature of evolution, where many early branches of life were pruned.

Redefining the Tree of Life

The discovery of the Burgess Shale fossils changed how scientists understood early animal evolution. Before these findings, the Cambrian Explosion was thought to involve the gradual appearance of simple ancestors of modern animal groups. The fossil record was dominated by hard shells and skeletons of familiar lineages. Soft-bodied animals were largely absent from this picture, leading to a skewed perception of Cambrian biodiversity.

The Burgess Shale shattered this view by revealing a surprising variety of complex and fully formed body plans that appeared suddenly in the fossil record. This showed that the base of the animal tree of life was not a simple, single trunk, but rather a dense, bushy structure with numerous, distinct branches.

This realization forced a major revision of evolutionary theory. It demonstrated that the history of life was not a straightforward march of progress from simple to complex. Instead, it was a history of explosive diversification followed by mass extinction, which winnowed down the range of body plans. The fossils from this site continue to inform our understanding of the origins of animal life.