Burgess Shale Fossils and their Importance

The Burgess Shale fossils have been called the world’s most significant fossil discovery, mainly because of their great age, their diversity and the incredible detail of their preservation. What makes them different from other fossil sites is that a series of geological factors resulted in these soft-bodied animals (mostly arthropods) having not only the hard parts of their bodies – bones, shells, teeth – but also the muscles, gills, digestive systems and other soft body parts preserved allowing scientists an unprecedented opportunity to observe not only these details but also the way the creatures lived and interacted.

The Burgess Shale fossils merit special interest for several reasons:

  • Their age - from the Cambrian period, 505 million years ago, shortly after an astonishing burst of biodiversity occurred in the ancient oceans.
  • Their exquisite preservation - amazingly fine details of the structure of the animals are seen in the fossils, which tell paleontologists much more about what the ancient animals looked like, and how they lived.
  • The fossils reveal important clues to the nature of evolution - all of the major types of animals (phyla) known today are represented in the Burgess Shale, plus others that cannot be placed in our modern classification system.

The story of the Burgess Shale begins more than 500 million years ago. Shortly before the Burgess Shale animals lived, there was an evolutionary “Big-Bang” called the Cambrian Explosion. After nearly 2 billion years of only simple, unicellular life-forms on earth, a full spectrum of complex animal forms appeared in the oceans in just 10-20 million years, a geologic blink-of-an-eye! The reason why is still of great debate and interest among scientists.

The Burgess Shale animals lived on or near the vicinity of a massive submarine carbonate wall, referred to as the Cathedral Escarpment which bounded a shallow warm carbonate seaway extending eastwards in to what is now Alberta. At the time the Burgess Shale fossils were formed, during the Cambrian period (about 505 - 510 million years ago), the North American continent straddled the Equator, and the surface of the land was completely devoid of complex life forms; that is, land plants and animals did not yet exist. Life on earth consisted only of marine plants and marine invertebrate animals.

Burgess Shale Cathedral Escarpment

Periodic mudslides would overcome the animals, transporting them in a turbulent cloud of mud to the base of the reef, where they were buried and died. Over the course of many millions of years, the animals were buried by sediments, and their delicate remains turned into fossils. The fossils were at one time covered by up to 10 kilometres of overlying rock, and would have been subjected to great heat and pressure at those depths. Beginning about 175 million years ago, mountain-building forces bulldozed and transported the fossils from their ocean burial-ground many kilometres eastward on faults, to their current position high on a mountain ridge in Yoho National Park.

The Cambrian period (545 – 490 million year ago) is unique in a geological sense. Although some scientists have theorized that multi-cellular life forms could have evolved as much as a billion years ago, the first multi-cellular life forms actually found in the fossil record lived about 600 million years ago. The fossil record, moreover, shows a significant diversification of life forms at the start of the Cambrian Period, beginning about 545 million years ago. Cambrian fossils are the first to show many of the features common to countless species today: heads, eyes, mouths and legs. So striking was the record of evolution during the Cambrian, in fact, that it has been nicknamed the Cambrian Explosion, and Earth's history prior to this time (about 85 percent of the planet's estimated life span) is often described simply as the Precambrian.

Researchers say that the ancestors of virtually all life forms on Earth, existing and extinct, can be found in the Burgess Shale, and therefore are a key to underst