Burgess shale
Burgess shale fossils
The Burgess Shale Lagerstätte is found in an area of the Canadian Rocky Mountains known as the Burgess Pass, and is located in British Columbia's Yoho National Park. Here we find fossils of creatures originating from the Cambrian explosion, an evolutionary burst of animal origins dating 545 to 525 million years ago. During this period, there was no life on land. The land underwent weathering and was subject to erosion. Thus arose mudslides, that periodically rolled into the seas and buried marine organisms. At the Burgess locality, because of such mudslides, an entire community of life slid into an undersea ravine at once and was deposited in a deep-water basin several hundred meters deep. The circumstances down there were very suitable for fossilization of soft tissues. It was cold, oxygen was not available, so micro-organisms did not have a chance to break down the soft parts. Over 60,000 unique fossils have been found. Most of them are arthropods, but many other fossils are found, such as worms, crinoids, sea cucumbers, and chordates. One of them deserves extra attention, and that is Pikaia gracilens.
Pikaia gracilens, Walcott, 1911
One of the most interesting of the multitude animal fossils found in the famous Burgess Shale is Pikaia. In detail, the fossils of Pikaia compressed within the Burgess Shale vaguely show the essential prerequisites for vertebrates such as traces of an elongate notochord, dorsal nerve cord and blocks of muscles down either side of the body. So this species must be closely related to the ancestor of all chordates. [Based on a recent redescription by Conway-Morris and Caron, 2011, it is not quite a chordate as it lacks a post-anal tail [or urostyle], and its notochord (if that is what it is) is rudimentary.]
