In 1909, Charles Walcott Discovered Fossils in the Burgess Shale

In 1909, Charles Walcott discovered fossils in the Burgess Shale of the Canadian Rockies. The area was formed by an underwater mudslide that provided an oxygen free environment, so decomposition was minimal. Why were these fossils so unique?

A) They were from the Cambrian period.
B) There were both vertebrate and invertebrate (soft bodied) animals fossilized.
C) Only invertebrate (soft bodied) animals were found and no vertebrate animals or plants.
D) Before this discovery, it was thought that mudslides could not occur underwater.
E) The first bird fossil, Archaeopteryx, was discovered in this area.