Although the Cambrian Explosion is
largely associated with animals having hard shells, the soft-bodied
biota also diversified during this period. The Middle Cambrian Burgess
Shale fauna, discovered by Charles Walcott in 1909, in British Columbia,
Canada, provides a rare glimpse of Cambrian soft-bodied animals.
This fauna is especially important because the fossil record is
biased toward organisms having hard parts. Another famous soft-bodied
occurrence is the Chengjiang Fauna, of Early Cambrian age, in Yunnan
Province, China. Although well-preserved soft-bodied fossils have
been found throughout the fossil record, they are rare occurrences
and are preserved only under very unusual environmental conditions.
The Burgess Shale was deposited in the ocean near an underwater
algal reef shelf. Occasional undersea landslides buried the animals
living there, and the fine mud prevented decay so that soft parts
were preserved. As a result, this fossil fauna records a host of
extinct soft-bodied organisms, many of which have no living counterparts.
Additionally, the Burgess Shale contains many shelled animals, some
of which preserve parts not normally seen, such as the legs and
antennae of trilobites and the setae of brachiopods; the exceptional
preservation allows paleontologists to understand the structure
and biology of the animals better. Some of the soft body plans of
Cambrian animals did not survive beyond that period. To our eyes,
some of these body plans look bizarre and it has taken a long time
for paleontologists to understand them. For example, the first reconstructions
of the onycophoran Hallucigenia were upside-down.
The sclerite-bearing Wiwaxia, the arthropod-like
predator Anomalocaris, and the five-eyed
Opabinia are all strikingly different from