FEATURE ARTICLE
The Origin of Animal Body Plans
Recent fossil finds and new insights into animal development are providing fresh perspectives on the riddle of the explosion of animals during the Early Cambrian
Douglas Erwin, James Valentine, David Jablonski
The Cambrian Explosion
Although abrupt geologically, the divergence of the early animal
groups was somewhat more drawn out than paleontologists recognized
even a decade ago. Recent fieldwork in Siberia and Mongolia has
demonstrated that skeletal fossils gradually became more common and
diverse in the earliest part of the Cambrian (known as the Manykaian
Stage). At the same time, trace fossils increased in diversity and
abundance, including the first trace fossils that reflect the
presence of animals with limbs. However, it is in rocks of the later
Tommotian and Atdabanian stages of the Early Cambrian, between about
530 and 525 million years ago, that fossil assemblages first include
most of the basic body plans of living animals. This is the
"Cambrian explosion," with the first appearance of
mineralized skeletons of such phyla as the Mollusca, Brachiopoda
(lamp shells), Arthropoda and Echinodermata. Trace fossils exhibit a
dramatically expanded range of animal activities, suggesting that as
skeletonized forms diversified, soft-bodied groups expanded as well.
Furthermore, soft-bodied phyla, such as Annelida, Onycophora and
Priapulida, which do not have mineralized skeletons, also make their
appearance, thanks largely to a beautifully preserved soft-bodied
fauna from the Late Early Cambrian of Chengjiang, Yunnan Province,
China. A number of forms from now-extinct phyla occur in these beds.
The Middle Cambrian Burgess Shale fauna from British Columbia,
Canada preserves many soft-bodied fossils similar to those of the
Chengjiang fauna, indicating that these forms were widespread and
persisted for many millions of years. These faunas serve to
emphasize the spectacular morphologic breadth that was achieved so
early in animal history. This fossil record raises many questions as
to how new body plans evolve and just how rapidly such novel
evolutionary innovations may be produced. Answering these questions
requires information from the field of evolutionary developmental biology.
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