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Potential scenario for metazoan evolution, representing the pre-placozoan and pre-sponge routes to the diversification of major metazoan groups. Pre-placozoans and pre-sponges develop following benthic colonization by a planktonic, swimming ball of cells on hard (pre-sponge) or soft (pre-placozoan) substrates. Relationships between non-poriferan metazoans, proposed to be derived from a pre-placozoan ancestor, are purposefully unresolved in this tree.
