An Ediacaran pre-placozoan alternative to the pre-sponge route towards the Cambrian explosion of animal life: a comment on Cavalier-Smith 2017.

Abstract

The evolution of the Metazoa is one of the most important and consequential events having taken place in our biosphere. Although Cavalier-Smith [1] has propounded a likely means by which animals evolved through an archetypal pre-sponge with choanocytes, an alternative route via immotile, non-suspension-feeding Ediacaran diploblastic organisms with a pre-placozoan grade of organization such as the Rangeomorpha [2] should also be considered (figure 1). Pre-placozoans differ from pre-sponges by their lack of basal pinacocytes and suspension-feeding capabilities, and by a feeding mode that relies on establishing symbiosis with, or directly phagocytosing chemoautotrophic bacteria [2]. Here we discuss how pre-sponges and pre-placozoans could have coexisted during early stages of metazoan evolution, resulting from planktonic, multicellular craspedid-like stem choanoflagellates settling on hard or soft substrates respectively. The conceptual approach to exploring the evolution of metazoans is corroborated by Ediacaran fossil evidence [2] and milestones in the fossil record that support the pre-placozoan route to animal diversification. Figure 1. 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. Given the comparative ease with which multicellular organisms can evolve [1], and the variety of benthic substrates available for colonization, all routes to the evolution of the Metazoa should be considered. The pre-sponges described by Cavalier-Smith evolved on hard substrates, and consisted of two cell types (flagellated choanocytes that retained their suspension-feeding function when they became benthic, and basal pinacocytes that developed as an adaptation for attachment to hard substrates, but which are not involved in feeding) forming a simple …