Early animal evolution: a morphologist's view

Abstract

Two hypotheses for the early radiation of the metazoans are vividly discussed in recent phylogenomic studies, the 'Porifera-first' hypothesis, which places the poriferans as the sister group of all other metazoans, and the 'Ctenophora-first' hypothesis, which places the ctenophores as the sister group to all other metazoans. It has been suggested that an analysis of morphological characters (including specific molecules) could throw additional light on the controversy, and this is the aim of this paper. Both hypotheses imply independent evolution of nervous systems in Planulozoa and Ctenophora. The Porifera-first hypothesis implies no homoplasies or losses of major characters. The Ctenophora-first hypothesis shows no important synapomorphies of Porifera, Planulozoa and Placozoa. It implies either independent evolution, in Planulozoa and Ctenophora, of a new digestive system with a gut with extracellular digestion, which enables feeding on larger organisms, or the subsequent loss of this new gut in the Poriferans (and the re-evolution of the collar complex). The major losses implied in the Ctenophora-first theory show absolutely no adaptational advantages. Thus, morphology gives very strong support for the Porifera-first hypothesis.

Keywords: animalia; large molecules; morphology; ultrastructure.

Figure 1.

Early animal phylogeny according to the Porifera hypothesis with an indication of gains of important characters. Neurotransmitters A and B, see the text.

Figure 2.

Early animal evolution according to the Ctenophora hypothesis (based on homoplasies), with an indication of gains of important characters. Homoplasies are in red.

Figure 3.

Early animal evolution according to the Ctenophora hypothesis (based on losses), with an indication of losses of important characters. Losses are in blue.