The origin of animal phyla and the new phylum Procoelomata

Abstract

A model of metazoan evolution presented previously (Bergstrom 1986 in Zoologica Scriptia 15) explains deuterostomian characters as derived from protostomian ones through loosening of the constraints in the spiralian type of morphogenesis. This fits phylogenies derived from studies of molecular sequences. The model helps explain (1) the well-known mixture of proto- and deuterostomian features in several groups; (2) the difficulties in making a phylogeny based on comparative anatomy, and (3) the fossil explosion in the Cambrian. Since protostomian features such as a ciliated locomotory sole and a pelagic larva with ciliary bands are widely distributed in branches of the phylogenetic tree, they must have been present in the stem of the tree. Most probably the stem forms were pseudosegmented, which helps explain how segmentation, oligomery and non-segmentation could evolve repeatedly in derived groups. Origination of new phyla involved macroevolutionary changes primarily in the mode of feeding and locomotion. The stem phylum, from which most other phyla appear to have been derived directly, is here named the Procoelomata. Machaeridian-type animals are referred to it. The Ediacaran-type Precambrian fossils cannot be placed in the metazoan evolutionary tree.