Evolutionary Waves: Patterns in the Origins of Animal Phyla.

Abstract

Concordant patterns of embryology, morphology and functional anatomy delimit grades of animal phyla, each of which contains a 'Major Phylum': PARACOELOMATA (nom.nov.) = acoelomates + pseudocoelomates, flexible hydrostatic skeleton, Nematoda; DEUTEROSTOMIA (including lophophorates) = enterocoelic coelom, rigid internal skeleton, Chordata; and PROTOSTOMIA with two subgrades, MONOMERIC P. = unsegmented, single coelom, molluscan blastular cross, partial rigid exoskeleton, Molluscs; and POLYMERIC P. = segmented, multiple coelom, annelid cross, rigid exoskeleton, Uniramia. Such groups are usually treated as arbitrary stages in mono- and limited-branch phylogenies, but recent studies show them to be real and significant because the only phylogenetic links are from each Paracoelomata and Protostomia Phylum to Turbellaria; and each Deuterostomia Phylum to Cnidaria-Ctenophora and/or enteropneust Hemichordata. Similar grades have often been explained by hypothetical common ancestors, which are unnecessary if the phyla arose during 'evolutionary waves'. These attribute the origin of each grade to the likelihood that its constituent phyla arose independently, about the same time, from the same ciliary powered ancestral stock which was preadapted to enabling a potential body cavity to be actualized while evolving a cylindrical, wholly muscle-powered, body with a hydrostatic skeleton. Because such a skeleton is functionally dependent upon other structural modifications, particularly of the body wall, it could appear only when these were also available. If the latter could be supplied in a number of ways, all opportunities would be exploited and a body cavity would appear several times. The morphology suggests that this did happen, so that a pseudocoelom and coelom evolved independently in each phylum where they occur. Because of evidence that Protostomia and Deuterostomia were never linked during evolution, the origin of the coeloms in the former are explained by the Gonocoelic Theory and in the latter by the Enterocoelic. This, with the recognition of the monomeric protostomes as a distinct group, establishes that segmentation arose at the same time as the coeloms, so that their origins are one problem and not two as usually thought. Finally, protistan data suggest that Turbellaria, and so Paracoelomata and Protostomia, arose from 'close mitosis' flagellates, as did Fungi; while Cnidaria, and so Deuterostomia, arose from 'open mitosis' flagellates. as did Plantae. Thus, the classic Animalia division into Protostomia and Deuterostomia may represent a Protista division such that the animal groups are closer to fungi and plants respectively than they are to each other.