Arguments concerning the basal evolution of the Bryozoa

Authors


Abstract

The number of tentacles per unit of body volume decreases with increasing body size in the Bryozoa. The ranges of zooid sizes and of tentacle numbers of the Phylactolaemata do considerably overlap with those of the Gymnolaemata s. l., but only the phylactolaemes form horseshoe-shaped lophophores. Therefore, the lophophore form in the Bryozoa does not simply depend on body sizes but on differences in the genomes in the two sub-classes. A lining-in of similar or similar seeming external shapes of zooids has no persuasive power unless it is combined with convincing arguments concerning the accompanying emendations of the internal anatomy. Economizations and attained degrees of functional effectivity provide main guide-lines for the argumentation and for testing the probability of discussed cases of evolutionary branching during attempts to reconstruct alterations of the internal anatomy. Recapitulative arrangements may play an important role in this context. Statistics on “phens” cannot help to solve these problems. Comparison of the forms of the body bending, of the modes of ontogenetioal displacement of the polypide, and of the arrangements of the body musculature in combination supports the interpretation that the Stenostomata and the Eurystomata have a common root with primarily erect, uncalcified forms and thus most probably are a monophyletic group of Gymnolaemata s. l. originating in phylactolaeme like ancestors. Omitting the Phylactolaemata (as a linking group with many plesiomorph features) in attempts to reconstruct the bryozoan evolution drastically increases the amount of morphological differences between the Gymnolaemata s. l. and the Phoronidae, which are commonly accepted to have pre-served the most morphological characteristics of the bryozoan ancestors. It must be warned of an overestimation of the possible role of the fossil record for the reconstruction of the bryozoan phylogeny, which strongly demands the aids by investigations also on Recent species.

Ancillary