Reid REH. (1970). Tetraxons and demosponge phylogeny. In: Fry WG (ed) The Biology of the Porifera. Zoological Society of London, Academic Press London, pp 63-89.

277320

Reid REH

1970

Publication

Proceedings of the 1st International Sponge Conference

Available for editors   [request]

According to Schulze (1887) and Dendy (1921) the regular tetraxon, 01' calthrops, is the central type of demosponge spicule and all monaxonid demosponges have been derived from choristids in phylogeny. Dendy's (1921) detailed development of this concept can provide a complete evolutionary explanation of demosponge spiculation. On the other hand, rejection of some of Dendy's concepts by later authors, including the writer, leaves many monaxonid sponges with no sure relationship to choristids. Such monaxonnids may never have had tetraxons as megascleres or even at any stage of phylogeny. Lévi (1957) has claimed that some are of altogether independent origin. In addition, some evidence can be interpreted as suggesting that the primitive demosponge spicules were monaxons, not tetraxons. This evidence includes (1) the origin of some triaene tetraxons from monaxons in ontogeny, (2) the occurrence of monaxonid sponges in the Cambrian period, long before the oldest known (Lower Carboniferous) choristids, and (3) the occurrence of tetraxon-like desmas in some Palaeozoic lithistids, which are probably of monaxonid origin. The writer agrees with previous authors who have rejected Dendy's concept of sigmatose microscleres as a homologous series but supports Dendy's treatment of the choristids against that of de Laubenfels (1936). The development of some triaenes from monaxons could be palingenetic but it could also be caenogenetic. Other cases of caeno-genetic change in ontogenetic prototypes can be demonstrated in spicules of the lithistid Tetracladina. The Palaeozoic record probably records only a very small fraction of the sponges which are likely to have existed in that period; the Lower Carboniferous choristids must also have ah'eady existed for long enough to evolve all the main types of tetraxon magascleres found in living choristids. Further, nothing is known of the microscleres of the Palaeozoic sponges, with which the early monaxonids and later choristers might, in fact, have no relationship. Four lines of evidence suggest early Palaeozoic Elxistence of sigmatomonaxonid sponges, Le. the group regarded by Levi as of independent origin. Last, variation in the desmas of Ordovician anthaspidellids suggests that their tetraxon-like desmas were monaxial or anaxial.

Porifera

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