Was Planktotrophic Larval Development Original In Metazoa or Not? – Evidence from Gastropods

Planktotrophic larval development has long been considered as original state in Metazoa. However various studies suggest that this is not so because 1) Cambrian univalves were too small to produce planktotrophic larvae, 2) mapping of planktotrophic larvae on current phylogenetic trees suggest “polyphyly” of this character, 3) early ontogenetic stages preserved in Cambrian invertebrates (especially molluscs) suggest non-planktotrophy, 4) Recent representatives of basal mollusc clades have non planktotrophic larval development. However, recently these traits of evidence have been challenged. Initial parts of some Cambrian small shelly fossils were interpreted as planktotrophic larval shells but this is not supported by size and morphology of these mollusc protoconchs. Small helcionellids were identified as casts of apices of large (cm) Cambrian mollusc limpets. However, the interpretation of these apices as larval shells is flawed because they are too large and they lack clear transitions to the adult stage. Even recent interpretations of modern monoplacophoran protoconchs as products of plankton-feeding larvae are unlikely because they are too large and were never found in the plankton. There is no convincing evidence for Early to Middle Cambrian planktotrophic larval shells. Planktotrophy probably evolved several times at the Cambrian/Ordovician transition and according to the current state of knowledge it seems unlikely that it is ancestral in Metazoa.