Critical Appraisal of the Early Fossil Record and Evolution of the Lophotrochozoa

Sammanfattning

The Cambrian explosion is an extensively studied subject. Yet, relationship among the mainlophotrochozoan clades -Annelida, Phoronida, Brachiopoda and Mollusca- remains particularlyunclear and controversial. Within these clades, the phylogeny is also poorly understood. A newproposition is presented for the evolution of the mollusc, unifying the Serialia and theAculifera/Conchifera hypotheses. CT-scans of Maikhanella multa from the Fortunian ofMongolia revealed the inner structure of the maikhanellid shell, and confirms that it is formedby the fusion of siphogonuchitid sclerites. Through ontogenesis, Maikhanella multa secretessclerites that merge basally to form bundles of sclerites, then plates. A new structure is identified:the basal layer, that is generated basally in the late ontogenesis. This structure isolates thesclerites and cuts the soft tissue connection from the rest of the body. This phenomenon resultsultimately in the erosion of the sclerites, forming the smooth apex often observed inMaikhanellidae.This study shows that the genera Totornatus and Canopoconus are ontogenetic stages ofMaikhanella, respectively younger and older than this latter. Growth lines are observed in thebasal layer of derived Maikhanellidae, revealing that the molluscan shell evolved from this taxa.Helcionelloids and Halkieriidae are also shown to be derived from the Maikhanellidae.Lophophorate, annelid and molluscan affinities have been claimed for the Hyolitha. It isshown in this work that none of these interpretations is satisfactory, and that the hyolithsremain a particularly problematic group of organisms.The importance of the evolution of the chaetae is highlighted, as this structure is observed inLophophorata, Annelida and Mollusca. Chaetae are probably homologous to the calcareoussclerites in the molluscs, as both structures share a microvillous secretion.A new phylogeny of the Lophotrochozoa is proposed, with a close relationship betweenLophophorata and Annelida, as well as a middle Cambrian crown group Mollusca origin.