The role of fossils and phylogeny in understanding the early evolution of annelids and mollusks (lophotrochozoans)

Particular questions regarding the evolution of annelids and molluscs are addressed. What was the ancestral body plan of the crown group, when did it diversify and how can hitherto problematic fossil stem groups inform us about evolution of the clades of interest? Chapter 1 outlines the questions addressed in this thesis. Chapter 2 describes the sequencing of 7 nuclear housekeeping genes from a series of molluscs and their use in interpreting the evolution of chitons and aplacophorans. These two groups form a monophyletic clade---Aculifera---and molecular divergence estimates indicate that this clade diversified at the Ordovician-Cambrian transition. This is consistent with some hypotheses about mollusk evolution that suggest that aplacophorans are derived and not plesiomorphic. Conchifera is consistently recovered as paraphyletic. This means that the ancestor of mollusks most likely resembled a monoplacophoran limpet. Chapter 3 discusses the nature of a problematic group, the Multiplacophora. These forms have been assigned recently to both the polyplacophoran stem and crown group. Using the molecular clock divergence estimates from chapter 2 Multiplacophora are shown to be a stem group that evolved some crown group characteristics convergently. Chapter 4 investigates the evolution of cephalopods in the light of molecular systematics using parts of the dataset from Chapter 2. The cephalopod crown group is younger than hitherto realized, which calls for an alternative hypothesis regarding the ancestor of the crown group, which is suggested to be the straight shelled orthocerids. Nectocaris was recently assigned to the cephalopod stem group, but this interpretation is rejected on the basis of incongruence with the fossil record and character polarity.;The second part of the thesis treats the evolution of annelids. In Chapter 5 a new fossil polychaete from the Early Cambrian is described that possess pygidial cirri. This is the first record of this structure in a stem annelid and is considered one of the defining features of the crown group. Many other Cambrian forms do not possess pygidial cirri and the new form may be closer to the crown group. Chapter 6 considers the nature of machaeridians and their affinities with the annelids, their mode of locomotion and some lesser-known features of the skeleton of one group of Machaeridia, the Plumulitidae. The Machaeridia was until recently a problematic taxon with no definite affmities to a specific phylum. These forms are here shown to be annelids within the crown group based on the evidence of soft tissues in a new Ordovician taxon. Machaeridians are unique annelids in that they evolved a biomineralized armour borne by dorsal parapodial cirri.