Phylogeny and evolution of basal living chitons (Mollusca: Polyplacophora: Lepidopleurida).

This group includes more than 130 recognised living species, distributed worldwide and primarily found in the deep sea; they are small and superficially homogenous in their external morphology. This thesis addresses seven aspects of the biology of these plesiomorphic chitons: (1) Chitons endemic to sunken wood in the South Pacific comprise the most speciose lepidopleuran local fauna.

Distribution patterns and faunal composition are examined and five new species are described; (2) The type species of Leptochiton (and of order Lepidopleurida) – L. asellus – is examined through histological sections and dissection to identify potential morphological characters for phylogenetic inference; (3) Placement of gonopores and nephridiopores in the pallial cavity are examined across species and their positional homology and phylogenetic informativeness is discussed; (4) Morphology – shell anatomy, girdle armature, and gills – is described encompassing 69 newly-described characters and is analysed using parsimony to assess existing dubordinal divisions; (5) Morphological phylogenetic hypotheses are tested by analysis of molecular sequences (complete 18S rRNA, complete 28S rRNA and COI); (6) Two new polyplacophoran ectoparasites are described, from separate populations of lepidopleurans; a parasitic foraminifera (North Atlantic) and an epibiotic bryozoan (South Pacific).