title: Evolutionary patterns and consequences of developmental mode in Cenozoic gastropods from southeastern Australia
Creators: name: Thomson, Kirstie
Publisher: name: University of Liverpool Repository
Year: 2014

description: Gastropods, like many other marine invertebrates undergo a two-stage life cycle. As the adult body plan results in narrow environmental tolerances and restricted mobility, the optimum opportunity for dispersal occurs during the initial larval phase. Dispersal is considered to be a major influence on the evolutionary trends of different larval strategies. Three larval strategies are recognised in this research: planktotrophy, lecithotrophy and direct development. Planktotrophic larvae are able to feed and swim in the plankton resulting in the greatest dispersal potential. Lecithotrophic larvae have a reduced planktic period and are considered to have more restricted dispersal. The planktic period is absent in direct developing larvae and therefore dispersal potential in these taxa is extremely limited. Each of these larval strategies can be confidently inferred from the shells of fossil gastropods and the evolutionary trends associated with modes of development can be examined using both phylogenetic and non-phylogenetic techniques. This research uses Cenozoic gastropods from southeastern Australia to examine evolutionary trends associated with larval mode. To ensure the species used in analyses are distinct and correctly assigned, a taxonomic review of the six families included in this study was undertaken. The families included in this study were the Volutidae, Nassariidae, Raphitomidae, Borsoniidae, Mangeliidae and Turridae. Phylogenetic analyses were used to examine the relationships between taxa and to determine the order and timing of changes in larval mode throughout the Cenozoic. Traditionally, planktotrophy has been considered the ancestral mode of development. However, using maximum parsimony and maximum-likelihood analysis, this research suggests that the ancestral developmental mode cannot be confidently determined in gastropods from southeastern Australia. Similarly, evidence that transitions between larval strategies might be reversible contradicts the general view that regaining the specialised structures associated with planktotrophy is so difficult that it is considered extremely unlikely to occur. When the timing of switches in larval mode was examined they were found to be scattered at different points in time rather than clustered to specific periods and therefore no inference can be made as to the likely factors driving transitions between larval modes. The correlation between mode of development and macroevolutionary trends was examined using non-phylogenetic techniques. The results do not concur with the hypothesis that species with planktotrophic larvae will exhibit wider geographic ranges, longer species durations and lower speciation rates then lecithotrophic or direct developing taxa. The analyses are thought to be hindered by a strong preservation bias and gaps within the fossil record. The quality of the fossil record and the congruence between phylogenies and stratigraphy is examined using the Stratigraphic Consistency Index, the Relative Completeness Index and the Gap Excess Ratio.