ANALYSES WITHIN A PHYLOGENETIC FRAMEWORK REVEALS MACROEVOLUTIONARY CONSEQUENCES OF LARVAL MODES WITHIN SPECIES OF ATHLETA (VOLUTOSPINA) (NEOGASTROPODA) FROM EOCENE DEPOSITS IN THE U.S. GULF COAST AND WESTERN EUROPE

The Eocene neogastropod clade Athleta (Volutospina) from the US Gulf Coast, England, and Paris Basin was used to examine evolutionary trends associated with larval mode. Previous studies on volutids have investigated potential connections between larval mode and evolutionary pattern in this clade, but not in an explicit phylogenetic framework. Theory predicts that planktotrophic larvae exhibit wider geographic distributions and longer durations than non-planktotrophic species as a result of greater dispersal ability and that non-planktotrophs speciate more frequently than planktotrophs. Larval modes were mapped onto a phylogeny to identify mode switches, determine the ancestral mode of development, and examine the macroevolutionary consequences of different modes of larval development.

When the developmental modes were mapped onto the phylogeny, planktotrophy was confirmed to be the plesiomorphic state. Non-planktotrophy arose independently at least 3 times in this clade and there is no evidence that larval mode reversals (back to planktotrophy) took place during this time.

When geographic location were applied to the phylogeny, 5 trans-oceanic dispersals were identified. 3 of these represent movement from the Gulf Coast to western Europe and 2 from Europe to the Gulf Coast.

Species with planktotrophic larvae had a median geologic duration of 8.2 million years compared to 3 million years for those with non-planktotrophic larvae. A Mann Whitney U-test indicated that the difference between planktotrophic and non-planktotrophic species durations is significant.

Patterns of species distribution revealed some striking differences in geographic spread between species with similar dispersal values. Yet when all taxa were considered together, the difference in median geographic range of planktotrophs and non-planktotrophs was not significant (Mann-Whitney U-test). Based upon the results, it seems larval mode is not a predictor of realized geographic range. However, when all species (regardless of larval mode) were divided into 2 groups (short-lived and long-lived), an interesting relationship is revealed. Short-lived species occupied smaller geographic ranges than long-lived species (Mann-Whitney U-test). Potential factors controlling geographic ranges as well as speciation rates will be discussed.