The evolutionary dynamics of sexual systems in deep time: An integrated biological and paleontological approach

This doctoral dissertation reports the results of a multi-faceted investigation into the evolutionary dynamics of sexual systems over geologic time using the fossil record of the bivalved branchiopod crustacea in the Order Spinicaudata. The difficulty of assessing the sex of fossils (and by extension, the sexual system employed in extinct organisms) is widespread, and in those taxa that do show sexual dimorphism (e.g., ammonites, some arthropods, and vertebrates), reproductive mechanisms are often invariant, making assessments of reproductive evolution impossible. In this study, new techniques have been developed that will allow the identification of sexes in fossil crustaceans within the taxon Spinicaudata ("clam shrimp"): an enigmatic group of crustaceans with a unique bivalved carapace. Clam shrimp are well-represented in the fossil record, and have a broad array of reproductive types: dioecy (males + females), androdioecy (males + hermaphrodites) parthenogenesis (asexual females), and selfing hermaphroditism. These projects combine the wealth of information about the evolutionary transitions among these various reproductive types gleaned from studies of extant clam shrimp, with the rich representation of clam shrimp throughout the fossil record (from the Devonian to the modern day) to address two canonical hypotheses of reproductive evolutionary theory: (1) that unisexual species should be short lived and less speciose than their outcrossing counterparts and (2) that androdioecy is an unstable, transitionary system that should not persist over long periods of time. These studies provide much needed reviews of existing paleontological and biological research regarding the Spinicaudata, assess their taphonomic fidelity using a number of experimental techniques, develops and tests a morphometric protocol for identifying sexes in fossil populations and subsequently extends these shape comparisons using a large collection of fossil clam shrimp taxa from across the world to characterize sex-specific shape differences among various fossil clam shrimp lineages. We then combine these methods to assess sex ratios in fossil clam shrimp, which are indicative of mating system type in modern taxa, thereby allowing us to assign mating system to fossil species. These combined techniques allow, for the first time, investigation of the two evolutionary questions in the studies noted above and to test theories of breeding system evolution that, to date, have remained elusive to empirical assessment and provide a foundation for future research utilizing the fossil record to perform transformative studies into the evolution of mating systems.