JURASSIC SPINICAUDATA OF NAMIBIA: EVIDENCE FOR THE EVOLUTION OF SEXUAL MATING SYSTEMS

Newly collected spinicaudatan (‘conchostracan’) specimens from a sedimentary interbed in the Kalkrand Formation of central Namibia provide insight into the evolution of freshwater taxa during the breakup of Gondwana as well as direct evident about ancient breeding systems. The Kalkrand Formation comprises a series of Early Jurassic (Toarcian) flood basalts that formed related to the initial stages of rifting of Africa from Antarctica. Sedimentologic analyses indicate a seasonal climate with frequent sheet flood events.

A monospecific assemblage of over 80 spinicaudatan specimens was collected from a layer interpreted as an ephemeral pond deposit with pedogenic overprinting. Two morphotypes, elongate and subcircular, occur. Each comprises approximately 50% of the population. The only aspect of morphology in which these two morphs differ is the height/length ratio. All other measures (ex., size, anterior shape, hinge length) are not statistically significant. Variation in the relative height of carapaces but no other metric is consistent with an interpretation of a dimorphic species. In modern dimorphic spinicaudatan species, the valves of female individuals are more circular than their male counterparts’ to provide room to store the egg clutch. The 50/50 ratio of carapace morphology indicates a dioecious mating system.

Spinicaudatan fossils are regularly preserved in sedimentary interbeds within continental flood basalt sequences related to the Mesozoic rifting of Gondwana. Spinicaudata from these deposits form a diverse suite of taxa, and would be expected to exhibit a similar suite of reproductive strategies or mating systems as modern spinicaudatan species. A comparison of published or potential data on mating systems in Mesozoic spinicaudatans records other instances of dioecy, such as the contemporary species Carapacestheria disgregaris from the Kirkpatrick Basalt of Antarctica. The relative frequency of sexual mating systems during the Mesozoic is reviewed.