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Evidence for a dioecious mating system in Early Jurassic Hardapestheria maxwelli gen. et sp. nov. (Crustacea, Branchiopoda, Spinicaudata) from the Kalkrand Formation of Namibia



A new spinicaudatan (clam shrimp), Hardapestheria maxwelli gen. et sp. nov., is described from the Jurassic Kalkrand Formation of central Namibia. Specimens were collected from a sedimentary interbed within a succession of flood basalts. These are the first spinicaudatans to be described from the Jurassic of south-western Africa. The new taxon is assigned to the family Eosestheriidae based on the combination of punctae and radial ornamentation on the carapace. Ornamentation on the growth bands in H. maxwelli differs from other eosestheriid genera because the punctate ornamentation is not restricted to the dorsal region of the carapace. Instead, all growth bands include a proximal region with punctae even though the distal portion of each growth band may exhibit anastomosing lirae. Among well-documented Mesozoic spinicaudatan genera, Hardapestheria is most closely related to Carapacestheria from the contemporaneous Kirkpatrick Basalt of Antarctica and Yanjiestheria from the Early Cretaceous of China. Hardapestheria maxwelli displays sexual dimorphism, which can be definitively related to a dioecious mating system with discrete male and female individuals. Review of additional early members of the Eosestheriidae suggests that the maintenance of two discrete sexes was the ancestral state for the clade. The ability to discriminate sexual mating system type unambiguously is rare in fossils, and this new species sheds light on the relationship between environmental stability and mating system evolution.