Evidence for a dioecious mating system in E arly J urassic H ardapestheria maxwelli gen. et sp. nov. ( C rustacea, B ranchiopoda, S pinicaudata) from the K alkrand F ormation of N amibia

Abstract A new spinicaudatan (clam shrimp), H ardapestheria maxwelli gen. et sp. nov., is described from the J urassic K alkrand F ormation of central N amibia. Specimens were collected from a sedimentary interbed within a succession of flood basalts. These are the first spinicaudatans to be described from the J urassic of south‐western A frica. The new taxon is assigned to the family E osestheriidae 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 M esozoic spinicaudatan genera, H ardapestheria is most closely related to C arapacestheria from the contemporaneous K irkpatrick B asalt of A ntarctica and Y anjiestheria from the E arly C retaceous of C hina. H ardapestheria 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 E osestheriidae 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.