Branchiopod crustacea are an ancient and enigmatic arthropod group. Likely originating in the Cambrian, the group is a ubiquitous and widely recognizable component of modern freshwater ecosystems with a relatively good freshwater fossil record, and are often indicators of temporary freshwater palaeoenvironments. The evolutionary history of the Branchiopoda, and in particular, the Order Spinicaudata (clam shrimp) is of particular interest in terms of sexual system evolution. The Spinicaudata provide an interesting problem for contemporary evolutionary theory in that the evolutionary instability and reduced lineage longevity associated with increased inbreeding (in the form of self fertilizing hermaphrodites) in this group is not apparent. New models presented here suggest at least three independent occurrences of sperm-producing females within the extant Spinicaudata, and four occurrences within the Class when considering the spinicaudatan sister group, the Notostraca (‘tadpole shrimp’). If the periodic occurrence of hermaphroditism is both an ancient and stable phenomena in this arthropod group, serious attention should be given in revision of the biological dogma that reduced outcrossing is always bad. When considered alongside other sexual systems within the branchiopoda, the persistence of a ‘sexual lability’ as an adaptation for reproductive assurance and environmental instability seems likely.

Combining these findings with ongoing integrated palaeontological and biological investigations to investigate the identification of different sexual systems within the fossil record aim to provide the first empirical observations of sexual system dynamics over evolutionary time.