THE FOSSIL RECORD OF LARGE BRANCHIOPOD CRUSTACEANS

Branchiopods are a small group of freshwater crustaceans that have a modest fossil record. The higher-level phylogeny of modern branchiopod crustaceans is relatively well understood with a topology of [Anostraca [Notostraca [Laevicaudata [Spinicaudata [Cyclestherida, Cladocera]]]]]. However, we do not have a firm grasp on divergence dates or character state transitions between the major clades. Rehbachiella from the upper Cambrian of Sweden is pretty firmly established as a stem group branchiopod. The anostracans and notostracans are two of the earliest branching lineage of branchiopods; they have stem group representatives in the Devonian known from the Rhynie Chert and the Strud Lagerstätte. Possible crown group representatives for both are known from the Mesozoic. The understudied Triassic Grès à Voltzia Lagerstätte contains possible stem group phyllopods and diplostracans. Laevicaudatans have a published record in the Permian and possible soft-part preservation known from the Jurassic. However, owing the character-poor nature of these fossils, it is impossible to tell if they represent crown-group laevicaudatans. Spinicaudatans have a rich record of carapaces. The earliest known total group spinicaudatans come from the Devonian (though there is reference to some possible late Silurian fossils). The leaiids are an enigmatic extinct diplostracan lineage thought to be closely related to the spinicaudatans. They have a record that extends from the Devonian to the Triassic. Cyclestherids have an enigmatic fossil record. There are no examples of cyclestherids preserved with soft-parts, so the only character used to assign fossils to this lineage is the shape of the carapace. According to that metric, cyclestherids have a record that begins in the Devonian.