Phylogeny and Fossil Record of Branchiopod Crustaceans: An Integrative Approach

Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans. It consists of several groups: anostracans, notostracans, paraphyletic 'conchostracans', and cladocerans. Branchiopods have a good fossil record that has been largely overlooked and never placed in a phylogenetic context. Reevaluation of the branchiopod fossil record along with focused taphonomy experiments has yielded interesting new insights into the evolution and diversification of this clade.;In order to evaluate the effect of taphonomy on specific features, fossil notostracans were investigated with parallel decay experiments on living relatives. They are of interest because of their 'primitive' nature (Triops cancriformis is cited as the oldest living species with putative Permian occurrences). The dorsal features of the notostracan carapace persisted throughout decay; after six weeks of decay, a carcass was indistinguishable from an exuvia. Furthermore, the elongate, 'antenniform' first thoracic limb, present in almost all living notostracans, did not disarticulated before the other limbs. This feature is in contrast to all known fossil notostracans, where the first thoracic limb is like the rest of the thoracic limbs.;Notostracans, rather than being emblematic of stasis, have a dynamic evolutionary history. Their fossil record suggests that they had a 'bivalved' posture ancestrally and that the differentiation of their anterior thoracic limbs was a relatively late development. The extinct sister group to notostracans, the geographically isolated kazacharthrans, do not seem to represent a monophyletic clade at all—they are more likely a part of the notostracan stem group. Fossil 'conchostracans' are more difficult to reconcile with the modern phylogenetic understanding of the group. Fossil 'conchostracans' typically preserve only the 'bivalved' carapace and its ornamentation, but the systematics of modern 'conchostracans' ignores these attributes. Documentation of carapace ornamentation in modern species will enable testing of the phylogenetic utility of them in fossil species. 'Conchostracan' growth lines may be a basal feature of diplostracans rather than of spinicaudatans. These new discoveries all contribute to a rich, emerging picture of branchiopod evolution. Furthermore, they demonstrate the utility of integrating fossils into phylogenies of modern animals to constrain ancestral states.