THE FOSSIL RECORD OF BRANCHIOPOD CRUSTACEANS: STASIS NO MORE?

Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans (dating from at least the Devonian). Branchiopoda consists of several groups: the anostracans, the notostracans, the paraphyletic ‘conchostracans’ and the cladocerans. Their thin, lightly mineralized exoskeleton requires special conditions for fossilization. Nonetheless, branchiopods have a very good fossil record that has been largely overlooked and never placed in a phylogenetic context. Re-evaluation of the branchiopod fossil record has yielded interesting new insights into the morphological evolution and diversification of this clade.

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 Notostraca stem group. Fossil ‘conchostracans’ are much 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’ essentially ignores these attributes. Documentation of the carapace ornamentation of modern species will enable testing of the phylogenetic utility of those features in the fossil representatives. ‘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 fossil forms into the phylogenies of modern animals in order to constrain ancestral states.