Meeting Abstract

26.5  Wednesday, Jan. 5  Living Fossils? The Phylogeny and Fossil Record of Branchiopod Crustaceans HEGNA, T. A.; Yale Univ., New Haven, CT thomas.hegna@yale.edu

Branchiopods are a small and morphologically diverse clade of predominantly freshwater crustaceans; likely crown-group representatives have been in existence since the Devonian Period (over 360 mya). Branchiopoda consists of several groups: the anostracans, the notostracans, the paraphyletic conchostracans and the cladocerans. They 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 have often been referred to as living fossils. Rather than being emblematic of stasis, they 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. Fossil conchostracans are more difficult to reconcile with a modern phylogenetic understanding of the group. Fossil conchostracans typically preserve only the carapace and its ornamentation, but systematics of modern conchostracansignores 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 are a much more ambiguous character—they may be a basal feature of diplostracans rather than of spinicaudatans. Inclusion of fossil taxa in a phylogenetic analysis based on morphological characters does not alter the inter-relationships of extant taxa, but it does alter our perception of character evolution within the clade. Understanding the course of character evolution within Branchiopoda may be useful for understanding the origin of Hexapoda.