Present data | To reveal a plausible phylogeny of pancrustaceans is today still a challenge. After the ambiguous results from single and multigene studies, phylogenomic data contributed a few insights to the position of single taxa. Namely the Remipedia as closest relatives to Hexapoda are highly supported. This result is also supported by older morphological data interpretation and recent evelopmental and neuroanatomical data.
Yet, molecular analyses including phylogenomic data generally place Branchiopoda closer to Hexapoda, while selected morphological, neuroanatomical, and larvaldevelopment data suggest a closer relationship of Malacostraca to Remipedia and Hexapoda. A similar pattern is found for the Cephalocarida, their close relationship to Remipedia in molecular analyses is not supported by recent neuroanatomical data (Stegner & Richter 2011). Interestingly, a previous comprehensive rDNA study on euarthropods (von Reumont et al. 2009) reconstructed a SG relationship of Cephalocarida and Branchiopoda, which is recently supported by Oakley and colleagues in a combined transcriptomic analyses including morphological and molecular data (Oakley et al. 2013). Paleontological data e.g. (Walossek 1993) would support this scenario. Thecostracan position is ambiguously resolved in molecular data, morphologically is the closer relationship to Malacostraca that is reconstructed in most trees less plausible than the alternative position as sistergroup to Copepoda (Meusemann et al. 2010, von Reumont et al. 2012).
A summary of the different phylogenetic hypotheses on the possible crustacean sistergroup to Hexapoda is given in the graphics (right). A: Polytomic clade of Remipedia + Malacostraca + Hexapoda supported by neuroanatomical and larval development studies (Fanenbruck et al. 2004; Fanenbruck & Harzsch 2005; Koenemann et al. 2009). B: Branchiopoda as sistergroup to Hexapoda as proposed by Glenner et al. (2006). C: Remipedia + Cephalocarida as sistergroup to Hexapoda as proposed by Regier and colleagues (Regier et al. 2010). D: The present hypothesis corroborated by recent molecular (von Reumont et al. 2009), mainly phylogenomic data (von Reumont et al. 2012, Oakley et al. 2013). Phylogenomic data for Cephalocarida is, however, still missing.
Supposed evolutionary scenario | The figure (right) summarizes new insights and open issues concerning crustacean phylogeny based on molecular and morphological data. Brown arrows and lines represent evolutionary lineages. Circles represent nodes that are strongly supported by morphological and molecular data. Dashed lines indicate more weakly supported relationships. Question marks indicate branches whose position is uncertain: variously because of ambiguity in the molecular data (red), conflict with morphological data (red), or a large gap in the fossil record (orange).