Evolution and Radiation of Crustacea

Abstract The evolution of the Crustacea following their origins in the Cambrian is outlined, with an overview of their paleontological history and global distributions into modern times. Major recent developments in arthropod evolution include recognition that Hexapoda is nested within Crustacea. Perspectives also changed during the last decades of the 20th century on the form of the crustacean ancestor, from being a long-bodied, serially homonomous form (like a remipede or cephalocarid) to a short-bodied, possibly ostracod-like form similar to Cambrian stem and crown group fossil forms. These changes have come through a shift to formal methods of phylogenetic analysis combined with the much larger volume of both morphological and molecular data now available. The most extensive current phylogenies typically recover the short-bodied Oligostraca (containing ostracods and a few minor groups) as basal crustaceans; Malacostraca and Maxillopoda are high in the tree; and Cephalocarida and Remipedia are derived forms as sister to Branchiopoda and Hexapoda, respectively. Each of these major groups can be understood through variations in tagmatization (differentiation of body segments into regions). The early crustacean fossil record (especially the Ordovician) is dominated by ostracods. Malacostracans, although having Cambrian origins, did not significantly radiate until the Mesozoic. Eumalacostraca continued to actively radiate in the Cenozoic and are now the most ubiquitous and morphologically disparate crustaceans. The processes driving crustacean evolution remain to be fully evaluated. Contingency and external factors are undoubtedly important, but most deep lineages of the Crustacea show pervasive macroevolutionary trends toward increasing tagmatization. These trends are apparently driven, meaning the formation of new body plans is not merely a contingent outcome—intrinsic factors may contribute to increasing tagmatization. Further data are required from ontogeny and developmental genetics, paleontology, and phylogenetics in order to better understand how crustaceans have evolved.