The fossil record of the Cladocera (Crustacea: Branchiopoda): Evidence and hypotheses

Waterfleas (Crustacea: Branchiopoda: Cladocera) are among the most successful invertebrates in current freshwater ecosystems, occupying a wide range of niches worldwide. Cladocerans are an important component of the zooplankton, occur in high abundance, and show a marked degree of morphological diversification and species richness with over 700 extant species, in > 100 genera. Though this group is of prime importance in the global aquatic food chain, comparably little is known of its early evolution. Until recently cladoceran evolutionary history has been clouded by a mixture of assumptions, erroneous fossil identifications and circular reasoning without context. Our knowledge of the cladoceran fossil record and our understanding of freshwater palaeoecosystems and their communities have made important progress over the last decades but still need further analysis of cladoceran palaeontological data and a discussion of molecular clock estimates. Here we present a critical review of the complete fossil record of the Cladocera including the first ever deep-time palaeolimnological framework for the group. Cladocera are key primary consumers whose evolution is crucial to the understanding of the Mesozoic Lacustrine Revolution that helped to shape aquatic ecosystems. The fossil record is placed in the context of major historical changes in the freshwater environment including the appearance of predator-prey relationships and niche availability that have impacted Cladocera evolution. The Cladocera exhibit a striking morphological stasis over millions of years yet they have overcome significant ecological shifts and they continue to show remarkable plasticity which have been the key to their survival and success in the present day. (C) 2016 Elsevier B.V. All rights reserved.