What are the Cladocera ?
The reliable interpretation of the results of any microfossil analysis of lake sediment is dependent on a thorough understanding of the systematics and ecology of the microfossil species. Unlike diatoms, Cladocera species do not have a cosmopolitan distribution, as demonstrated by D.G. Frey and others. This fact was not appreciated by early investigators with the result that many species reported (particularly from North America) were given the same name as European species to which they bore a resemblance. Now a growing amount of literature is demonstrating that "species" which were once claimed to be cosmopolitan are in fact groups or complexes of morphologically similar species with each member having a more restricted distribution than the original species or complex.
We are fortunate that there is a long history of Cladocera research in Europe, with Georg Ossian Sars producing some of the earliest works from Norway. Regional ecological studies and taxonomic keys have been produced for several countries including Britain, Ireland, France, Germany, Italy, Spain, Romania. This means that we can be relatively confident in our interpretation of changing cladoceran assemblages in paleoecological studies of European sites. However, studies in other areas of the world should be made with reference to taxonomic and ecological studies of the regional cladoceran fauna.