An Introduction to the Invertebrates, Second Edition - tiera.ru

180 CRUSTACEA13.7 How are Crustacea related to each other?Faced with so much diversity, there is bound to be controversyabout theories based on morphology. While recent fossil finds andmolecular evidence can help, many uncertainties remain: for a start,which present-day crustaceans are closest to the primitive condition?Probably the earliest crustaceans were very small and unspecialised.A recently found early Cambrian fossil, Ercaia minuta, was 2 4mminlength and had 13 similar segments, all with similar appendagesbut we cannot assume that it represents the primitive structure justbecause it is the earliest crustacean known. Unspecialised smallpresent-day crustaceans such as Cephalocarida or Remipedia seemto be obvious candidates, but these groups may be secondarilysimple (for example Remipedia have brains as elaborate as thoseof some Malacostraca). Concerning further crustacean evolution, also,uncertainty remains. Groups recognised in recent classificationsas being monophyletic, using morphological and molecular evidence,are Branchiopoda, Cephalocarida, Remipedia and Malacostraca.Opinions differ about the other main groups: some workers combineCopepoda, Cirripedia, Mystacocarida, Branchiura and perhapsPentostomida as ‘Maxillopoda’, with or without the inclusion ofOstracoda. Figures 13.3 to 13.6 illustrate all these groups, but cannotindicate the diversity which many of them represent.Crustacea, in summary, are a very large and diverse group ofessentially aquatic arthropods. The arthropod groups to be considerednext have members far more thoroughly adapted to life on land.