THE OLDEST ARTHROPODS AND THE ORIGIN OF THE CRUSTACEA

In the last decade new finds in the Cambrian of well-preserved faunas have given us a wealth of new material. This has meant a virtual explosion in our knowledge of the earliest arthropods, although only a fraction of the material has as yet been described. Most significantly, perhaps, the valuable descriptions of the Orsten material have shed some light on the origination of crustaceans through the sequential acquisition of crustacean characters in a group of 'stem-group crustaceans' and even more basic forms. An attempt is made to outline an evolutionary tree leading on one side to trilobitomorphs and chelicerates, on the other to crustaceans and related forms. It involves numbers of synapomorphies added successively, and it appears unlikely that true crustaceans were resent already in the earliest Cambrian.