The origin of Hexapoda: A crustacean perspective

There have been many attempts recently, from many different perspectives, t o produce a phylogeny of arthropods. Many of these analyses employ data fro m molecular sequences and developmental genetic studies, and these often st and at odds with each other as well as those derived from consideration of morphology and paleontology. Carcinologists often have a distinctive viewpo int. While molecules and genetics can suggest a close hexapod-crustacean co nnection, morphology and paleontology typically advocate a basal position f or the hexapod-myriapods with a separate schizopod lineage aligning crustac eans more closely with a trilobite-cheliceriform clade. It is not a questio n of who is right or wrong. It is only that caution should prevail when int erpreting data that place undue emphasis on single types of evidence. Never theless, the issue of possible hexapod-crustacean affinities could stand as a viable alternative hypothesis among many. We could then ask just which g roups of crustaceans might be closely related to the hexapods? Current phyl ogenetic analyses of crustaceomorphs indicate that we should consider five groups in this regard: 1) stem-group crustaceomorphs from the Cambrian, 2) Remipedia, 3) maxillopodans, 4) phyllopodans, and 5) Malacostraca. Each of these groups presents distinctive problems as well as possibilities as sist er taxa to hexapods. Some crustaceans obviously make better candidates than others. Nevertheless, we can posit which groups we might further explore i n regard to accumulating molecule sequences, planning developmental genetic studies, or uncovering ultrastructural data that will allow us to test cur rent multiple alternative hypotheses.