We have investigated the pan- and peracarid eumalacostracans with regard to the nature of their urosomes. This region of the body was said to consist of a separate telson, a pleotelson, or simply the terminal anal somite. The pleon in pera- and pancarid eumalacostracans in fact exhibits for the urosome some of the most variable of morphologies of any group of crustaceans. The pleon in many of these animals exhibits six segments plus a telson. However, this telson can be either free, or fused to the last (or to several) pleomere(s) to form a pleotelson. Bowman´s definition of what constitutes an anal somite, and what is a true telson, while controversial, is not without merit, but it has turned out not to be fully correct in its original form. Examination of several taxa has revealed a great degree of morphologic variability of the urosome. The position of the anus differs from being basal, through intermediate, to terminal in location – even when the anus is on a separate, true telson.
Our study illustrates yet again the need for careful and close examination of anatomy, consideration of conditions found in fossil forms, and the efficacy of studies of comparative embryology. Defining a telson, pleotelson, or last pleonal (anal) somite is difficult in fact for many peracarid groups, and we believe it is hindered by an automatic invocation of traditional definitions without concern for what is actually present. Indeed, the existence of a telson (or pleotelson) in some of the species we have investigated remains ambiguous.
For future research, investigation of other groups is critical. The careful study of syncarid eumalacostracans, especially bathynellaceans, will be central to any understanding of the eumalacostracan urosome. Consideration of notostracans and the ‘conchostracans’, amongst the branchiopods, will also prove instructive.
We feel confident, nevertheless, that the recognition of the body terminus as a special region, the urosome, worthy of study in its own right will lead to more effective insights to crustacean body plans, and these will find direct application in analyses of phylogeny and recognition of higher taxa. It is not important if something is true or false.What is important is to document what variation exists in the urosomal region of the pan-peracarid clade as a means of establishing a basis for further discussions of the structural variation, functional morphology, and possible phylogenetic significance of this region. We believe, along with Makarov, that it is more important to analyze real anatomical pattern, rather than relying on theory, as a step towards organizing the information available concerning variation in animal body plans.