Systematics of the Eucestoda: advances toward a new phylogenetic paradigm,and observations on the early diversification of tapeworms and vertebrates

Evolutionary relationships of the Eucestoda have received intense but spora dic attention over the past century. Since 1996, the landscape has dramatic ally changed with respect to our knowledge of the phylogenetic relationship s among the tapeworms. The 2nd International Workshop for Tapeworm Systemat ics (IWTS) held in Lincoln, Nebraska in October of that year provided the c atalyst for development of novel hypotheses for inter- and intra-ordinal ph ylogeny. The working-group structure of the 2nd IWTS and results of phyloge netic studies are briefly introduced in the present manuscript. Higher-leve l phylogenies derived from parsimony analysis of independent data bases rep resenting comparative morphology or molecular sequences were largely congru ent and supported monophyly for the Eucestoda, The Caryophyllidea are basal ; difossate forms such as the Pseudophyllidea are primitive; tetrafossates including the Tetraphyllidea, Proteocephalidea, Nippotaeniidea, Tetrabothri idea and Cyclophyllidea are derived; and hypotheses differed in the placeme nt of the Trypanorhyncha and the Diphyllidea. These studies may provide a f oundation for resolution of inter- and intra-ordinal relationships for the tapeworms. Additionally, the first comprehensive phylogenetic hypotheses fo r the Pseudophyllidea, Diphyllidea, Trypanorhyncha, the paraphyletic Tetrap hyllidea + Lecanicephalidea, Proteocephalidea and Cyclophyllidea were devel oped during and subsequent to the 2nd IWTS. The stage is now set for contin ued and rapid advances in our understanding of the eucestodes. These studie s have also served to re-emphasise the rich genealogical diversity of tapew orms and the temporally deep history for their origin. A co-evolutionary hi story and radiation of eucestodes may involve deep co-speciation with verte brate host taxa, accompanied by some level of colonisation and extinction, extending into the Palaeozoic, minimally 350-420 million years ago.