100 Years of Deuterostomia (Grobben, 1908): Cladogenetic and Anagenetic Relations within the Notoneuralia Domain

Results from molecular systematics and comparative developmental genetics changed the picture of metazoan and especially bilaterian radiation. According to this new animal phylogeny (introduced by Adoutte et al. 1999/2000), Grobbens (1908) widely favoured protostome-deuterostome division of the Bilateria can be upheld, but only with major rearrangements within these superphyla. On the cladogenetic level, the Protostomia are split into two unexpected subgroups, the Lophotrochozoa and Ecdysozoa. The deuterostomes are split into the subgroups Chordata and Ambulacraria, which is not novel since Grobben (1908) introduced the Deuterostomia in this way (together with the Chaetognatha as a third line). However, many details of the new deuterostome phylogeny do not fit traditional, morphology-based reconstructions. As a consequence, three relatively unexpected proposals for early deuterostome evolution are favoured today: An ambulacraria-scenario, a xenoturbellid-scenario, and a chordate-scenario. The first two proposals are often discussed in the literature, while the chordate-scenario is almost completely neglected. Therefore, the paper presented focuses on the chordate scenario, i.e. the hypothesis of an acrania-like ur-deuterostomian. It is argued that the acrania-hypothesis is clearly preferable when biomechanic options of a polysegmented, hydroskeletal body plan are taken into account. The so called hydroskeleton hypothesis, rooted in the work of W. F. Gutmann, is the most detailed anagenetic scenario which depicts an acrania-like ur-deuterostome.