Alternative hypotheses on the phylogenetic position of the Tardigrada within the Ecdysozoa and the evolution of segmental ganglia. (A) Tree illustrating controversial views on the phylogenetic position of tardigrades as either the sister group of arthropods, onychophorans, onychophorans plus arthropods, or one of the cycloneuralian taxa (modified from ref. ). (B–H) Alternative scenarios on the evolution of segmental ganglia (dark-blue dots) in panarthropods, depending on the phylogenetic position of tardigrades favoured. Segmental ganglia might be either homologous structures (B, D, F, H) or they might have evolved independently in tardigrades and arthropods (C, E, G). The scenario in B is most parsimonious, whereas those in D, F and H require additional losses (magenta bars with an “R”) in onychophorans and/or other ecdysozoans. Asterisks in G and H indicate that a ventral chain of ganglia-like thickenings also occurs in kinorhynchs, although their homology with segmental ganglia of tardigrades and arthropods is uncertain (see text for further details).
Mayer et al. BMC Evolutionary Biology 2013 13:230 doi:10.1186/1471-2148-13-230