THE ARCHAEOGASTROPODA A CLADE, A GRADE OR WHAT ELSE

The classic concept of Thiele's Archaeogastropoda includes the Docoglo ssa (now Patellogastropoda), Neritacea (now Neritimorpha), Cocculinace a (now Cocculiniformia), Zeugobranchia and Trochacea (now Vetigastropo da). The recent discovery of many new archaeogastropod groups mainly f rom deep waters, and in particular from the hydrothermal vent habitat, necessitates a reevaluation of the taxon. Archaeogastropoda can still be clearly defined by protoconch characters and by the diagnostic str eptoneurous and hypoathroid nervous system (close association of pleur al and pedal ganglia). This definition also includes the Neritimorpha and the architaenioglossate groups. The amount of convergence is very high among archaeogastropods, most characters of advanced gastropods o ccurred several times in parallel in evolution. As in Thiele's time, t he taxon ''Archaeogastropoda'' is regarded as the basic gastropod stem group and should be classified as a paraphyletic taxon. Whether this stem group gave rise to a single or two lines of higher gastropods is still a matter of debate.