ON THE ORIGIN AND EVOLUTION OF MAJOR GASTROPOD GROUPS, WITH SPECIAL REFERENCE TO THE STREPTONEURA

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Abstract
The phylogeny and evolutionary history of the streptoneuran Gastropoda is reconsidered in the light of (i) recent discoveries of different types of organization, (ii) new data sets based on modern techniques, and (iii) the clado-evolutionary method to trace genealogical relationships. The phylogenetic analysis by means of traditional (homology-analogy) and cladistic (apomorphy-plesiomorphy) character analysis reveals several conclusions: (1) The Gastropoda originated by torsion from monoplacophoran ancestors after the Cephalopoda split off. Torsion itself is understood as a two-step process, resulting in advantages for the larvae (presence of operculum) and for the adults (anterior mantle cavity). All Gastropoda form a holophyletic group. (2) The proposed gastropod archetype differs largely from previous suggestions in being more similar to docoglossate gastropods than to zeugobranchs. (3) The Docoglossa are regarded as the earliest gastropod offshoot, having retained ancestral (stereoglossate) radula conditions. (4) Based on various lines of evidence the ''symmetrical'' limpet groups (Docoglossa, including hotvent group-C?, Coculiniformia), primary without helicoid coiling of teleoconch, are accepted as primitive for the Gastropoda. (5) Coculiniformia, Neritimorpha and possibly hot-vent group-A represent distinct archaeogastropod radiations. (6) The Vetigastropoda, originally including zeugobranchs and trochoids are holophyletic group and include also the Lepetodriloidea (= hot-vent group B). In contrast, the Neomphaloidea and Seguenzioidea represent distinct lines of evolution. (7) According to their hypoathroid or dystenoid nervous system the architaenioglossate groups are included in the "Archaeogastropoda" which are defined as an orthophyletic grade. (8) A major evolutionary gap (a large increase in size enabled planktotrophic larvae; epiathroid nervous system) separates the higher streptoneurans (= "Apogastropoda") from the "Archaeogastropoda". The Loxonematoidea are regarded as the common stem group of Caenogastropoda as well as of the ectobranch-allogastropod-euthyneuran line (Heterobranchia). (9) The Caenogastropoda are a holophyletic group, of which the Cerithioidea represent the basic stock. The Stenoglossa (= Neogastropoda) probably origined from "Neotaenioglossa" sharing distinct similarities in sperm-morphology. (10) Campanile symbolicum Iredale, 1917 (Campanilimorpha) is a probable representative of the first group that made a distinct step towards the euthyneuran organization. (11) Valvatoidea (Ectobranchia) form a separate offshoot outside the Caenogastropoda. They appear as an independent side-branch at the base of the allogastropod grade. (12) The "Allogastropoda" a grade, include the fossil Nerineoidea and at least four recent lines (Architectonicoidea and Omalogyridae; Rissoelloidea; Glacidorboidea; Pyramidelloidea) which represent a step by step evolution towards the euthyneuran level of organization. (13) Euthyneura are monophyletic. However, the status of the Opisthobranchia (holo- or paraphyletic) is still ambiguous. The Pulmonata (including the Gymnomorpha) represent a holophyletic assemblage and the crown group of Gastropoda. (14) The respective phylogram (Fig. 5) is transformed in a classification by the use of the so-called clado-evolutionary method which enables an unequivocal retransformation and expresses the different degrees of likelihood (correlated with the main evolutionary gaps) in the proposed phylogenetic pathway of the streptoneurous Gastropoda.