384 FOSSIL INVERTEBRATES/Gastropods Figure Schematic diagram showing the four possible relationships between shell coiling and body asymmetry Orthostrophy means that anatomically dextral (or sinistral) animals occupy dextrally (or sinistrally) coiled shells The term hyperstrophy is used when the handedness of the shell and soft body is different (see text for explanation) Thiele, integrated earlier classifications and divided the gastropods into three subclasses: Prosobranchia, Opisthobranchia, and Pulmonata In addition, the Prosobranchia were divided into three orders: Archaeogastropoda, Mesogastropoda, and Neogastropoda Thiele’s system was used by zoologists and palaeontologists for most of the twentieth century However, during recent decades, numerous new data on the anatomy of various gastropod groups have been accumulated, mainly by the application of new methods (e.g., transmission electron microscopy) At the same time, studies of the deep-sea faunas associated with hydrothermal vents have brought the discovery of new gastropod groups with unusual anatomical features The evaluation of this newly gathered data in the light of the existing classification revealed a need for its revision Recent analyses of numerous morphological and developmental characters of living gastropods have resulted in a new classification scheme (Figure 8), which has been independently supported by results from molecular studies The placement of fossil gastropods into this classification of living gastropods has been difficult because of the lack of necessary anatomical characters Recent studies have revealed that Patellogastropoda (¼ Docoglossa, Cyclobranchia) represents the sister group to all other living gastropods Living patellogastropods with limpet-shaped shells are exclusively marine and occur mostly on rocky shores in all continents The Patellogastropoda and their coiled ancestors have been united into the subclass Eogastropoda All other living gastropods and their ancestors have been placed in the subclass Orthogastropoda, comprising four main groups of living gastropods: Neritimorpha, Archaeogastropoda, Caenogastropoda, and Heterobranchia (Figure 8) The Neritimorpha (¼ Neritopsina) is an ancient gastropod group with a long fossil record (Figures 2, 5, and 9), which colonized many different marine (shallow- and deep-water), freshwater, and terrestrial environments The Palaeozoic Cyrtoneritimorpha, with openly coiled early shells (Figures 2H and 3D), may represent a closely related group The living Archaeogastropoda unites the Vetigastropoda (Figures 3B and 3C) and several smaller groups, such as the Neomphaloidea, which occur in faunas associated with deep-sea hydrothermal vents The Archaeogastropoda have colonized almost all marine and estuarine environments There are also a number of extinct, mainly Palaeozoic groups (Figure 2) with uncertain relationships to living archaeogastropods The Palaeozoic Euomphaloidea (¼ Euomphalomorpha; Figures 2P and 3E), known mainly from shallowwater, marine environments, may be a sister or basal group of the Archaeogastropoda The Caenogastropoda and Heterobranchia are sister groups which are united in the taxon Apogastropoda Both groups are highly diverse and have colonized almost all marine, freshwater, and terrestrial environments The Palaeozoic Subulitoidea and Peruneloidea (Perunelomorpha) (Figures 2I and 3L) may be ancestral or basal groups of the Caenogastropoda or of all Apogastropoda The extant Caenogastropoda unites the two orders Architaenioglossa and Sorbeoconcha Terrestrial Cyclophoroidea and freshwater Ampullarioidea form the Architaenioglossa On the other hand, the mostly marine Sorbeoconcha represents a highly diverse group uniting