similar to, or in some cases the same as, traditional Linnaean names for taxa above the level of superfamily. Whenever monophyly (a single ancestry) has not yet been tested and confirmed, or where a traditional taxon of gastropods has already been discovered to be paraphyletic (that it excludes some of its descendants) the term "group" or "informal group" has been used. Both Linnaean taxa and clades are invalid if it turns out they are polyphyletic, in other words if they consist of more than one lineage. In this taxonomy 611 valid families are recognized. Of these, 202 families are exclusively fossil, and this is indicated here with a dagger †. Superfamily names are standardized so that they all end in the suffix "-oidea", also commonly used for superorders and subclasses, replacing the "-acea" ending found especially in the older literature. Classification of families into subfamilies however is often not well resolved, and should be regarded as the best possible hypothesis. The publication Bouchet & Rocroi (2005) also includes a nomenclator of about 2400 suprageneric taxa (taxa above the level of genus) of gastropods, from the subtribe to the superfamily. A full bibliographic reference is provided for each taxon, giving the name of the authority, the original publication, the date of that publication, the type genus for the taxon, its nomenclatural status, and its validity under the rules of the ICZN. Since the publication of this taxonomic system in 2005, various changes have been published by other authors, for more information see changes in the taxonomy of gastropods since 2005. Main clades, groups and informal groups Here the information is displayed in the form of a cladogram (an evolutionary tree of descent.) It is worth bearing in mind however that this taxonomy is provisional: many of the taxa are still only known as "groups" or "informal groups", and these are very likely to be changed as more information becomes available. This cladogram is based on the following information. The list format used below makes clear which taxa are informal groups rather than clades: Paleozoic molluscs of uncertain systematic position Clade Patellogastropoda Clade Vetigastropoda Clade Cocculiniformia Clade Neritimorpha Paleozoic Neritimorpha of uncertain systematic position Clade †Cyrtoneritimorpha Clade Cycloneritimorpha Clade Caenogastropoda Caenogastropoda of uncertain systematic position Informal group Architaenioglossa Clade Sorbeoconcha Clade Hypsogastropoda Clade Littorinimorpha Informal group Ptenoglossa Clade Neogastropoda Clade Heterobranchia Informal group Lower Heterobranchia Informal group Opisthobranchia Clade Cephalaspidea Clade Thecosomata