The earliest caenogastropods appear in late Silurian and early Devonian rocks from about 400 million years ago. From this initial appearance, the caenogastropods have radiated into the most diverse gastropod group living today. This diversity spans every aspect of biodiversity and includes morphology, habitat, behavior, and reproductive mode, among others. Partly because of this tremendous diversity, they were mistakenly divided into two groups: the Mesogastropoda and the Neogastropoda. However, today the term "mesogastro-pod" is understood to be just a level or grade of gastropod evolution, not an evolutionary lineage. In contrast, the Neogastropoda do represent a monophyletic group within the Caenogastropoda and first appear during the Cretaceous, although the apex of their diversification is in the Upper Cretaceous and during the Tertiary. Another recognized clade within the Caenogastropoda is the Architaenoglossa. Members of the Architaenoglossa were previously considered to be "lower mesogastropods," but are now recognized as the sister taxa of the Sorbeoconcha (which also includes the Neogastropoda). Thus, the Caenogastropoda are recognized as containing the Architaenoglossa and the Sorbeoconcha. The Architaenoglossa contain the freshwater group Am-pullarioidea and the terrestrial group Cyclophoroidea. The Sorbeoconcha consist of all remaining caenogastropods. Finer level relationships within the Caenogastropoda remain problematic despite ongoing morphological and molecular studies. This lack of resolution based on morphological characters occurs in part because the earlier variation seen in or gan systems and structures of other groups (e.g., the kidney/heart complex, radula) has become relatively fixed in caenogastropods (and especially in the Sorbeoconcha). Where novelty has evolved, it is often similar across taxa and confounds convergent evolution. In molecular studies, appropriate molecular markers for the divergence times within this taxon have yet to be identified.
The Caenogastropods are composed of the Architaenoglossa and the Sorbeoconcha. The Architaenoglossa include the Ampullarioidea and Cyclophoroidea, and the Sorbeo-concha consist of the Cerithiomorpha, Littorinimorpha, Pte-noglossa, and Neogastropoda. There are about 100 families within these smaller clades.
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