Evolution of cephalopods

The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata,^[1] and purported stem-group representatives present in the earliest Cambrian lagerstätten.^[2][3]

The class developed during the middle Cambrian, and underwent pulses of diversification during the Ordovician period^[4] to become diverse and dominant in the Paleozoic and Mesozoic seas. Small shelly fossils such as Tommotia were once interpreted as early cephalopods, but today these tiny fossils are recognized as sclerites of larger animals,^[5] and the earliest accepted cephalopods date to the Middle Cambrian Period. During the Cambrian, cephalopods are most common in shallow near-shore environments, but they have been found in deeper waters too.^[6] Cephalopods were thought to have "undoubtedly" arisen from within the tryblidiid monoplacophoran clade.^[7] However genetic studies suggest that they are more basal, forming a sister group to the Scaphopoda but otherwise basal to all other major mollusc classes.^[8][9] The internal phylogeny of Mollusca, however, is wide open to interpretation – see mollusc phylogeny.