Evolution of cephalopods

The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata.

The class developed during the middle Cambrian, and underwent pulses of diversification during the Ordovician period 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, 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. Cephalopods were thought to have "undoubtedly" arisen from within the tryblidiid monoplacophoran clade. 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. The internal phylogeny of Mollusca, however, is wide open to interpretation – see mollusc phylogeny.

The cephalopods were once thought to have evolved from a monoplacophoran-like ancestor with a curved, tapering shell, and to be closely related to the gastropods (snails). The similarity of the early shelled cephalopod Plectronoceras to some gastropods was used to support this view. The development of a siphuncle would have allowed the shells of these early forms to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the floor, and separated the true cephalopods from putative ancestors such as Knightoconus, which lacked a siphuncle. Negative buoyancy (i.e. the ability to float)^{[clarification needed]} would have come later, followed by swimming in the Plectronocerida and eventually jet propulsion in more derived cephalopods. However, because chambered shells are found in a range of molluscs – monoplacophorans and gastropods as well as cephalopods – a siphuncle is essential to ally a fossil shell conclusively to the cephalopoda. Chambered gastropods can be distinguished from cephalopod shells by the absence of a siphuncle, the irregular spacing of septa, the layering of the shell and (in younger or unmetamorphosed rocks) its microstructure, and the relatively thick width of the shell. The earliest such shells do not have the muscle scars which would be expected if they truly had a monoplacophoran affinity.

Understanding of early cephalopod origins is by necessity biased by the available fossil material, which on the whole consists of shelly fossils. Critical fossils are detailed below; since their stratigraphic age has guided the interpretation of the fossils, they are listed in descending order of age.

With the exception of the shelly genera Ectenolites and Eoclarkoceras, none of the 30+ Cambrian cephalopod genera are known to have survived into the Ordovician. Cambrian cephalopods differ from their descendants by account of their small size (a few centimetres in length); long, tapering shells; smooth shell surfaces; closely spaced septa; and lack of deposits in their body chamber; several more specific features are also only seen in certain groups of Cambrian cephalopod.

Tannuella is the oldest fossil to have been assigned to the cephalopods, dating from the Early Cambrian (Atdababian and Botomian), ~522 million years ago. Its position in this group is suggested based on its shape and the presence of chambers. Under this hypothesis, it would be a precursor to the hypseloconids and then genera such as Knightoconus that eventually gave rise to the cephalopods.

Knightoconus is a Late Cambrian monoplacophoran thought to represent an ancestor to the cephalopods. It had a chambered, conical shell, but lacked a siphuncle. Although earlier molluscan fossils are also septate, Knightoconus is the latest septate mollusc before the first siphunculate cephalopods – a point that has been taken to prove its relevance to the Cephalopoda. The absence of this siphuncle has been taken as evidence against cephalopod ancestry – how, it is argued, could a siphuncle evolve to penetrate existing septa? The prevailing argument suggests that a strand of tissue remained attached to the previous septum as the mollusc moved forwards and deposited its next septum, producing an obstacle to the complete closure of the septum and becoming mineralised itself. 10 or more septa are found in mature individuals, occupying around a third of the shell – septa form very early and have been found in specimens as small as 2 mm in length. Septa are uniformly spaced, which is inconsistent with a gastropod affinity. Unlike monoplacophoran fossils, there is no evidence of muscle scarring in Knightoconus fossils.

Plectronoceras is arguably the earliest known crown-group cephalopod, dating to the Upper Cambrian. Its 14 known specimens hail from the basal Fengshan Formation (north-east China) of the earliest Fengshanian stage. None of the fossils are complete, and none show the tip or opening of the shell. Approximately half of its shell was filled with septa; 7 were recorded in a 2 cm shell. Its shell contains transverse septa separated by about half a millimetre, with a siphuncle on its concave side. Its morphology matches closely to that hypothesised for the last common ancestor of all cephalopods, and the Plectronocerida have been said to be the ancestors of the Ellesmerocerids, the first "true cephalopods".