Evolution of cephalopods

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

[1] The class developed during the middle Cambrian, and underwent pulses of diversification during the Ordovician period[2] to become diverse and dominant in the Paleozoic and Mesozoic seas.

The cephalopods were once thought to have evolved from a monoplacophoran-like ancestor[8] with a curved, tapering shell,[9] and to be closely related to the gastropods (snails).

[10] 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.

[14] 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.

[14] Tannuella is the oldest fossil to have been assigned to the cephalopods, dating from the Early Cambrian (Atdababian and Botomian), ~522 million years ago.

[13] The earliest true cephalopod order to emerge was the Ellesmerocerida, which were quite small; their shells were slightly curved, and the internal chambers were closely spaced.

[13] On the basis of muscle scars preserved in such genera as Paradakeoceras and Levisoceras, these animals are reconstructed with a straight body and dorsal shell, with the head at the anterior, concave surface of the shell, and the funnel (consisting of a pair of folds in the foot at the rear), not juxtaposed with the head as in later, oncocerid-like forms.

[2] By the mid Ordovician these orders are joined by the Orthocerids, whose first chambers are small and spherical, and Lituitids, whose siphuncles are thin.

[2] The mid Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100–200 m.[2] The wide-siphuncled Actinocerida and the Discocerida both emerged during the Darriwilian.

[20] In the Early Palaeozoic, their range was far more restricted than today: They were mainly constrained to sub-littoral regions of shallow shelves of the low latitudes, and usually occur in association with thrombolites.

[21] A number of fossils have historically been considered to represent components of the cephalopods' history, but been reinterpreted on the basis of additional material.

However discoveries of more detailed fossils showed that Volborthella’s small, conical shell was not secreted but built from grains of the mineral silicon dioxide (silica); neither was it septate.

One such genus is Shelbyoceras, which was reclassified based on a depressed groove that forms a band around the shell, which is similar to a feature seen in Hypseloconus.

[36] The Kirengellids are a group of shells that, whilst originally aligned to the monoplacophoran ancestry of the cephalopods, have been reinterpreted as brachiopods.

It is thought that competitive pressure from fish forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonise shallow waters.

The Early Devonian Naefiteuthis has been interpreted as the earliest fossil coleoid, and its shell may be in a partly internalized state.

[35] The Mazon Creek biota contains a decapod, Jeletzkya, which had ten arms, but the status of its shell is ambiguous as it has not been extracted from the concretion that preserves the only fossil.

[42]: 289 The preservation of cephalopod soft parts is not entirely unusual; soft-bodied fossils, especially of coeloids (squid), are relatively widespread in the Jurassic,[46] but phosphatized remains are unknown before this period.

[47] On the other hand, soft parts – including a possible ink sac – are known from the Paleozoic Hunsrück Slate and Francis Creek shale.