Actinocerida
| Actinocerida Temporal range: Lower Ordovician–Devonian |
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| General form of an Actinocerid | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Subclass: | Nautiloidea |
| Order: |
Actinocerida Teichert, 1933 |
The Actinocerida comprise an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial canals and a narrow space along the inner side of the connecting ring known as a paraspatium. (Teichert 1964) Septal necks are generally short and cyrtochoanitic, some being recumbent, some hook shaped. Most grew to lengths of about 60 to 90 cm (2.0 to 3.0 ft) but some, like the Huroniidae of the Silurian grew significantly larger.
The Actinocerida inhabited shallow to quite deep waters, where they alternated between swimming and lying on the bottom. They were predatory, and able to control their buoyancy to a greater degree than their contemporaries.
The derivation of the Actinocerida remains enigmatic. They first appear late in the Early Ordovician (Cassinian Stage, late Floian) with the Georginidae but don't become well established until the beginning of the early Whiterockian Stage (Dapingian) of the Middle Ordovician (Flower 1868,1976) The Georginidae, introduced and described by Mary Wade in 1977 (Wade 1988)., based on te genus Georgina, are known from the upper Canadian Coolibah Formation of the Georgina Basin in Northern Australia.How the Georginidae relate to older stocks is unclear. The Upper Cambrian Protactinocerida have been suggested as being ancestral but none are known to have gone beyond the near end the Cambrian extinction, which makes any connection hypothetical.
Polydesmia was once thought to be the ancestral form of the actinocerids, and was derived from the ellesmeroceriid Bathmoceras. However, it turned out based on a reassessment of Lower Ordovician and Whiterockian formations in northeastern China that Polydesmia is antedated by Wutinoceras, its assumed primitive nature rather a derived condition.
Actinocerids first appeared early in the Middle Ordovician, with the exception of the Georginidae, which are known from the Cassinian in Northern Australia. They reached their greatest diversity in the Middle Ordovician with more than 20 genera, then declined somewhat in the Late Ordovician and more so in the Early Silurian; made a slight come back in the Middle Silurian but not to Late Ordovician numbers; and declined more or less steadily from the Late Silurian into the Devonian
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