Mollusca		Cephalopoda
CYRTOSOMA		Cephalopoda

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Cephalopoda

CYRTOSOMA  
|--+--SCAPHOPODA
|  `--CEPHALOPODA  
|     |--Plectronocerida  
|     `--+--Ellesmerocerida  
|        |--Endocerida  
|        |--Actinocerida  
|        |--Pseudorthocerida  
|        |--Discosorida  
|        `--+--NAUTILOIDEA
|           `--NEOCEPHALOPODA
|              |--Ammonoidea
|              `--COLEOIDEA
|                 |--DECAPODIFORMES
|                 `--VAMPYROPODA
`--Gastropoda

Biology

All cephalopods are carnivorous, feeding primarily on fish, other mollusks, Crustacea, and worms.  The head projects into a crown of prehensile tentacles - ranging from 8 in the octopus to 80 or 90 in the living nautilus.  These tentacles are actually a specialized form of the standard molluscan foot, and used for grasping prey.  Once the prey is snared it is bitten with strong beak-like jaws and pulled into the mouth by the radula.

Cephalopods are quite large by molluscan standards  (most species being between 6 and 70 cm including tentacles), with the giants of the group - such as the modern day Architeuthis, the giant squid, with a body length (including tentacles) of up to 16 meters, the Ordovician endocerid nautiloid Cameroceras, with a straight shell up to 10 metres in length, and the Cretaceous ammonoid Pachydiscus seppenradensis, with a coiled shell 3 metres in diameter - the largest invertebrates ever to live, with weights of one to two tons.  Such giant cephalopods play or played a similar ecological role of top predator to that of Devonian arthrodire placoderms, Mesozoic pliosaurs and Cenozoic toothed whales.

Cephalopods have a highly developed nervous system, unequalled among the invertebrates, and correlated with locomotor dexterity and carnivorous lifestyle (predators generally always have larger brains than prey animals).  There is a high level of cephalization (development and concentration of sensory and neural centers in the head).  The nerve ganglia are concentrated and more or less fused to form a brain that encircles the esophagus.  A bundle of giant nerve fibres tied to the mantle give them very rapid reflexes.  They are visual creatures, changing colour to express mood.  The eyes of the Coleoidea are very elaborate, with a retinal structure remarkably like that found in vertebrates.  The eye of the giant squid is the largest of any animal - 40 cm across. Nautiloids have smaller and more primitive eyes.

As with vertebrates, the brain is partitioned into different areas that control particular functions.  for example the brain centres for both forward swimming and closing of the suckers are located in the cerebral ganglia.  These creatures are most intelligent and highly evolved of the mollusks, indeed they are the most intelligent of all the invertebrates, exhibiting complex patterns of behavior.  Octopi for example can easily be trained to distinguish between classes of objects.

Evolutionary History

The cephalopods first appeared in the Furongian.  The first forms had gently curved shells.  During the Ordovician the group underwent an astonishing evolutionary radiation, possibly due to the new ecological niches made possible by the extinction of anomalocarids at the end of the Cambrian.   Some eight new orders appeared.  There was tremendous diversity among them.  Some had long straight shells, short straight ones, curved, lightly coiled, and tightly coiled ones evolved.  The internal structure of the shell differed greatly as well, mostly in the structure of the siphuncle.  Most were probably relatively slow movers, at least compared to today's forms.   The largest ones had huge straight shells that reached 3 to 5 or even 10 metres in length.  All these early forms are classed under the paraphyletic and probably artificial taxon Nautiloidea.

The Nautiloid Cephalopods remained unchallenged through the Ordovician and Silurian, finally giving way to the large predatory fish of the late Devonian.  About this time the ammonoids began to take over from the nautiloids.  The ammonoids are rare in the early Devonian, but by the end of the period and the beginning of the Carboniferous they increase greatly in diversity.  During this time, all but two the remaining Nautiloid orders die out.  The Coleoidea meanwhile make their first appearance in the Late Mississippian (middle Carboniferous) but remain rare.

 The end-Permian extinction exterminated all but a single family of Ammonoids.  But these adaptable mollusks recovered strongly as the Mesozoic dawned, and the Ceratite lineage appeared with a great evolutionary radiation during the Triassic.  So successful were these creatures that the Triassic period has been called "The Age of Ceratites".  Over 80 families are known from this time.  Another mass extinction at the end of the Triassic saw the demise of the Ceratites, along with the last remaining straight-shelled Nautiloids (the pseudorthocerids).  At this time new groups of Ammonoids with much more complex sutures ("ammonites" in the strict sense) took over.

As well as ammonites, the squid-like Belemnites, representing the Coleoidea, also underwent a huge evolutionary radiation as the Jurassic dawned.  The first representatives of modern coleoid groups like octopus and squid were other groups of cephalopods that appeared during the Jurassic, but being soft-bodied and only very rarely preserved it is not certain from the fossil record how common they were.  But there is no denying that Ammonoids, belemnoids, and proto- modern-style coleoids all formed a very significant part of the Jurassic and Cretaceous nektonic marine ecosystems.  The Ammonoids and belemnoids were to remain highly successful until the end of the Cretaceous, where the same extinction event that killed off the dinosaurs and the other the Mesozoic megafauna also exterminated the Ammonoids.  A few belemnoids straggled on until the Eocene, but they were now heavily out-competed by the modern Coleoidea (octopus, squid, cuttlefish, etc).

The Coleoidea remain an important and remarkably successful group of marine invertebrates to this day.  Meanwhile, only a few species of pearly nautilus continue as the last survivors of the once important Nautiloidea.

Systematics

The class Cephalopoda has been traditionally divided into three subclasses on the basis  of shell structure, or two subclasses on the basis of gills and other soft parts.    As shown here:

Tetrabranchia four gills primitive forms external shell	Nautiloidea
	Ammonoidea
Dibranchiata two gills advanced forms reduced or absent shell	Coleoidea

The trouble with this scheme is that it is impossible to know from the fossil shells alone whether all the extinct forms had two or four gills, or anything really much about their soft body structure.   Moreover just from shell structure alone it became obvious that the simplistic three-fold classification was wanting.  Certainly the Nautiloidea appear to be not a single subclass but a very amorphous, paraphyletic group, so much so that the term Nautiloidea now really means "all cephalopods that are not ammonoids or coleoids"

Thus, while there is some agreement regarding recent cephalopods, the classification of the various extinct forms is very uncertain, precisely due to this fact that (with one or two rare exceptions known from the Devonian, and a Jurassic ammonite that preserved muscle attachment scars) the soft body parts are not known.  The following list basically follows Curt Teichert "Main Features of Cephalopod Evolution", pp.19-20, in The Mollusca vol.12, Paleontology and Neontology of Cephalopods, ed. by M.R. Clarke & E.R. Trueman, Academic Press, Harcourt Brace Jovanovich, 1988, except that (a) Teichert's two Subclasses Endoceratoidea and Actinoceratoidea have been discarded, since the two orders Endocerida and the Actinocerida are probably not so distinct from their contemporaries as to justify such a high taxonomic ranking; and (b) I have incorporated it with the Palcephalopoda/Neocephalopoda Hypothesis.  For a list of various cephalopod classifications see Theo Engeser's pageThe Classification of the Fossil Nautiloidea s.l. and The problems of characterizing higher taxonomic units in the Cephalopoda

Class Cephalopoda

(Nautiloidea "sensu lato") here

Subclass: Palcephalopoda

Mostly equivalent to the old term "Nautiloidea".  Reproduction strategy mostly K-selected with a few well-developed large offspring and long-lived adults. Yolk-rich development (fewer larger eggs).  Radula with 9 teeth and 4 marginalia ("Lateradulata").  Four gills.  Early forms probably had ten arms but later many more arms developed.  The arms lack adhesive suckers.  The phragmocone is well developed and large, originally slightly curved with the siphuncle was situated between the center and the ventral surface.  Siphuncle generally large with internal deposits, with many specialized forms developed that can be distinguished according to the structure of the siphuncle

          Infraclass unnamed ("Ellesmeroceroidea"? or several infraclasses?)

          Infraclass Nautiloidea Agassiz, 1947 (= Nautiloidea "sensu stricto")

Subclass: Neocephalopoda

Evolved from Palcephalopoda.  Includes a number of lineages with reduced internal shells, and even some (e.g. octopoids and their relatives) that discarded their shell altogether.  Radula with seven teeth and two marginalia per row ("Angusteradulata").  Reproduction mostly strategy r-selected with many small planktonic offspring, although some advanced forms (e.g. Octopoda) k-selected with yolk-rich development.  Mostly ten arms, which generally possess hooks (Belemnites) or adhesive suckers (advanced Coloidea).  Siphuncle (in those types that retain their shell) thin and empty.  Additional layers on outside of shell.  Phragmocone (the shell) originally straight with the siphuncle was situated at or near the center.  Later the position of the siphuncle shifted to the ventral surface, (Bactritida), the shell became coiled (Ammonoidea), internal or reduced or absent (Coleoidea).   Most extinct forms presumably (like recent forms) with only two gills

          Infraclass Orthoceratoidea Kuhn, 1940

Ammonoidea here

          Infraclass Ammonoidea Agassiz, 1947

(Coleoidea) here

Links

Links

The Cephalopod Page - Cool site.  Beautiful photos, well-written and informative text, and heaps of links.  Recommended.  Mostly recent species but there is a page on Ammonites.

Cephalopoda

Old Calamari - Kevin Bylund's site on fossil Cephalopoda. Covers Nautiloids and Ammonoids of Utah (arranged according to geological age), as well including some useful cephalopod and paleontology links.

Cephalopods

All the recent cephalopod shells are composed of aragonitic layers : prismatic layers, nacreous layers and spherulitic prismatic layers.  It must be noticed that the "prismatic" layers in Cephalopod shells are not similar to those of other mollusks ; they are always aragonitic, and composed of small fibres, resembling to the fibres of the coral skeletons.