Cephalopoda
From Palaeos
CYRTOSOMA |--+--SCAPHOPODA | `--CEPHALOPODA | |--Plectronocerida | `--+--Ellesmerocerida | |--Endocerida | |--Actinocerida | |--Pseudorthocerida | |--Discosorida | `--+--NAUTILOIDEA | `--NEOCEPHALOPODA | |--Ammonoidea | `--COLEOIDEA | |--DECAPODIFORMES | `--VAMPYROPODA `--Gastropoda
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[edit] Biology
The name Cephalopoda literally means "head feet" and refers to the fact that these animals have a foot (actually a cluster of tentacles) directly abutting their head. The group includes cuttlefish, octopods, squid, the pearly nautilus, and a large number of ancient (mostly Paleozoic and Mesozoic) forms. All are active marine predators (although some early types were drifters), able to swim swiftly, and easily competing with fish in the marine habitat. There are 650 living species, but more than 7,500 fossil forms are known (and as in all cases like this this number is obviously a gross underestimate of the real number of cephalopod species that have ever lived through the Phanerozoic time). Like fish they are equipped with highly developed eyes and other sense organs, include both active swimmers and bottom-dwellers, and in many cases have a streamlined body for more efficient locomotion. Swimming is by rapidly expelling water from the mantle cavity. The water is forced out through a funnel or siphon - the hyponome - actually a tube-like flap of modified foot, thus driving the animal in the opposite direction. This is the key to the so-called "jet-propulsion" of these animals The funnel is highly maneuverable and can be directed in any direction, allowing motion backwards or forwards. However, the fastest movement is backward escape swimming, with powerful contractions of the mantle ejecting water through the forward facing funnel. A cloud of "ink" can also be ejected as a sort of underwater smoke screen to hide the fleeing animal.
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. Octopods can easily be trained to distinguish between classes of objects.
[edit] Evolutionary History
The cephalopods first appeared in the late Cambrian. 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 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.
[edit] 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.
[edit] Class Cephalopoda
[edit] 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?)
Order Plectronocerida Flower, 1964
Order Yanhecerida Chen & Qi, 1979 (or included in Plectronocerida or Ellesmerocerida?)
Order Protactinocerida Chen & Qi, 1979 (or included in Plectronocerida or Ellesmerocerida?)
Order Ellesmerocerida Flower, 1950
Order Endocerida Teichert, 1933
Order Injetocerida Balashov, 1960 (or included in Endocerida?)
Order Discosorida Flower, 1950
Order Actinocerida Teichert, 1933
Order Pseudorthocerida
Infraclass Nautiloidea Agassiz, 1947 (= Nautiloidea "sensu stricto") Order Tarphycerida Flower, 1950 Suborder Tarphycerina Flower, 1950 Suborder Barrandeocerina Flower Order Oncocerida Flower, 1950 Order Nautilida Agassiz, 1847
[edit] 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 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 Order Orthocerida Kuhn, 1940 Order Ascocerida Kuhn, 1949
Infraclass Ammonoidea Agassiz, 1947 Order Bactritida Shimanskiy Order Anarcestida Miller & Furnish 1954 Order Goniatitida Hyatt 1884 Order Clymeniida Wedekind 1927 Order Prolecanitida Miller & Furnish 1954 Order Ceratitida Hyatt 1884 Order Phyllocerida Kuhn, 1940 Order Lytocerida Hyatt 1889 Order Ammonitida Agassiz 1847 Order Ancylocerida Wedman 1966
Infraclass Coleoidea Bather, 1888 Order Boletzkiyida Bandel, Reitner & Stürmer 1983 Order Aulococerida Wedman 1966 Order Belemnitida Zittel 1885 Order Phragmoteuthida Jeletzky 1964 Order Teuthida Naef 1916 Order Belemnoteuthida Stolley 1919 Order Sepiida Naef 1916 Order Vampyromorpha Grimpe 1917 Order Octopodida Leach 1818
[edit] Phylogeny
<==Cephalopoda | i. s.: Tapashanites | Pseudotirolites asiatica | Pseudogastrioceras | Gattendorfia | Ammonellipsites | Pseudogrammoceras | Denckmannia | Esericeras | Pleydellia subcompta | Lolliguncula | Belemnella | |--B. casimirovensis | |--B. lanceolata | `--B. occidentalis | Belemnitella | |--B. junior | |--B. langei | |--B. minor | `--B. schmidti Christensen & Schulz 1997 | Dimitobelus | |--D. hectori | `--D. stimulus | Rhabdoceras suessi Oravecz 1961 | Phoenixites frechi | Holzapfeloceras | |--*H. convolutum (Holzapfel 1895) | `--H. croyi House 1978 | Sobolewia | |--S. nuciformis (Whidborne 1890) | `--S. rotella (Holzapfel 1985) | Parodiceras | Werneroceras | Maenioceras terebratum (Sandberger & Sandberger 1856) | Tornoceras | Gomiprotomeroceras (nom. inv.) | `--G. acutum Sobolew 1914 | Wedekindella brilonensis (Kayser 1872) | Pseudoprobeloceras polonicum (Sobolew 1914) | Archoceras | |--A. tataense Bensaïd 1974 | `--A. varicosum (Drevermann 1901) | Ponticeras | Koenenites lamellosus (Sandberger & Sandberger 1856) | Acanthoclymenia genundewa (Clarke 1898) | Pharciceras | Epitornoceras mithracoides (Frech 1887) | Linguatornoceras [incl. Truyolsoceras] | |--L. clausum (Glenister 1958) | |--L. compressum (Clarke 1899) | `--‘Truyolsoceras’ undulatum (Sandberger & Sandberger 1856) | Manticoceras | |--M. adorfense (Wedekind 1913) | |--M. ammon (Keyserling 1844) | |--M. drevermanni (Wedekind 1913) | |--M. intumescens (Beyrich 1837) | |--M. inversum Wedekind 1913 | |--M. lamed (Sandberger & Sandberger 1850) | `--M. sinuosum | Aulatornoceras | |--A. auris (Quenstedt 1846) | `--A. belgicum (Matern 1931) | Beloceras acutodorsatum Dybczyński 1913 | Crickites holzapfeli Wedekind 1913 | Crassotornoceras belgicum (Matern 1931) | Siemiradzkia | Arkelloceras tozeri Frebold 1951 | Parachondroceras filicostatum Imlay 1967 | Imlayoceras Frebold 1963 | Talkeetnites Imlay 1980 | Longaeviceras | Chamoussetia | Xenocephalites [incl. Cadoceras (Oligocadoceras Meledina 1977)] | |--‘Arcticoceras’ crassicostatum | `--‘Arcticoceras’ loveanum | Goniocamax lundgreni | Gonioteuthis praewesfalica | Actinocamax verus | |--A. v. verus | `--A. v. antefragilis | Teicherticeras | |--T. desideratum | `--T. lissovi | Talenticeras | Mimosphinctes | Miklukhoceras pamiricum Pavlov 1967 | Perrinitidae | Prostacheoceras Ruzhencev 1937 [Cycloloboidea, Vidrioceratidae] | |--*P. juresanensis (Maximova 1935) [=Marathonites juresanensis] | |--P. alter Leonova 1989 | `--P. oshense (Toumanskaya 1938) | Paraceltites Gemmellaro 1887 | Stacheoceras Gemmellaro 1887 | Epiglyphioceras Spath 1930 | Schreyerites binodosus (Hauer 1851) | Paraceratites | |--P. brembanus (Mojsisovics 1882) | `--P. trinodosus (Mojsisovics 1882) | Synpharciceras clavilobum (Sandberger & Sandberger 1850) | Acrimeroceras falcisulcatum Becker 1993 | Prolobites | |--P. aktubensis Bogoslovsky 1969 | `--P. delphinus (Sandberger & Sandbergber 1850) | Clistroceras globosum Nassichuk 1967 | Nathorstites maconnelli (Whiteaves 1889) | Mayaites (Araucanites) stipanicici Westermann & Riccardi 1975 |--Plectronoceras |--Nautilida |--+--Endoceratoidea | `--Actinoceratoidea `--+--Bactritida [Bactritoidea] |--Aulacoceratida |--Ammonoidea `--Coleoidea
* Type species of genus indicated
[edit] References
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Credits
text © M. Alan Kazlev 1998-2002; taxonomy and references CKT071126
