Systematic Paleobiology
Kingdom: Animalia
Phylum: Mollusca
Class : Cephalopoda
Cuvier, 1797
- Subclass: Nautiloidea
- Subclass: Ammonoidea
- Subclass: Coleoidea
The Cephalopoda comprises a class of molluscs characterized by muscular arms, or tentacles, that project from the head and surround the mouth. Three divisions, or subclasses, are involved, the Ammonoidea which are extinct and the Nautiloidea and Coleoidea which have modern representatives.
The Nautiloidea and Ammonoidea have external, chambered shells and are thus ectocochliate. The Nautiloidea have their beginning in the Late Cambrian and after a near extinction diversified into some 8 orders in the Ordovician only to more or less steadily decline there after. The Ammonoidea, which are derived from the nautiloid Orthocerida, have their beginning in the Devonian and after great diversification went extinct at the end of the Cretaceous.
Coleoids differ in that their shells, if present, are internal. They include recent squid, octopus, and cuttlefish.
Evolution[]
Cephalopods made their first appearance in the Late/Upper Cambrian with the ancestral Nautiloidea and diversified in the following Ordovician [1] to become diverse and dominant in the Paleozoic and Mesozoic seas.[2] During the Late Cambrian, cephalopods were most common in shallow near-shore environments, but they have been found in deeper water sediments as well.[3] Cephalopods were thought to have arisen from within the tryblidiid Monoplacophora.[4] 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.
Fossil orthoconic nautiloid from the Ordovician of Kentucky; an internal mold showing siphuncle and half-filled camerae, both encrusted.
Cephalopods are thought to have evolved from a monoplacophoran-like ancestor[6] with a curved, tapering shell,and to be closely related to gastropods (snails). The development of a siphuncle allowed their shells to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the sea floor, and separates the true cephalopods from putative ancestors such as Knightoconus, which lacked a siphuncle. Buoyancy (i.e. the ability to float) came later, followed by swimming in the Plectronocerida and eventually jet propulsion in more derived cephalopods.[7] However, because chambered shells are found in a range of molluscs - monoplacophora and gastropods as well as cephalopods - a siphuncle is essential to ally a fossil shell conclusively to the cephalopoda. The earliest such shells do not have the muscle scars which would be expected if they truly had a monoplacophoran affinity.
The earliest cephalopod order to emerge was the Plectronocerida, which were quite small, their shells slightly curved, the internal chambers closely spaced.[1] Early cephalopods had fine shells which could not cope with the pressures of deep water.[1]which gave rise to the Ellesmerocerida. These were supplemented in the mid Tremadoc by larger typically straight shelled Endocerida with large siphuncles and coiled Tarphycerida with more narrow siphuncles.[1] By the mid Ordovician these orders are joined by the orthocerids characterized by longer chambers and narrow central siphuncles with thin connecting rings and lituitids which begin coiled but become straight during growth. The oncocerids also appear during this time; they are restricted to shallow water sediments and have short conchs (shells).[1] 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 -200m.[1] The wide-siphuncled Actinocerida and the Discosorida both emerged during the Darriwilian.[1] The direction of curvature would prove to be crucial to future evolution of various lineages. Endogastric curvature with the hyponomic sinus on the inside curvature prohibited the development of coiled forms, whereas exogastric curvature with the hypononic sinus on the outer curvature allowed for the evolution of coiling.
Cephalopods are predators at or near the top of the food chain.[8]
An ammonitic ammonoid with the body chamber missing, showing the septal surface (especially at right) with its undulating lobes and saddles.
The Ammonoidea and Coleoidea, (including extinct Belemnoidea and modern Neocoleoidea), both diverged from the nautiloidea during the middle Paleozoic between 450 and 300 million years ago, although the coleoids may be polyphyletic.[9] 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. However, some of the straight-shelled nautiloids evolved into belemnites, out of which some evolved into squid and cuttlefish. The loss of the shell may also have resulted from evolutionary pressure to increase manoeuvrability, resulting in a more fish-like habit.[9] This pressure may have increased as a result of the increased complexity of fish in the late Palaeozoic, increasing the competitive pressure.[9]Internal shells still exist in many non-shelled living cephalopod groups but most truly shelled cephalopods, such as the ammonites, became extinct at the end of the Cretaceous.
The tentacles of ancestral cephalopods developed from the molluscan foot and is thought to have involved five pairs which surrouned the mouth [10]
The preservation of cephalopod soft parts is not entirely unusual; soft-bodied fossils, especially of coeloids (squid), are relatively widespread in the Jurassic,[11] but phosphatised remains are unknown before this period.[12]
Classification[]
Pyritized fossil of Vampyronassa rhodanica, a vampyromorphid from the Lower Callovian 164.7 Ma.
The classification shown here follows largely from Current Classification of Recent Cephalopoda [7] (May 2001), plus that regarding fossil groups from several sources. The three subclasses are traditional, corresponding to the three orders of cephalopods proposed by Bather.[13]
Shevyrev, (2005) [14] subdivided the cephalopoda into eight subclasses, shown with their respective orders
Ellesmeroceratoidea Flower 1950, with orders
- Plectronocerida
- Protactinocerida
- Yanhecerida
- Ellesmerocerida
Subclass Endoceratoidea Teichert, 1933, including
- Endocerida
- Intejocerida
Subclass Actinoceratoidea Teichert, 1933 for the
- Actinoceratida
Subclass Nautiloidea Agassiz, 1847, including
- Basslerocerida
- Tarphycerida
- Lituitida
- Discosorida
- Oncocerida
- Nautilida
Subclass Orthoceratoidea Kuhn, 1940, including
- Orthocerida
- Ascocerida
- Dissidocerida
- Bajkalocerida
Subclass Bactritoidea Shimansky, 1951 Subclass Ammonoidea Zittel, 1884 Subclass Coleoidea Bather, 1888
Thomas Berthold and Theo Engeser [15] divided the Cephalopoda into two primary groups based on the radula, referred to as Palcephalopoda and Neocephapoda. Most nautiloids belong in the Palcephalopoda. The Neocephalopoda includes modern coleoids, belemnoids, ammonoids, and many orthocerid families.[16]
References[]
- ^ a b c d e f g Pulsed cephalopod diversification during the Ordovician. Palaeogeography, Palaeoclimatology, Palaeoecology 1 March 2009. [1]
- ^ name="Dzik1981">J. Dzik, 1981. Origin of the Cephalopoda. Acta Palaeontologica Polonica 26:2 p161-191
- ^ Ed Landing 2009. The Oldest Cephalopods from East Laurentia. Journal of Paleontology Vol 83
- ^ Main features of cephalopod evolution, The Mollusca vol.12. Palaeontology and Neontology of Caphalopods M.R. Clarke and E.R.; Trueman Eds. Academic Press 1988.
- ^ Giribet et al. Evidence for a clade composed of molluscs with serially repeated structures: monoplacophorans are related to chitons. Proceedings of the National Academy of Sciences of the United States of America. [2](Relevance questioned)
- ^ H. Lemche and K.G. Wingstrand 1959. The Anatomy of Neopilina galatheae Lemche, 1957 (Mollusca, Tryblidiacea). Galathea Rep. Vol 3,
- ^ Kroger B. 2007. Some Lesser Known Features of the Ancient Cephalopod Order Ellesmerocerida (nautiloidea, Cephalopoda). Palaeontology Vol 50, issue 3 pp 565–572 doi 10.1111/j.1475-4983.2007.00644.x
- ^ Peter Boyle & Paul Rodhouse, 2004. Cephalopods: ecology and fisheries. Blackwell Publishers, Ames,Iowa. [3]
- ^ a b c Wilbur, Karl M.; Trueman, E.R.; Clarke, M.R., eds. (1985), The Mollusca, 11. Form and Function, New York: Academic Press, ISBN 0-12-728702-7
- ^ Shigeno et al 2008 Evolution of the cephalopod head complex by assembly of multiple molluscan body parts: Evidence from Nautilus Embryonic Development. Journal of Morphology Vol 269, no 1. pp 1-17. [4]}}
- ^ Kear, A.J., D.E.G Briggs,& D.T. Donovan 1995. Decay and fossilization of non-mineralized tissue in coleoid cephalopods. Palaeontology 38:1 pp104-132. [5]
- ^ doi:10.1144/gsjgs.150.6.1035
- ^ Bather, F.A. 1888b Professor Blake and Shell-Growth in Cephalopoda. Annals and Magazine of Natural History. Series 6, Vol. 1 pp 421–426
- ^ Shevyrev, A.A. 2005. The Cephalopod Macrosystem: A Historical Review, the Present State of Knowledge, and Unsolved Problems: 1. Major Features and Overall Classification of Cephalopod Mollusks. Paleontological Journal V39:6 pp606-614
- ^ Berthold & Engeser 1987. Phylogenetic analysis and systematization of the Cephalopoda (Mollusca). Verhandlungen Naturwissenschaftlichen Vereins in Hamburg. (NF) Vol 29, pp187=-220
- ^ Engeser, Theo 1997. Fossil Nautiloidea Page.[6]
Further reading[]
- A comprehensive overview of Paleozoic cephalopods: Barskov et al 2008.
Cephalopods in the marine ecosystems of the Paleozoic. Paleontological Journal Vol 42, pp 1167–1284