Neocephalopods are a group of

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

s that include the

coleoid Coleoidea or Dibranchiata is one of the two subclasses of cephalopod molluscs containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e. octopus, squid and cuttlefish). Unlike its extant sister group Nautiloide ...

s and all extinct species that are more closely related to

extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...

coleoids than to the

nautilus A nautilus (; ) is any of the various species within the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae and the suborder Nautilina. It comprises nine living species in two genera, the type genus, ty ...

. In

cladistic Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...

terms, it is the

total group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

of Coleoidea. In contrast, the palcephalopoda are defined as the sister group to the neocephalopoda.

Taxonomic history

The name Neocephalopoda was first published (in Lehmann & Hillmer, 1980) as an Infraclass, which (in a reversal of the usual Linnean hierarchy) included the Subclasses Bactritoidea, Ammonoidea, and Coleoidea. Neocephalopoda was a new name for a group that had been recognized previously and called Angusteradulata (Lehmann, 1967), based on a discovery of an ammonoid

radula The radula (; : radulae or radulas) is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters ...

similar to modern coleoids. It was contrasted with the Lateradulata, which included the nautilus and most fossil nautiloids. These names were applied by later workers running cladistic analyses on the cephalopods. Berthold & Engeser (1987) adopted the name Angusteradulata for the

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

comprising ammonoids and coleoids. Later, Engeser (1996) included not only the coleoids, ammonoids, and bactritids, but also some orthocerid families: Michelinoceratidae,

Sphaerorthoceratidae Sphaeorthoceratidae is an extinct family of orthocone cephalopods that lived in the Paleozoic Era. They were similar to the nautiloid ''Michelinoceras'' in the Apex (mollusc), apex, but had shorter necks to their Septum (marine biology), septa t ...

, Arionoceratidae, "and probably other groups as well." Although the name Angusteradulata appeared earlier than Neocephalopoda, both names were coined by Lehmann, who preferred the latter. Engeser and Lehmann later both agreed that Neocephalopoda was a more suitable name than Angusteraduata.

Features of the neocephalopods

The following characters are thought to be shared, at least ancestrally, by neocephalopods. Most are from Engeser (1996) or from his Fossil Nautiloidea page. * The embryonic shell is small (about 1.5 to 3 mm), somewhat spherical, and separated from the rest of the shell by a constriction. This may reflect an

r-selected In ecology, selection theory relates to the selection of combinations of traits in an organism that trade off between quantity and quality of offspring. The focus on either an increased quantity of offspring at the expense of reduced individua ...

strategy of producing a large number of small offspring, as seen in most modern coleoids, rather than the

K-selected In ecology, selection theory relates to the natural selection, selection of combinations of Trait (biological), traits in an organism that trade off between quantity and quality of offspring. The focus on either an increased quantity of offsprin ...

strategy of the nautilus. * The

protoconch A protoconch (meaning first or earliest or original shell) is an embryonic or larval shell which occurs in some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod. In older texts it is also called " ...

, or initial chamber, lacks a cicatrix, a scar left by the original organic plate during the embryonic development of the nautilus and its fossil relatives. * The radula is more similar to coleoids, with 9 elements per row, than to the nautilus, with 13 elements per row. Ammonoids have been found with coleoid-like radulas, as have

orthoconic An orthocone is the long, cone-shaped shell belonging to several species of ancient nautiloid cephalopod—the prehistoric ancestors of today's marine cephalopod mollusks, including the cuttlefishes, nautiluses, octopuses and squids.; During the ...

nautiloids dating back to the

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

Period (Gabbott, 1999).

Classification

Although there is wide agreement that bactritids included the ancestors of ammonoids and coleoids, and hence that all three groups belong in one clade, the relations among some neocephalopods remain problematic. The greatest obstacles may lie among the nautiloids: * The classification of orthocerids is still jumbled and requires revision based on features of the embryonic shell. * Since cephalopod fossils from the

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

and Early

Periods are rare and usually incomplete, we do not know the ancestral states of many characters, making it difficult to assess which derived characters unite the neocephalopods and which other nautiloid groups belong among them. Nevertheless, recent descriptions of shells with preserved apical ends have aided in the classification of major taxa. For instance, ascocerids and pseudorthocerids have a cicatrix and so do not belong among the neocephalopods. Conversely, true orthocerids and lituitids lack a cicatrix and have a round protoconch, which suggest neocephalopod affinities. In contrast, some workers have suggested that

crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, parti ...

cephalopods did not evolve until approximately the

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

Period. If that is correct, then all early Paleozoic cephalopods must be

stem Stem or STEM most commonly refers to: * Plant stem, a structural axis of a vascular plant * Stem group * Science, technology, engineering, and mathematics Stem or STEM can also refer to: Language and writing * Word stem, part of a word respon ...

cephalopods and not neocephalopods.

Footnotes

References

*Berthold, Thomas, & Engeser, Theo. 1987. Phylogenetic analysis and systematization of the Cephalopoda (Mollusca). ''Verhandlungen Naturwissenschaftlichen Vereins in Hamburg''. (NF) 29: 187-220. *Engeser, Theo. 1996. The Position of the Ammonoidea within the Cephalopoda. In: ''Ammonoid Paleobiology'', Vol. 13 of ''Topics in Geobiology'', ed. by Neil Landman ''et al.'', Plenum Press, New York. Chapter 1, pp. 3–19. * * *Lehmann, U., & Hillmer, G. 1980. ''Wirbellose Tierre der Vorzeit Leitfaden der Systematischen Paläontologie.'' Ferdinand Enke Verlag, Stuttgart, Germany, 1–340. *Lehmann, U., & Hillmer, G. 1983. ''Fossil Invertebrates.'' Transl. by Janine Lettau from Lehmann & Hillmer 1980. Cambridge University Press, New York.

External links

The Palcephalopoda/Neocephalopoda Hypothesis
from Dr. Theo Engeser's Fossil Nautiloidea Page {{Taxonbar, from=Q1980811 Cephalopod taxonomy