Neocephalopods are a group of

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

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

s that include the

coleoid Subclass (biology), Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, ...

s and all extinct species that are more closely related to

extant Extant 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 * Extant taxon, a taxon which is not extinct, ...

coleoids than to the

nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in t ...

. In

cladistic Cladistics (; ) 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 typically shared derived char ...

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 (, ; plural radulae or radulas) is an anatomical structure used by molluscs 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 ...

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 A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

comprising ammonoids and coleoids. Later, Engeser (1996) included not only the coleoids, ammonoids, and bactritids, but also some orthocerid families: Michelinoceratidae, Sphaerorthoceratidae, 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, ''r''/''K'' 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 individ ...

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

K-selected In ecology, ''r''/''K'' 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 individ ...

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 an unusually long straight shell of a nautiloid cephalopod.; During the 18th and 19th centuries, all shells of this type were named ''Orthoceras'', creating a wastebasket taxon, but it is now known that many groups of nautiloids d ...

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. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

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 Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

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, partic ...

cephalopods did not evolve until approximately the

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 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 shortest period of the Paleozo ...

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

stem Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushro ...

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