Neocephalopoda
Encyclopedia
Neocephalopods are a group of cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

 mollusks that include the coleoids and all extinct species that are more closely related to extant coleoids than to the nautilus
Nautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...

. In cladistic terms, it is the total group of Coleoidea. In contrast, the palcephalopoda are defined as the sister group to the neocephalopoda.

The difference between modern coleoids and nautiluses is great today, but the fossil neocephalopods show that many of the differences accumulated piecemeal. These fossil forms include belemnoids, ammonoids, the bactritids, and some orthocerid-type nautiloid
Nautiloid
Nautiloids are a large and diverse group of marine cephalopods belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and...

s.

Taxonomic history

The name Neocephalopoda was first published (in Lehmann & Hillmer, 1980) as an Infraclass, which (in a reversal of the usual Linnean
Linnean
Linnean is closely associated to the alternative spelling Linnaean, and can refer to any of the following all of which are related to the original system of scientific taxonomy of biological species or its author:-Linnaean:*Carolus Linnaeus...

 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
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

 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
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 comprising ammonoids and coleoids. Later, Engeser (1996) applied it to the total-group coleoids as described above. Thus he included not only the coleoids, ammonoids, and bactritids, but also some orthocerid families: Michelinoceratidae, Sphaerorthoceratidae
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, but had shorter necks to their septa that were orthochoanitic or suborthochoanitic....

, 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
    R/K selection theory
    In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity or quality of offspring...

     strategy of producing a large number of small offspring, as seen in most modern coleoids, rather than the K-selected
    R/K selection theory
    In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity or quality of offspring...

     strategy of the nautilus.

  • The protoconch
    Protoconch
    A protoconch is an embryonic or larval shell of some classes of molluscs, e.g., the initial chamber of an ammonite or the larval shell of a gastropod...

    , 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
    Orthocone
    An orthocone is a usually long straight shell of a nautiloid cephalopod. During the 18th and 19th centuries, all shells of this type were named Orthoceras, but it is now known that many groups of nautiloids developed or retained this type of shell....

     nautiloids dating back to the Ordovician
    Ordovician
    The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

     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
    Cambrian
    The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

     and Early Ordovician
    Ordovician
    The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

     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
Ascocerida
The Ascocerida are comparatively small, bizarre nautiloids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longiconic juvenile portion and an inflated breviconic adult portion that separate sometime in...

 and pseudorthocerids
Pseudorthocerida
Pseudorthocerida are generally straight longiconic nautiloids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous parietal lining.. Cameral deposits are common and concentrated ventrally...

 have a cicatrix and so do not belong among the neocephalopods. Conversely, true orthocerids
Orthocerida
Orthocerida is an order of extinct nautiloid cephalopods also known as the Michelinocerda that lived from the Early Ordovician possibly to the Late Triassic . A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous...

 and lituitids
Lituitida
The Lituitida are the Lituitidae of the Treatise , reranked as an order and combined with other orthoceratoids.. They are considered to be more closely related to the Orthocerida than to the Ascocerida or Pseudorthocerida which are also included.Lituitids are characterized by smooth to annulate...

lack a cicatrix and have a round protoconch, which suggest neocephalopod affinities.

External links

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