Nautiloid

Nautiloids are a group of marine mollusks in the subclass Nautiloidea, which all possess an external shell, the best-known example being the modern nautiluses. They flourished during the early Paleozoic era, where they constituted the main predatory animals, and developed an extraordinary diversity of shell shapes and forms. Some 2,500 species of fossil nautiloids are known, but only a handful of species survive to the present day.

Taxonomic relationships
The nautiloids are among the group of animals called the cephalopods (class Cephalopoda), which also includes ammonoids, belemnites and modern coleoids such as octopus and squid. The cephalopods are an advanced class of a larger group of animals called the mollusks (phylum Mollusca), which includes gastropods and bivalves.

Traditionally, the most common classification of the cephalopods has been a three-fold division (by Bather, 1888), into the nautiloids, ammonoids, and coleoids. This article is about nautiloids in that broad sense, sometimes called Nautiloidea sensu lato.

Cladistically speaking, nautiloids are a paraphyletic assemblage united only by shared primitive (plesiomorphic) features that are not found in other cephalopods. In other words, they are a grade group that gave rise to both ammonoids and coleoids, and are defined by the exclusion of both those descendent groups. Both ammonoids and coleoids are thought to be descended from the bactritids, which in turn arose from straight-shelled orthocerid nautiloids.

The ammonoids (a group which includes the ammonites and the goniatites) are extinct cousins of the nautiloids that evolved early in the Devonian period, some 400 million years ago. Also in the Devonian or Early Carboniferous, the bactritids separately gave rise to the first coleoids, in the form of early belemnoids. Hence, all cephalopods living today are descended from Paleozoic nautiloids.

Some workers apply the name Nautiloidea to a more exclusive group, called Nautiloidea sensu stricto. This taxon consists only of those orders that are clearly related to the modern nautilus. The membership assigned varies somewhat from author to author, but usually includes Tarphycerida, Oncocerida, and Nautilida.

Characteristics
There are three key features which are common to the shells of the nautiloids. These are the internal chambers, the siphuncle and the sutures of the shell, features that are also found in the shells of all ammonoids.

The thin walls between the internal chambers (camerae) of the shell are called the septa. As the nautiloid grew, it would detach its body from the walls of the shell, move forward, and secrete a new septum behind it. Each septum added created a new camera in the shell. The body of the animal itself occupied the last chamber of the shell - the living chamber.

The septa were perforated by the siphuncle, which ran through each of the internal chambers of the shell. Surrounding the fleshy tube of the siphuncle were structures made of Aragonite (a polymorph of Calcium Carbonate - which during fossilisation was converted to Calcite): septal necks and connecting rings. Some of the earlier nautiloids deposited calcium carbonate in the empty chambers (called cameral deposits) or within the siphuncle (endosiphuncular deposits), a process which may have been connected with controlling buoyancy. The nature of the siphuncle and its position within the shell are important in classifying nautiloids.

Sutures (or suture lines) are visible as a series of narrow wavy lines on the surface of the shell, and they appear where each septa contacts the wall of the outer shell. The sutures of the nautiloids are simple in shape, being either straight or slightly curved. This is different from the "zigzag" sutures of the goniatites and the highly complex sutures of the ammonites.

Modern nautiloids
Much of what is known about the extinct nautiloids is based on what we know about the modern nautiluses, such as the Chambered Nautilus which is found in the south west Pacific Ocean, from Samoa to the Philippines, and the in the Indian Ocean off of the coast of Australia. It is not usually found in waters less than 100 meters deep and may be found as far down as 500 to 700 meters (2,300 feet).

Nautiluses are free swimming animals that possess a head with two simple lens-free eyes, arms (or tentacles). They each have a smooth shell, with a large body chamber, which is divided into chambers that are filled with an inert gas (similar to air but with more nitrogen and less oxygen) making the animal buoyant in the water. As many as 90 tentacles are arranged in two circles around their mouth. The animal has jaws which are horny and beak-like, and it is a predator, feeding mainly on crustaceans.

Empty nautilus shells may drift a considerable distance and have been reported from Japan, India and Africa. Undoubtedy the same applies to the shells of fossil nautiloids, the gas inside the shell keeping it buoyant for some time after the animal's death so that the empty shell was carried some distance from where the animal lived before it finally sank to the sea-floor.

Nautiluses propel themselves by jet propulsion, expelling water from an elongated funnel called the hyponome, which can be pointed in different directions to control their movement. They do not have an ink sac like that found in belemnites and some of the other cephalopods, and there is no evidence to suggest that the extinct forms possessed an ink sac either. Unlike the extinct ammonoids, the modern nautiluses lack any sort of plate for closing their shell. With one exception, no such plate has been found in any of the extinct nautiloids either.

The coloration of the shell of the modern nautiluses is quite prominent, and, although it is somewhat rare, the shell coloration has been known to be preserved in fossil nautiloids. They often show color patterns on the dorsal side only, which suggests the living animals swam horizontally.

Fossil record
Nautiloids are often found as fossils in early Palaeozoic rocks (less so in more recent strata). The shells of fossil nautiloids may be either straight (i.e., orthoconic as in Orthoceras and Rayonnoceras), curved (as in Cyrtoceras) coiled (as in Cenoceras), or rarely a hellical coil (as in Lorieroceras). Some species' shells -- especially in the late Paleozoic and early Mesozoic -- are ornamented with spines and ribs, but most have a smooth shell.

The rocks of the Ordovician period in the Baltic coast and parts of the United States contain a variety of nautiloid fossils, and specimens such as Discitoceras and Rayonnoceras may be found in the limestones of the Carboniferous period in Ireland. The marine rocks of the Jurassic period in Britain often yield specimens of Cenoceras, and nautiloids such as Eutrephoceras are also found in the Pierre Shale formation of the Cretaceous period in the north-central United States.

Specimens of the Ordovician nautiloid Endoceras have been recorded measuring up to 3.5 meters (13 feet) in length, and Cameroceras is (somewhat doubtfully) estimated to have reached 11 meters (36 feet). These large nautiloids must have been formidable predators of other marine animals at the time they lived.

In some localities, such as Scandinavia and Morocco, the fossils of orthoconic nautiloids accumulated in such large numbers that they form Orthoceras limestones. Although the term Orthoceras now only refers to a Baltic coast Ordovician genus, in prior times it was employed as a general name given to all straight-shelled nautiloids that lived from the Ordovician to the Triassic periods (but were most common in the early Paleozoic era.

Evolutionary history
Nautiloids are first known from the late Cambrian Fengshan Formation of northeastern China, where they seem to have been quite diverse (at the time this was a warm shallow sea rich in marine life). However, although four orders have been proposed from the 131 species named, there is no certainty that all of these are valid, and indeed it is likely that these taxa are seriously oversplit.

Most of these early forms died out, but a single family, the Ellesmeroceratidae, survived to the early Ordovician, where it ultimately gave rise to all subsequent cephalopods. In the Early and Middle Ordovician the nautiloids underwent an evolutionary radiation, perhaps due to the new ecological niches made available by the extinction of anomalocarids at the end of the Cambrian. Some eight new orders appeared at this time, covering a great diversity of shell types and structure, and ecological lifestyles.

Nautiloids remained at the height of their range of adaptations and variety of forms throughout the Ordovician, Silurian, and Devonian periods, with various straight, curved and coiled shell forms coexisting at the same time. Several of the early orders became extinct over that interval, but others rose to prominence.

Nautiloids began to decline in the Devonian, perhaps due to competition with their descendants and relatives the Ammonoids and Coleoids, with only the Nautilida holding their own (and indeed increasing in diversity). Their shells became increasingly tightly coiled, while both numbers and variety of non-Nautilid species continued to decrease throughout the Carboniferous and Permian.

The massive extinctions at the end of the Permian were less damaging to nautiloids than to other taxa and a few groups survived into the early Mesozoic, including pseudorthocerids, bactritids, nautilids and possibly orthocerids. The last straight-shelled forms were long thought to have disappeared at the end of the Triassic, but a possible orthocerid has been found in Cretaceous rocks. Apart from that exception, only a single nautiloid suborder, the Nautilina, continued throughout the Mesozoic, where they co-existed quite happily with their more specialised ammonoid cousins. Most of these forms differed only slightly from the modern nautilus. They had a brief resurgence in the early Tertiary (perhaps filling the niches vacated by the ammonoids in the end Cretaceous extinction), and maintained a worldwide distribution up until the middle of the Cenozoic Era. With the global cooling of the Miocene and Pliocene, their geographic distribution shrank and these hardy and long-lived animals declined in diversity again. Today there are only six living species, all belonging to two genera, Nautilus (the pearly nautilus), and Allonautilus.

Classification
The following 1988 classification by Curt Teichert, updates the 1964 version in the Treatise on Invertebrate Paleontology, and is based mostly on shell structure (Teichert 1988, p.19).

Subclass Orthoceratoidea Kuhn, 1940
 * Order Plectronocerida Flower, 1964 (Cambrian)
 * Order Yanhecerida Chen & Qi, 1979 (Cambrian)
 * ?Order Protactinocerida Chen & Qi, 1979 (Cambrian)
 * Order Ellesmerocerida Flower, 1950
 * Suborder Ellesmerocerina Flower, 1950 (Cambrian to Ordovician)
 * Suborder Cyrtocerina Flower, 1964 (Ordovician)
 * Order Orthocerida Kuhn, 1940 (Ordovician to Triassic)
 * Order Ascocerida Kuhn, 1949 (Ordovician to Silurian)

Subclass Actinoceratoidea Teichert, 1933
 * Order Actinocerida Teichert, 1933 (Ordovician to Carboniferous)

Subclass Endoceratoidea Teichert, 1933
 * Order Endocerida Teichert, 1933 (Ordovician to Silurian)
 * ?Order Injetocerida Balashov, 1960 (Ordovician)

Subclass Nautiloidea Agassiz, 1847
 * Order Tarphycerida Flower, 1950
 * Suborder Tarphycerina Flower, 1950 (Ordovician to Silurian)
 * Suborder Barrandeocerina Flower (Ordovician to Devonian)
 * Order Oncocerida Flower, 1950 (Ordovician to Carboniferous)
 * Order Discosorida Flower, 1950 (Ordovician to Devonian)
 * Order Nautilida Agassiz, 1847
 * Suborder Rutocerina Shimanskiy, 1957 (Devonian to Triassic)
 * Suborder Lirocerina Shimanskiy, 1957 (Carboniferous to Triassic)
 * Suborder Nautilina Agassiz, 1847 (Triassic to Recent)

A further order, Bactritida, are sometimes considered nautiloids close to the Orthocerida, sometimes very primitive ammonoids, and sometimes placed in a subclass of their own, called Bactritoidea.

Since 1988, two other orders have gained recognition by some workers: the Pseudorthocerida and the Dissidocerida, both previously included in the Orthocerida.

A more recent interpretation by Theo Engeser (Engeser 1997-1998) suggests that nautiloids, and indeed cephalopods in general, fall into two main groups, the Palcephalopoda (including all the nautiloids except Orthocerida and Ascocerida) and the Neocephalopoda (the rest of the cephalopods).