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The Nautilus, shown below, is the last surviving genus of the nautiloids.  Nautiloids are a group of marine animals which possess an external shell.  Nautiloids have heads with well developed eyes and grasping tentacles.  They swim by squirting water out from the body cavity.  Essentially all that is known about the extinct nautiloids is based on what we learned from the modern Nautilus. 
Nautiloids first appeared in the Cambrian period, some 570 million years ago, and flourished in the ancient seas of the Paleozoic era.  There are some 2,500 species of fossil nautiloids, but only a handful of species survive to the present day.  The nautiloids are part of the group of animals called the cephalopods, which also includes ammonoids and belemnites. The ammonoids, a group which includes the ammonites and the goniatites, evolved from the nautiloids early in the Devonian period some 400 million years ago.  To learn more about ammonites and goniatites, just click on the appropriate icon below. 
The cephalopods are an advanced class of a larger group of animals called the molluscs, which includes bivalves and gastropods.  To learn more about bivalves and gastropods, just click on the appropriate icon below. 
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.  The siphuncle is a tube which runs through each of the internal chambers of the shell.  Nautiloids are classified by the nature of the siphuncle and its position within the shell.  The thin walls between the internal chambers of the shell are called the septa, and as the nautiloid grew, it would move its body forward in the shell.  As it moved it body forward, the nautiloid would secret a new septa behind it, thereby adding a new chamber to the shell. 
An example of these chambers can be seen in the image above of the cross-section of a modern Nautilus.  These chambers can also be seen below in the fossils of ammonites millions of years older.
 Sutures are features found in the shells of all ammonoids.  Sutures 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, compared to the “zigzag” sutures of the goniatites and the highly complex sutures of the ammonites.
The shells of fossil nautiloids may be either straight, curved, coiled or in a helical coil.   Some nautiloids lost their coil and grew straight bodies. These include the Orthoceras 
and the Rayonnoceras.  They can be curved, such as Cyroceras; or coiled, as in Cenoceras.  They can also be in a helical coil such as in Lorieroceras.  Some shells are ornamented with spines and ribs, but most have a smooth shell. 
Orthoceras, often found as black-white fossils, are the fossilized remains of a squid shell approximately 350 million years old.  This primitive squid built a straight chambered shell, and when it died, the shell dropped to the bottom of the ocean where it fossilized inside a black limestone.  They are mined in the northern Sahara desert of Morocco.  Orthoceras is a common straight shelled nautiloid from the rocks of the Ordovician period.
The rocks of the Ordovician period, in the Baltic coast and parts of the United Stated,
contain a variety of nautiloid fossils.  Specimens such as Discitoceras and Rayonnoceras may be found in the limestones of the Carboniferous period in the Republic of Ireland.  The marine rocks of the Jurassic period in the United Kingdom often yield specimens of Cenoceras.  Nautiloids, such as Eutrephoceras, are found in the Pierre Shale formation of the Cretaceous period in the Midwestern part of the United States.  Specimens of the nautiloid Endoceras, from the Ordovician period, have been recorded measuring up to 3.5 meters (13 feet) in length. 
Nautiloids began to decline in both their numbers, and variety of species, in the upper 
part of the Paleozoic era.  This decline continued throughout the Mesozoic and 
Cenozoic eras.  Their shells became increasingly tightly coiled, and most of the more 
recent forms, differ slightly from the modern Nautilus.  With the exception of a few 
species from the Triassic period, all of the nautiloids found after the Paleozoic era are 
coiled.  In some localities, the shells of fossil nautiloids accumulated in such large numbers that they form Orthoceras limestones. Fossil nautiloids have a worldwide distribution up until the middle of the Cenozoic era, after which their geographic distribution shrinks, and their numbers decline to just four species.
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