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Nautiloids are a group of marine animals which all possess an external shell, the most well known example being the modern nautiluses. They first appeared in the Cambrian period, some 570 million years ago, and flourished in the ancient seas of the Palaeozoic era. There are some 2,500 species of fossil nautiloid, but only a handful of species survive to the present day. 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.
The nautiloids are part of the group of animals called the cephalopods (class Cephalopoda), which also includes ammonoids, belemnites and nautiloids. The cephalopods are an advanced class of a larger group of animals called the molluscs (phylum Mollusca), which includes gastropods and bivalves.
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 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 secreting septa behind it, thereby adding new chambers to the shell. The body of the animal itself occupied the last chamber of the shell - the living chamber. Some of the earlier nautiloids deposited calcium carbonate in the empty chambers, a process which may have been connected with controlling buoyancy.
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, compared to the 'zigzag' sutures of the goniatites and the highly complex sutures of the ammonites.
Essentially all that 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 well developed 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 animals 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 a elongated funnel called the hyponome, which can be pointed in different directsions 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, and no such plate has been found in any of the extinct nautiloids.
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.
The shells of fossil nautiloids may be either straight (as in Orthoceras and Rayonnoceras), curved (as in Cyroceras) or coiled (as in Cenoceras), or a hellical coil (as in Lorieroceras). Some shells 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 the Republic of Ireland. The marine rocks of the Jurassic period in the United Kingdom often yield specimens of Cenoceras, and nautiloids such as Eutrephoceras are also found in the Pierre Shale formation of the Cretaceous period in the midwestern part of the United States.
Specimens of the Ordovician nautiloid Endoceras have been recorded measuring up to 3.5 meters (13 feet) in length. These large nautiloids must have been formidable predators of other marine animals at the time they lived.
Nautiloids reached the height of their range of adaptations and variety of forms during the Ordovician and Silurian periods, with various straight, curved and coiled shell forms coexisting at the same time. Nautiloids began to decline in both their numbers and variety of species in the upper part of the Palaeozoic era, and this decline continued throughout the Mesozoic and Cenozoic eras. Their shells became increasingly tightly coiled and most of the more recent forms differ only slightly from the modern Nautilus. With the exception of a few species from the Triassic period, all of the nautiloids found after the Palaeozoic era are coiled.
In some localities, such as Scandinavia and Morocco, the shells of fossil nautiloids accumulated in such large numbers that they form Orthoceras limestones. Orthoceras is a common straight shelled nautiloid from the marine rocks of the Ordovician period.
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.
See also
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