Ammonoidea

The Ammonoidea constitute a subclass of extinct cephalopods found in marine sediments from the Early Devonian through the Cretaceous. Baring an external shell, ammonoids superficially resemble nautiloids, such as the modern Nautilus. however, based on similarity of the radulas, they are more closely related to modern coleoids, squid, octopus, and cuttlefish.

Ammonoids are commonly referred to in the vernacular as Ammonites, although the term is often reserved for members of the order Ammonitida. For derivation and history of the term see Ammonite

Ammonoid shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled forms (known as "heteromorphs"). Often with short stratigraphic ranges, they make excellent index fossils, making it possible to link the rock layer in which they are found to specific stage or substage within a given geologic period.

Classification
Originating from bactritoid nautiloids, ammonoid cephalopods first appeared in the Early Devonian (circa 400 million years ago) and became extinct by the close of the Cretaceous (65 Ma) along with other marine fauna as well as the dinosaurs. The classification of ammonoids is based in part on the ornamentation and structure of the septa that form the chambers of the shell While nearly all nautiloids show gently curving sutures, the ammonoid suture line (the intersection of the septum with the outer shell) was folded, forming saddles (or peaks) and lobes (or valleys).

Suture patterns
Three major types of suture patterns in Ammonoidea have been noted:
 * Goniatitic - numerous undivided lobes and saddles; typically 8 lobes around the conch. This pattern is characteristic of the Paleozoic ammonoids.
 * Ceratitic - lobes have subdivided tips, giving them a saw-toothed appearance, and rounded undivided saddles. This suture pattern is characteristic of Triassic ammonoids and appears again in the Cretaceous "pseudoceratites".
 * Ammonitic - lobes and saddles are much subdivided (fluted); subdivisions are usually rounded instead of saw-toothed. Ammonoids of this type are the most important species from a biostratigraphical point of view. This suture type is characteristic of Jurassic and Cretaceous ammonoids but extends back all the way to the Permian.

Orders and suborders


The Ammonoidea can be divided into eight orders, listed here starting with the most primitive and going to the more derived. Note that in some classifications these are referred to as suborders, included in only three orders, the Goniatitida, Ceratitida, and Ammonitida
 * Anarcestida, Devonian
 * Clymeniida. Upper Devonian
 * Goniatitida., Middle Devonian - Upper Permian
 * Prolecanitida, Upper Devonian - Upper Triassic
 * Ceratitida, Permian - Triassic
 * Phylloceratida, Triassic - Cretaceous
 * Lytoceratida, Jurassic - Cretaceous
 * Ammonitida, Lower Jurassic - Upper Cretaceous

Life


Because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are very rarely preserved in any detail. Nonetheless, much has been worked out by examining ammonoid shells and by using models of these shells in water tanks.

Many ammonoids probably lived in the open water of ancient seas, rather than at the sea bottom. This is suggested by the fact that their fossils are often found in rocks that were laid down under conditions where no bottom-dwelling life is found. Many of them (such as Oxynoticeras) are thought to have been good swimmers with flattened, discus-shaped, streamlined shells, although some ammonoids were less effective swimmers and were likely to have been slow-swimming bottom-dwellers. Ammonites and their kin probably preyed on fishes, crustaceans and other small creatures, while they themselves were preyed upon by such marine reptiles as mosasaurs. Fossilized ammonoids have been found showing teeth marks from such attacks. They may have avoided predation by squirting ink, much like modern cephalopods; ink is occasionally preserved in fossil specimens.

The soft body of the animal occupied the open chamber at the end of the shell. The smaller earlier walled off segments  were used to control buoyancy by filling them with gas. Thus the smaller sections of the coil would have floated above the larger sections.

The chambered part of the ammonite shell is called a phragmocone. The phragmocone contains a series of progressively larger chambers, called camerae (sing. camera) that are divided by thin walls called septa (sing. septum). Only the last and largest chamber, the body chamber, was occupied by the living animal at any given moment. As it grew, it added newer and larger chambers to the open end of the phragmocone. A thin narrow tube, called a siphuncle, containing living tissue, extending from the animal's body, passed through the septa, by which through hyperosmotic active transport  water could be emptied from the chambers, enabling it to control the buoyancy of the shell and thereby rise or descend in the water column.

A primary difference between ammonites and nautiloids is that the siphuncle of ammonites (excepting Clymeniina) runs along the ventral margin (i.e., the inner surface of the outer rim) of the shell), while the siphuncle of nautiloids is more varied in position and form. In ‘’Nautilus’‘ it runs more or less through the center of the septa and camerae.

Sexual dimorphism


One feature found in shells of the modern Nautilus is the variation in the shape and size of the shell according to the gender of the animal, the shell of the male being slightly smaller and wider than that of the female. This sexual dimorphism is thought to be an explanation for the variation in size of certain ammonite shells of the same species, the larger shell (called a macroconch) being female, and the smaller shell (called a microconch) being male. This is thought to be because the female required a larger body size for egg production. A good example of this sexual variation is found in Bifericeras from the early part of the Jurassic period of Europe.

It is only in relatively recent years that the sexual variation in the shells of ammonites has been recognized. The macroconch and microconch of one species were often previously mistaken for two closely related but different species occurring in the same rocks. However, these "pairs" were so consistently found together that it became apparent that they were in fact sexual forms of the same species.

Variations in shape
The majority of ammonite species feature a shell that is a planispiral flat coil, but other species feature a shell that is nearly straight (as in baculites). Still other species' shells are coiled helically, superficially like that of a large gastropod (as in Turrilites and Bostrychoceras). Some species' shells are even initially uncoiled, then partially coiled, and finally straight at maturity (as in Australiceras). These partially uncoiled and totally uncoiled forms began to diversify mainly during the early part of the Cretaceous and are known as heteromorphs.

Perhaps the most extreme and bizarre looking example of a heteromorph is Nipponites, which appears to be a tangle of irregular whorls lacking any obvious symmetrical coiling. However, upon closer inspection the shell proves to be a three-dimensional network of connected "U" shapes. Nipponites occurs in rocks of the upper part of the Cretaceous in Japan and the USA.

Ammonites vary greatly in the ornamentation (surface relief) of their shells. Some may be smooth and relatively featureless, except for growth lines, and resemble that of the modern Nautilus. In others various patterns of spiral ridges and ribs or even spines are shown. This type of ornamentation of the shell is especially evident in the later ammonites of the Cretaceous.

Aptychus


Like the modern nautilus, many ammonites were probably able to withdraw their body into the living chamber of the shell and developed either a single horny plate or a pair of calcitic plates with which they were able to close the aperture, or opening, of the shell. The plates are collectively termed the aptychus or aptychi in the case of a pair of plates, and anaptychus in the case of a single plate. The paired aptychi were symmetrical to one another and equal in size and appearance. –(ed note: separate explanation of aperture omitted)

Anaptychi are relatively rare as fossils. They are found representing ammonites from the Devonian through the Cretaceous periods.

Calcified aptychi only occur in ammonites from the Mesozoic era. They are almost always found detached from the shell, and are only very rarely preserved in place. Still, sufficient numbers have been found closing the apertures of fossil ammonite shells as to leave no doubt as to their identity as part of an ammonite. What exact function they serve is however not certain. One long-standing and widespread interpretation of them as a form of operculum has more recently been contested. The latest studies suggest that the anaptychus may have in fact formed part of a special jaw apparatus).

Soft parts
Although ammonites do occur in exceptional lagerstatten such as the Solnhoffen, their soft part record is surprisingly bleak - beyond a tentative ink sac and possible digestive organs, no soft parts are known at all. It can be tentatively assumed that they had numerous tentacles, each quite weak, and engulfed prey almost whole.

Size
Few of the ammonites occurring in the lower and middle part of the Jurassic period reach a size exceeding 23 centimeters (9 inches) in diameter. Much larger forms are found in the later rocks of the upper part of the Jurassic and the lower part of the Cretaceous, such as Titanites from the Portland Stone of Jurassic of southern England, which is often 53 centimeters (2 feet) in diameter, and Parapuzosia seppenradensis of the Cretaceous period of Germany, which is one of the largest known ammonites, sometimes reaching 2 meters (6.5 feet) in diameter. The largest documented North American ammonite is Parapuzosia bradyi from the Cretaceous with specimens measuring 137 centimeters (4.5 feet) in diameter, although a new British Columbian specimen, if authentic, would appear to trump even the European champion.

Distribution


Starting perhaps late in the Silurian, ammonoids were extremely abundant, especially as ammonites during the Mesozoic era. Many genera evolved and ran their course quickly, becoming extinct in a few million years. Due to their rapid evolution and widespread distribution, ammonoids are used by geologists and paleontologists for biostratigraphy. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.

When ammonites are found in clays their original mother-of-pearl coating is often preserved. This type of preservation is found in ammonites such as Hoplites from the Cretaceous Gault clay of Folkestone in Kent, England.

The Cretaceous Pierre Shale formation of the United States and Canada is well known for the abundant ammonite fauna it yields, including Baculites, Placenticeras, Scaphites, Hoploscaphites, and Jeletzkytes, as well as many uncoiled forms. Many of these also have much or all of the original shell, as well as the complete body chamber, still intact. Many Pierre Shale ammonites, and indeed many ammonites throughout earth history, are found inside concretions. Other fossils, such as many found in Madagascar and Alberta (Canada), display iridescence. These iridescent ammonites are often of gem quality (ammolite) when polished. In no case would this iridescence have been visible during the animal's life; additional shell layers covered it.

The majority of ammonoid specimens, especially those of the Paleozoic era, are preserved only as internal molds; the outer shell (composed of aragonite) having been lost through fossilization. It is only in these internal-moldic specimens that the suture lines can be observed; in life the sutures would have been hidden by the outer shell.

Ammonoid Extinction
The extinction of the ammonites along with other marine animals and of course, dinosaurs, has been attributed to a bolide impact, marking the end of the Cretaceous Period. However regardless of what effect an impact may have had, many of these groups, including ammonoids, were already in serious decline. Previously ammonoid cephalopods barely survived several earlier major extinction events, often with only a few species surviving from which a multitude of forms diversified.

Eight or so species from only two families made it almost to the end of the Cretaceous, the order having gone though a more or less steady decline since the middle of the period. Six other families made it well into the upper Maastrichtian (uppermost stage of the Cretaceous) but were extinct well before the end. All told 11 families entered the Maastrictian, a decline from the 19 families known from the Cenomanian in the middle of the Cretaceous.

One reason given for their demise is that Cretaceous ammonites, being closely related to Coleoids, had a similar reproductive strategy in which a huge number of eggs were laid in a single batch at the end of the life span. These, along with juvenile ammonites, are thought to have been part of the plankton at the surface of the ocean where they were killed off by the effects of an impact. Nautiloids, exemplified by Nautilus, are thought on the other hand to have had a similar reproductive strategy in which eggs were laid in smaller batches many times during the life span on the sea floor well away from any direct effects of such a bolide strike, and thus survived.

Terminological note
The words "ammonite" and "ammonoid" are both used quite loosely in common parlance to refer to any member of subclass Ammonoidea. However, in stricter usage the term "ammonite" is reserved for members of suborder Ammonitina (or sometimes even order Ammonitida).

References and further reading

 * Neal L. Larson, Steven D Jorgensen, Robert A Farrar and Peter L Larson. Ammonites and the other Cephalopods of the Pierre Seaway. Geoscience Press, 1997.
 * Lehmann, Ulrich. The Ammonites: Their life and their world.  Cambridge University Press, New York, 1981.  Translated from German by Janine Lettau.
 * Monks, Neale and Palmer, Phil. Ammonites. Natural History Museum, 2002.
 * Walker, Cyril and Ward, David. Fossils. Dorling, Kindersley Limited, London, 2002.
 * A Broad Brush History of the Cephalopoda by Dr. Neale Monks, from The Cephalopod Page.
 * Ammonite maturity, pathology and old age By Dr. Neale Monks, from The Cephalopod Page. Essay about the life span of Ammonites.
 * Cretaceous Fossils Taxonomic Index for Order Ammonoitida
 * Deeply Buried Sediments Tell Story of Sudden Mass Extinction