Ammonite
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? AmmonitesFossil range: Late Silurian - Cretaceous
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 A large ammonite, 0.6 metre (2 feet) across,
outside the Portland Museum, Portland, England. |
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Orders and Suborders
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Order Ammonitida
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Ammonites are an extinct group of marine animals ( subclass Ammonoidea) in the phylum Mollusca and class Cephalopoda. Their closest living relative is probably not the modern Nautilus, which they resemble, but rather the Subclass Coleoidea (octopus, squid, and cuttlefish). Their fossil shells take the form of flat spirals (though there are some rarer helically spiraled and non-spiraled forms, called heteromorphs) and are responsible for the animals' name as they somewhat resemble a tightly coiled ram's horn (the god Ammon was commonly depicted as a man with ram's horns). Plinius the Elder (died 79 AD near Pompeii) called fossils of these animals ammonis cornua, "horn of Ammon". Often the name of ammonite species ends in ceras, Greek (κέρας) for "horn" (e.g. Pleuroceras).
Classification
Originating from within the bactritoid nautiloids, Ammonites first appeared in the Late Silurian to Early Devonian (~400 million years ago) and became extinct at the close of the Cretaceous along with the dinosaurs (65 million years ago). The classification of ammonites is based in part on the ornamentation and structure of the septa comprising their shells' gas chambers; by these and other characteristics we can divide this subclass into three orders and eight known suborders. While nearly all nautiloids show gently curving sutures, the ammonite suture line (the intersection of the septum with the outer shell) was folded, forming saddles (or peaks) and lobes (or valleys).
Several types of suture patterns are 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.
- 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 of Ammonites from an stratigraphical point of viwe. This suture type is characteristic of Jurassic and Cretaceous ammonoids.
Here are the orders and suborders listed from most primitive to more derived:
Orders and suborders:
- Goniatitida (Devonian to Permian) - round saddles, pointed lobes
- Anarcestina (Devonian only)
- Clymeniina (Upper Devonian only)
- Goniatitina (Devonian to Upper Permian)
- Ceratitida (Carboniferous to Triassic) - round saddles, serrated lobes
- Prolecanitina (Upper Devonian to Upper Triassic)
- Ceratitina (Permian to Triassic)
- Ammonitida (Permian to Cretaceous) - folded saddles and lobes, fractal patterns
- Phylloceratina (Lower Triassic to Upper Cretaceous)
- Ammonitina (Lower Jurassic to Upper Cretaceous)
- Lytoceratina (Lower Jurassic to Upper Cretaceous)
- Ancyloceratina (Upper Jurassic to Upper Cretaceous) - the heteromorph ammonites
Life
Because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are practically never preserved in any detail. Nonetheless, a lot has been worked out by examining ammonite shells and by using models of these shells in water tanks.
Many ammonites 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 benthic (bottom-dwelling) life is found. Many of them are thought to have been good swimmers with flattened, discus-shaped, streamlined shells (such as Oxynoticeras), although some were less effective swimmers and were likely to have been slow-swimming bottom dwellers. Ammonites probably preyed on fishes and crustaceans and were themselves preyed upon by marine reptiles (such as Mosasaurs). Fossilized shells are sometimes found showing teeth marks from such attacks.
Shell anatomy and diversity
Basic shell anatomy
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 coil.
A thin living tube called a siphuncle passed through the septa, extending from the ammonite's body into the empty shell chambers. Through a hyperosmotic active transport process, the ammonite emptied water out of these shell chambers. This enabled 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 periphery of the septa and camerae (i.e., the inside of the outer axis of the shell), while the siphuncle of nautiloids 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 to 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 ammonites have a shell that is a planispiral flat coil, but some have a shell that is partially uncoiled, partially coiled and partially straight (as in Australiceras), nearly straight (as in baculites), or coiled helically - superficially like that of a large gastropod (as in Turrilites and Bostrychoceras). 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 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.
The 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 opening of the shell. The opening of the shell is called the aperture. 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 aptychi were identical and equal in size.
Anaptychi are relatively rare as fossils. They are found representing ammonites from the Devonian period through those of the Cretaceous period.
Calcified Aptychi only occur in ammonites from the Mesozoic era and are normally found detached from the shell and are 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 intended purpose. (This long-standing and wide-spread interpretation of the function of the aptychus has long been disputed. The latest studies suggest that the anaptychus may have in fact formed part of a special jaw apparatus).
Large numbers of detached aptychi occur in certain beds of rock (such as those from the Mesozoic in the Alps). These rocks are usually accumulated at great depths. The modern Nautilus lacks any calcitic plate for closing its shell, and only one extinct nautiloid genus is known to have borne anything similar. Nautilus does, however, have a leathery head shield (the hood) which it uses to cover the opening when it retreats inside.
There are many forms of aptychus, varying in shape and the sculpture of the inner and outer surfaces, but because they are so rarely found in position within the shell of the ammonite it is often unclear to which species of ammonite many aptychi belong. A number of aptychi have been given their own genus and even species names independent of their unknown owners' genus and species, pending future discovery of verified occurrences within ammonite shells.
Size
Few of the ammonites occurring in the lower and middle part of the Jurassic period reach a size exceeding 23 centimetres (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 centimetres (2 feet) in diameter, and Pachydiscus seppenradensis of the Cretaceous period of Germany, which is one of the largest known ammonites, sometimes reaching 2 metres (6.5 feet) in diameter. The largest documented North American ammonite is Parapuzosia bradyi from the Cretaceous with specimens measuring 137 centimetres (4.5 feet) in diameter, although a new British Columbian specimen appears to trump even the European champion.
Ammonite distribution
Starting from the late Silurian, ammonites were extremely abundant, especially in the Mesozoic seas. Many genera evolved and ran their course quickly, becoming extinct in a few million years. Due to their rapid evolution and widespread distribution, ammonites are useful for 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.
Due to their nektonic (free-swimming) habits, ammonites often happened to live directly above seafloor waters so poor in oxygen as to prevent the establishment of animal life on the seafloor. When upon death the ammonites fell to this seafloor and were gradually buried in accumulating sediment, bacterial decomposition of these corpses often led to local changes in redox conditions sufficient to lower the local solubility of minerals dissolved in the seawater, notably phosphates and carbonates. The resulting concentric precipitation of minerals around a fossil is called a concretion and is responsible for the outstanding preservation of many ammonite fossils.
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, 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 ammonite specimens, especially those of the Paleozoic Era, are preserved only as internal molds; that it to say, the outer shell (composed of aragonite) has been lost through fossilization. It is only in these specimens that the suture lines can be observed; in life the sutures would have been hidden by the outer shell.
The ammonites survived several major extinction events, with often only a few species surviving. But each time, this handful would diversify into a multitude of forms. Ammonites fossils become less abundant during the upper part of the Cretaceous, and none survive into the Cenozoic era. The last surviving lines died out about 65 million years ago along with the dinosaurs in the Cretaceous-Tertiary extinction event.
Trivia
In medieval times, ammonites were believed to be petrified snakes. They were frequently fitted with carved snake-like heads and sold to pilgrims. A famous example of this links the ammonite fossils common in the Jurassic sediments around Whitby, North Yorkshire with the legend that St. Hilda turned a plague of snakes into stone. Even today, tourists can buy ammonite fossils with heads carved onto them to make them look more snake-like.
It is said that the original discus used by the ancient Greeks in their Olympics was in fact a fossilized ammonite; a number of ammonite generic names include an explicit reference to the discus shape (e.g., Sphenodiscus).
In India, ammonite fossils are identified with the god Vishnu and are used in various ceremonies. They are mostly collected in Nepal, from the bed of the River Gandaki where it cuts through Jurassic sediments. These fossils are known as "shaligram shila" [1].