Science Fair Project Encyclopedia
Ammonites are an extinct group of marine animals (subclass Ammonoidea) in the phylum Mollusca and class Cephalopoda. Their closest living relative is probably the modern nautilus, whom they resemble. Their fossil shells have the form of flat spirals (though there are some rarer 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 on ceras, Greek (κέρας) for "horn" (e.g. Pleuroceras).
The ammonite shell contained a series of progressively larger chambers divided by thin walls called septa, with only the last and largest chamber occupied by the living animal. A thin living tube called a siphuncle passed through the septa, extending from the ammonite's body into the empty shell chambers. The ammonite secreted gas into these shell chambers, enabling it to control the buoyancy of the shell. As it grew, it added newer and larger chambers to the open end of the coil.
Ammonites first appeared in the late Silurian, early Devonian Periods (~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 their shells, which divide this subclass into three orders and eight known suborders. Unlike the nautiloids, the suture line between the chamber wall and the outer shell was folded, forming saddles (or peaks) and lobes (or valleys). Here they are listed from most primitive to more advanced:
- Goniatida (Devonian to Permian) - round saddles, pointed lobes
- Ceratida (Carboniferous to Triassic) - round saddles, serrated lobes
- Ammonitida (Permian to Cretaceous) - folded saddles and lobes, fractal patterns
- Ancyloceratina (Upper Jurassic to Upper Cretaceous) - the heteromorph ammonites
- Anarcestina (Devonian only)
- Clymeniina (Upper Devonian only)
- Goniatitina (Devonian to Upper Permian)
- Prolecanitina (Upper Devonian to Upper Triassic)
- Ceratitina (Permian to Triassic)
- Phylloceratina (Lower Triassic to Upper Cretaceous)
- Lytoceratina (Lower Jurassic to Upper Cretaceous)
- Ammonitina (Lower Jurassic to Upper Cretaceous)
Ammonites were extremely abundant, especially in the Mesozoic seas. 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.
One feature found in shells of the modern nautilus is the variation in the size of the shell according to the gender of the animal, the shell of the male being slightly smaller 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.
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 and belemnites), or coiled helically - superficially like that of a large gastropod (as in Turrilites and Bostrychoceras). These partially uncoiled and totally uncoiled forms began to appear 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.
Because ammonites and their close relatives are extinct, little is known about their way of life. Nonetheless, a lot has been worked out by using models of ammonite shells in water tanks. Most ammonites probably lived at the surface of ancient seas; 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 bottom dwellers. Ammonites probably preyed on fishes and crustaceans. Ammonites were themselves preyed upon by marine reptiles (such as Mosasaurs) and their fossilized shells are sometimes found showing the teeth marks from such attacks.
Many ammonites were 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 termed the anaptychus in the case of a single plate, and are collectively termed the aptychus or aptychii in the case of a pair of plates. The aptychus were equal in size.
Anaptychus are rare as fossils. They are found in ammonites from the Devonian period through those of the Cretaceous period.
Aptychus only occur in ammonites from the Mesozoic era and are normally found detached 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*. Large numbers of detached aptychus 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 sort of plate for closing its shell, and none has been found in any of the extinct nautiloids. (*However, this long standing and wide-spread interpretation of the function of the aptychus has recently been disputed. The latest studies suggest that the apatychus may have in fact formed part of a special jaw apparatus).
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 unclear as to which species of ammonite many aptychus belong.
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. Other fossils, such as many found in Madagascar, display iridescence.
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 the largest known ammonite sometimes reaching 2 metres (6.5 feet) in diameter. The largest known North American ammonite is Parapuzosia bradyi from the Cretaceous with specimens measuring 137 centimetres (4.5 feet) in diameter.
The Cretaceous Pierre Shale formation of the United States and Canada is well known for the abundant ammonite fauna it yields, including Baculites, Placenticeras, Hoploscaphites, and Jeletzkytes, as well as many uncoiled forms. Ammonites fossils become less abundant during the upper part of the Cretaceous, and none survive into the Cenozoic era.
In medieval times, ammonites were believed to be petrified snakes. They were frequently fitted with carved snake-like heads and sold to pilgrims.
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.
- Monks, Neale and Palmer, Phil. Ammonites. Natural History Museum, 2002.
- Walker, Cyril and Ward, David. Fossils. Dorling, Kindersley Limited, London, 2002.
- Fossils and the geological timescale
- Ammolite - a gemstone formed from fossil ammonite shells.
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