An extinct, nautilus-like mollusc; found extensively as fossil shells from the Devonian to the Upper Cretaceous periods; shell external in life, typically a flattened spiral, divided internally by transverse walls. (Class: Cephalopoda. Subclass: Ammonoidea.)
For the explosive "Amonite", see Ammonal.|
iAmmonites Fossil range: Late Silurian - Cretaceous |
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Artist's reconstruction of a live ammonite. |
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| Scientific classification | ||||||||
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| Orders and Suborders | ||||||||
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Order Ammonitida Ammonitina Acanthoceratina Ancyloceratina Phylloceratina LytoceratinaOrder Goniatitida Goniatitina Anarcestina ClymeniinaOrder Ceratitida Ceratitina Prolecanitina |
Ammonites are an extinct group of marine animals of the subclass Ammonoidea in the class Cephalopoda, phylum Mollusca.
Classification
Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Late Silurian to Early Devonian (circa 400 million years ago) and became extinct at the close of the Cretaceous (65 m.y.a.) along with the dinosaurs.
Suture patterns:
Goniatitic - numerous undivided lobes and saddles;Orders and suborders:
Goniatitida (Devonian to Permian) -- have round saddles, pointed lobes Anarcestina (Devonian only) Clymeniina (upper Upper Devonian only) Goniatitina (Devonian to Upper Permian) -- includes the true goniatites Ceratitida (Carboniferous to Triassic) -- have round saddles, serrated lobes Prolecanitina (Upper Devonian to Upper Triassic) Ceratitina (Permian to Triassic) -- includes the true ceratites Ammonitida (Permian to Cretaceous) -- have folded saddles and lobes, fractal patterns Phylloceratina (Lower Triassic to Upper Cretaceous) Ammonitina (Lower Jurassic to Upper Cretaceous) -- includes the true ammonites Lytoceratina (Lower Jurassic to Upper Cretaceous) Ancyloceratina (Upper Jurassic to Upper Cretaceous) -- the heteromorph ammonitesLife
Because ammonites and their close relatives are extinct, little is known about their way of life. Nonetheless, a lot 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.
Shell anatomy and diversity
Basic shell anatomy
The chambered part of the ammonite shell is called a phragmocone.
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.
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 inner surface 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.
It is only in relatively recent years that the sexual variation in the shells of ammonites has been recognized.
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).
Ammonites vary greatly in the ornamentation of their shells. 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. 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.
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.
Distribution
Starting from the late Silurian, ammonoids were extremely abundant, especially as ammonites during the Mesozoic era.
Due to their free-swimming and/or free-floating 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. The resulting spontaneous 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 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.
The majority of ammonoid specimens, especially those of the Paleozoic era, are preserved only as internal molds;
The ammonoids survived several major extinction events, with often only a few species surviving. Ammonite fossils became less abundant during the latter part of the Mesozoic, with none surviving into the Cenozoic era. That no ammonites survived the extinction event at the end of the Cretaceous, while some nautiloid cousins survived, might be due to differences in ontogeny.
Trivia
In medieval times, ammonites were believed to be petrified snakes. 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.
There is a Pokémon named after the ammonites. (Omanyte)
Terminological Note
The words "ammonite" and "ammonoid" are both used quite loosely in common parlance to refer to any member of Subclass Ammonoidea.
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