   #copyright

Ammonite

2007 Schools Wikipedia Selection. Related subjects: Mineralogy

   iAmmonites

                  Fossil range: Late Silurian - Cretaceous

   Artist's reconstruction of a live ammonite.
   Artist's reconstruction of a live ammonite.
                 Scientific classification

   Kingdom:  Animalia
   Phylum:   Mollusca
   Class:    Cephalopoda
   Subclass: Ammonoidea
             Zittel, 1884

                            Orders and Suborders

   Order Ammonitida
     * Ammonitina
     * Acanthoceratina
     * Ancyloceratina
     * Phylloceratina
     * Lytoceratina

   Order Goniatitida
     * Goniatitina
     * Anarcestina
     * Clymeniina

   Order Ceratitida
     * Ceratitina
     * Prolecanitina

   Ammonites are an extinct group of marine animals of the subclass
   Ammonoidea in the class Cephalopoda, phylum Mollusca. 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.
   Ammonites' closest living relative is probably not the modern Nautilus
   (which they outwardly resemble), but rather the subclass Coleoidea (
   octopus, squid, and cuttlefish). Their fossil shells usually take the
   form of planispirals, although there were some helically-spiraled and
   non-spiraled forms (known as "heteromorphs"). Their spiral shape begot
   their name, as their fossilized shells somewhat resemble tightly-coiled
   rams' horns. Plinius the Elder (died 79 A.D. near Pompeii) called
   fossils of these animals ammonis cornua ("horns of Ammon") because the
   Egyptian god Ammon ( Amun) was typically depicted wearing ram's horns.
   Often the name of an ammonite genus ends in ceras, which is Greek
   (κέρας) for "horn" (for instance, Pleuroceras).

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. The classification of
   ammonoids 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 subclass Ammonoidea into three orders and
   eight known suborders. 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).

   Three major types of suture patterns in Ammonoidea have been noted.

   Suture patterns:
     * 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.

   The three orders and various suborders of Ammonoidea are herein listed
   from most primitive to more derived.

   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 ammonites

Life

   Jeletzkytes, a Cretaceous ammonite from the USA
   Enlarge
   Jeletzkytes, a Cretaceous ammonite from the USA

   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 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.

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 variety of ammonite forms, from Ernst Haeckel's 1899 Kunstformen der
   Natur (Artforms of Nature)
   Enlarge
   A variety of ammonite forms, from Ernst Haeckel's 1899 Kunstformen der
   Natur (Artforms of Nature)

   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 inner surface of the
   outer axis of the shell), while the siphuncle of nautiloids runs more
   or less through the centre of the septa and camerae.

Sexual dimorphism

   Ammonite species, Jurassic era
   Enlarge
   Ammonite species, Jurassic era

   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.
   Asteroceras, a Jurassic ammonite from England
   Enlarge
   Asteroceras, a Jurassic ammonite from England

   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.

Distribution

   A specimen of Hoploscaphites from the Pierre Shale of South Dakota.
   Much of the original shell has survived.
   Enlarge
   A specimen of Hoploscaphites from the Pierre Shale of South Dakota.
   Much of the original shell has survived.

   Starting from the late 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.
   An iridescent ammonite from Madagascar.
   Enlarge
   An iridescent ammonite from Madagascar.

   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. When upon
   death the ammonites fell to this seafloor and were gradually buried in
   accumulating sediment, bacterial decomposition of these corpses often
   tipped the delicate balance of local redox conditions sufficiently to
   lower the local solubility of minerals dissolved in the seawater,
   notably phosphates and carbonates. 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 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; that it to say, the outer
   shell (composed of aragonite) has 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.

   The ammonoids survived several major extinction events, with often only
   a few species surviving. Each time,however, this handful would
   diversify into a multitude of forms. Ammonite fossils became less
   abundant during the latter part of the Mesozoic, with none surviving
   into the Cenozoic era. The last surviving lines disappeared along with
   the dinosaurs 65 million years ago in the Cretaceous-Tertiary
   extinction event. 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. If their extinction was due to an
   meteor strike, plankton around the globe could have been severely
   diminished, thereby dooming ammonite reproduction during its planktonic
   stage.

Trivia

   Fossilized ammonite from Morocco
   Enlarge
   Fossilized ammonite from Morocco

   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" .

   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. However,
   in stricter usage the term "ammonite" is reserved for members of
   Suborder Ammonitina (or sometimes even Order Ammonitida).
   Retrieved from " http://en.wikipedia.org/wiki/Ammonite"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
