Ammonite facts for kids
Temporal range: Devonian – Cretaceous
Anarcestida; Clymeniida; Goniatitida; Prolecanitida; Ceratitida; Phylloceratida; Lytoceratida; Ammonitida; Ancyloceratina
These creatures lived in the seas from at least 400 to 65 million years ago. They became extinct at the K/T extinction event. 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. Nine orders are recognised in the Ammonoidea: five in the Palaeozoic and four in the Mesozoic.
Ammonites' widely-known fossils show a ribbed spiral-form shell, in the end compartment of which lived the tentacled animal. Their name came from their spiral shape as their fossilized shells somewhat resemble tightly-coiled rams' horns. Pliny the Elder (d. 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.
Ammonites first appeared in the early Devonian period. They evolved from a small, straight shelled Bactridian, which was an early Nautiloid. They quickly evolved into a variety of shapes and sizes, including some shaped like hairpins. During their evolution the ammonites faced no fewer than four catastrophic events that would eventually lead to their extinction. The first event occurred in the Upper Devonian, and the second at the end of the Permian (250 million years ago), when only two lines survived the P/Tr extinction event. The surviving species radiated and flourished throughout the Triassic perod. At the end of this period (206 million years ago) they faced near extinction again, when only one genus survived. This event marked the end of the Triassic and the beginning of the Jurassic, during which time the number of ammonite species grew once more. The final catastrophe occurred at the end of the Cretaceous period when all species were annihilated and the ammonites became extinct.
The young ammonites lived in the plankton, near the sea surface. They ate mostly small fry as they were growing. This made them especially vulnerable to any event which upset the plankton zone.
Ammonites began life as tiny planktonic creatures less than 1mm in diameter. In their infancy they would have been vulnerable to attack from other predators, including mosasaurs and fish. However, their shell gave their soft parts some protection. The existence of sexual dimorphism, with larger females and smaller males, has been much discussed.p244 The matter is still open, but at least in some species deposits are found with two sizes and no intermediates.
As the shell grew, the back compartments were sealed with a semi-permeable membrane. A single tube, the siphuncle, passed through the centre of each septum and connected the chambers The animal could add or withdraw gas as it needed for buoyancy. On the inside of the shell, the compartments are marked by elaborate sutures. These can be seen easily on those fossils which are internal moulds, as most are.p241 Ammonites were active predators, and they themselves were often eaten by fish and marine reptiles. The fossils are almost always found with the outer compartment broken off, probably as a result of just such an attack.
Ammonites swam by jet propulsion, as do most other cepalopods. Water would have come into the mantle cavity, passed over the gills, and was squirted out. Nautilus also has an escape mechanism, where a contraction of the branchial (gill) chamber causes the animal to jump out of the way of a predator. It would be reasonable to suppose that ammonites had a similar mechanism.
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 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.
One feature found in shells of the modern Nautilus is the variation in the shape and size of the shell according to the sex 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 strght (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.
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.
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 Parapuzosia 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 2.3-metre (7.5-foot) British Columbian specimen, if authentic, would appear to trump even the European champion.
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.
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 a bolide strike, plankton around the globe could have been severely diminished, thereby dooming ammonite reproduction during its planktonic stage.
In medieval Europe, fossilised ammonites were thought to be petrified snakes, and were called "snakestones" or, more commonly in medieval England, "serpentstones". They were taken to be evidence for the actions of saints such as Saint Hilda and Saint Patrick. Traders would occasionally carve the face of a snake into the empty, wide end of the ammonite fossil and sell them to the public. Ammonites from the Gandaki river in Nepal are known as saligrams, and are believed by Hindus to be a concrete manifestation of God or Vishnu.
Images for kids
Fossil shell of ammonite Placenticeras whitfieldi showing punctures caused by the bite of a mosasaur, Peabody Museum of Natural History, Yale
Baculites ammonite from the Late Cretaceous of Wyoming: The original aragonite of the outer conch and inner septa has dissolved away, leaving this articulated internal mold.
Ammonite Facts for Kids. Kiddle Encyclopedia.