Ammonoids are a group of
extinct marine
mollusc animals in the subclass Ammonoidea of the
class Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, ...
a. These molluscs, commonly referred to as ammonites, are more closely related to living
coleoids (i.e.,
octopuses,
squid and
cuttlefish) than they are to shelled
nautiloids such as the living ''
Nautilus
The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina.
It comprises six living species ...
'' species. The earliest ammonites appeared during the
Devonian, with the last species vanishing during the
Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden extinction event, mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million y ...
.
Ammonites are excellent
index fossils, and linking the rock layer in which a particular species or genus is found to specific
geologic time periods is often possible. Their
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
shells usually take the form of
planispirals, although some helically spiraled and nonspiraled forms (known as
heteromorphs) have been found.
The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled
rams' horns.
Pliny the Elder
Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. He wrote the encyclopedic ...
( 79 AD near Pompeii) called fossils of these animals ("
horns of Ammon
The horns of Ammon were curling ram horns, used as a symbol of the Egyptian deity Ammon (also spelled Amun or Amon). Because of the visual similarity, they were also associated with the fossils shells of ancient snails and cephalopods, the latter n ...
") because the Egyptian god Ammon (
Amun) was typically depicted wearing rams' horns. Often, the name of an ammonite genus ends in -''ceras'', which is from () meaning "horn".
Diagnostic characters
Ammonites (subclass Ammonoidea) can be distinguished by their septa, the dividing walls that separate the chambers in the phragmocone, by the nature of their sutures where the septa join the outer shell wall, and in general by their
siphuncles.
Septa
Ammonoid septa characteristically have bulges and indentations and are to varying degrees convex when seen from the front, distinguishing them from nautiloid septa, which are typically simple concave, dish-shaped structures. The topology of the septa, especially around the rim, results in the various suture patterns found.
Suture patterns
While nearly all nautiloids show gently curving sutures, the ammonoid suture line (the intersection of the septum with the outer shell) is variably folded, forming saddles ("peaks" that point towards the aperture) and lobes ("valleys" which point away from the aperture). The suture line has four main regions. The external or ventral region refers to sutures along the lower (outer) edge of the shell, where the left and right suture lines meet. The external saddle lies directly on the lower midline of the shell and is edged by external lobes. On suture diagrams the external saddle is supplied with an arrow which typically points towards the aperture. The lateral region involves the first saddle and lobe pair past the external region as the suture line extends up the side of the shell. Additional lobes developing towards the inner edge of a whorl are labelled umbilical lobes, which increase in number through ammonoid evolution as well as an individual ammonoid's development. Lobes and saddles which are so far towards the center of the whorl that they are covered up by succeeding whorls are labelled internal lobes and saddles. Three major types of suture patterns are found in the Ammonoidea:
*
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
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
"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
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
and Cretaceous ammonoids, but extends back all the way to the
Permian
The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Pale ...
.
File:Ammonite Goniatites plebeiformis Goniatitic suture.jpg, '' Goniatites plebeiformis'' showing Goniatitic suture
File:Ammonite Protrachyceras pseudoarchelonus Ceratitic suture.jpg, '' Protrachyceras pseudoarchelonus'' showing Ceratitic suture
File:Ammonite Lytoceras sutile Ammonitic suture.jpg, ''Lytoceras sutile
''Lytoceras sutile'' is an ammonite species belonging to the family Lytoceratidae. These cephalopods were fast-moving nektonic carnivores.
Fossil record
Fossils of ''Lytoceras sutile'' have been found from the Jurassic to the Cretaceous (age ra ...
'' showing Ammonitic suture
Siphuncle
The
siphuncle in most ammonoids is a narrow tubular structure that runs along the shell's outer rim, known as the venter, connecting the chambers of the
phragmocone to the body or living chamber. This distinguishes them from living nautiloides (''Nautilus'' and ''
Allonautilus'') and typical
Nautilida, in which the siphuncle runs through the center of each chamber. However the very earliest nautiloids from the Late Cambrian and Ordovician typically had ventral siphuncles like ammonites, although often proportionally larger and more internally structured. The word "siphuncle" comes from the
New Latin
New Latin (also called Neo-Latin or Modern Latin) is the revival of Literary Latin used in original, scholarly, and scientific works since about 1500. Modern scholarly and technical nomenclature, such as in zoological and botanical taxonomy ...
''siphunculus'', meaning "little siphon".
Classification
Originating from within the
bactritoid
The Bactritida are a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period (407 million years ago) with questionable origins in Pragian stage before 409 million y ...
nautiloids, the ammonoid cephalopods first appeared in the
Devonian (''circa'' 409 million years ago (Mya)) and became
extinct shortly after
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
(66 Mya). The classification of ammonoids is based in part on the
ornamentation and structure of the
septa comprising their shells' gas chambers.
Orders and suborders
The Ammonoidea can be divided into six orders, listed here starting with the most primitive and going to the more derived:
*
Agoniatitida, Lower Devonian – Middle Devonian
*
Clymeniida, Upper Devonian
*
Goniatitida, Middle Devonian – Upper Permian
*
Prolecanitida, Upper Devonian – Upper Triassic
*
Ceratitida, Upper Permian – Upper Triassic
*
Ammonitida, Lower Jurassic – Lower Paleocene
In some classifications, these are left as suborders, included in only three orders:
Goniatitida,
Ceratitida and
Ammonitida.
Taxonomy of the ''Treatise on Invertebrate Paleontology''
The ''
Treatise on Invertebrate Paleontology'' (Part L, 1957) divides the Ammonoidea, regarded simply as an order, into eight suborders, the Anarcestina, Clymeniina, Goniatitina and Prolecanitina from the Paleozoic; the Ceratitina from the Triassic; and the Ammonitina, Lytoceratina and
Phylloceratina from the Jurassic and Cretaceous. In subsequent taxonomies, these are sometimes regarded as orders within the subclass Ammonoidea.
Life
Because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are very rarely preserved in any detail. Nonetheless, much 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, because their fossils are often found in rocks laid down under conditions where no
bottom-dwelling
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning ...
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.
Synchrotron analysis of an aptychophoran ammonite revealed remains of
isopod and
mollusc larvae in its buccal cavity, indicating at least this kind of ammonite fed on
plankton
Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cr ...
.
They may have avoided predation by squirting
ink, much like modern cephalopods; ink is occasionally preserved in fossil specimens.
The soft body of the creature occupied the largest segments of the shell at the end of the coil. The smaller earlier segments were walled off and the animal could maintain its buoyancy by filling them with gas. Thus, the smaller sections of the coil would have floated above the larger sections.
Many ammonite shells have been found with round holes once interpreted as a result of limpets attaching themselves to the shells. However, the triangular formation of the holes, their size and shape, and their presence on both sides of the shells, corresponding to the upper and lower jaws, is more likely evidence of the bite of a medium-sized
mosasaur
Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Ma ...
preying upon ammonites.
Some ammonites appear to have lived in
cold seeps and even reproduced there.
Shell anatomy and diversity
Basic shell anatomy
The chambered part of the ammonite shell is called a
phragmocone. It contains a series of progressively larger chambers, called
camera
A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with a ...
e (sing. camera) that are divided by thin walls called
septa (sing. septum). Only the last and largest chamber, the
body chamber
The body whorl is part of the morphology of the shell in those gastropod mollusks that possess a coiled shell. The term is also sometimes used in a similar way to describe the shell of a cephalopod mollusk.
In gastropods
In gastropods, th ...
, 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. Where the outer
whorl
A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs).
Whorls in nature
File:Photograph and axial plane flo ...
of an ammonite shell largely covers the preceding whorls, the specimen is said to be ''
involute
In mathematics, an involute (also known as an evolvent) is a particular type of curve that is dependent on another shape or curve. An involute of a curve is the locus of a point on a piece of taut string as the string is either unwrapped from o ...
'' (e.g., ''
Anahoplites''). Where it does not cover those preceding, the specimen is said to be ''
evolute
In the differential geometry of curves, the evolute of a curve is the locus of all its centers of curvature. That is to say that when the center of curvature of each point on a curve is drawn, the resultant shape will be the evolute of that c ...
'' (e.g., ''
Dactylioceras'').
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
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
of the shell and thereby rise or descend in the water column.
A primary difference between ammonites and nautiloids is 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
The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina.
It comprises six living species ...
'' 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
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...
is thought to be an explanation for the variation in size of certain ammonite shells of the same species, the larger shell (the macroconch) being female, and the smaller shell (the 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
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
period of
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located enti ...
.
Only recently has sexual variation in the shells of ammonites 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, because the dimorphic sizes are so consistently found together, they are more likely an example of sexual dimorphism within the same species.
Whorl width in the body chamber of many groups of ammonites, as expressed by the width:diameter ratio, is another sign of dimorphism. This character has been used to separate "male" (Largiventer conch "L") from "female" (Leviventer conch "l").
Variations in shape
150px, Heteromorph ammonite '' ''">Didymoceras stevensoni''
The majority of ammonite species feature planispiral, flat-coiled shells, but other species feature nearly straight (as in
baculites) shells. Still other species' shells are coiled helically, similar in appearance to some
gastropods (e.g., ''
Turrilites
''Turrilites'' is a genus of helically coiled ammonoid cephalopods from the lower part of the Upper Cretaceous (Cenomanian and Turonian); generally included in the Ancyloceratina. Previously (Arkell, 1957) it was included in the ammonoid suborder ...
'' and ''
Bostrychoceras''). Some species' shells are even initially uncoiled, then partially coiled, and finally straight at maturity (as in ''
Australiceras
''Australiceras'' is an extinct ammonite genus from the upper part of the Early Cretaceous, Aptian stage, included in the family Ancyloceratidae.
Description
''Australiceras'' has an evolute shell, coiled with all whorls showing in keeping wit ...
''). 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 symmetric coiling. Upon closer inspection, though, the shell proves to be a three-dimensional network of connected "U" shapes. ' occurs in rocks of the upper part of the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
in Japan and the United States.
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.
Aptychus
Some ammonites have been found in association with a single horny plate or a pair of calcitic plates. In the past, these plates were assumed to serve in closing the opening of the shell in much the same way as an
operculum, but more recently they are postulated to have been a jaw apparatus.
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 paired aptychi were symmetric to one another and equal in size and appearance.
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
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...
era. They are almost always found detached from the shell, and are only very 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 identity as part of the anatomy of an ammonite.
Large numbers of detached aptychi occur in certain beds of rock (such as those from the Mesozoic in the
Alps
The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, ...
). 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 one kind of aptychus belongs. 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.
Soft-part anatomy
Although ammonites do occur in exceptional
lagerstatten such as the
Solnhofen Limestone, their soft-part record is surprisingly bleak. Beyond a tentative ink sac and possible digestive organs, no soft parts were known until 2021.
They likely bore a radula and beak, a marginal siphuncle and ten arms.
They operated by direct development with sexual reproduction, were carnivorous, and had a
crop
A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydropo ...
for food storage. They are unlikely to have dwelt in fresh or brackish water.
Many ammonites were likely
filter feeder
Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...
s, so adaptations associated with this lifestyle like sieves probably occurred.
A 2021 study found ammonite specimens with preserved hook-like suckers, providing a general shape to ammonite tentacles. A contemporary study found an ammonite isolated body, offering for the first time a glimpse into these animals' organs.
Size
The smallest ammonoid was ''
Maximites
''Maximites'' is a genus of Pennsylvanian (geology), Late Carboniferous ammonoids. Adult specimens were smallest organisms, the smallest known ammonoids, only at about in diameter of shells. Fossils are found in various Late Carboniferous marin ...
'' from the
Upper Carboniferous. Adult specimens reached only in shell diameter. Few of the ammonites occurring in the lower and middle part of the Jurassic period reached a size exceeding 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 in diameter, and ''
Parapuzosia seppenradensis'' of the Cretaceous period of Germany, which is one of the largest-known ammonites, sometimes reaching in diameter. The largest-documented
North American ammonite is ''
Parapuzosia bradyi
''Parapuzosia bradyi'' is a gigantic species of ammonite, reaching diameters of more than by . It is the largest species of ammonite in North America. It had a moderately involute shell with flat sides. The inner whorls are slightly oval-shape ...
'' from the Cretaceous, with specimens measuring in diameter.
Distribution
Starting from the mid-Devonian, ammonoids were extremely abundant, especially as ammonites during the
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...
era. Many
genera evolved and ran their course quickly, becoming extinct in a few million years. Due to their rapid
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
and widespread distribution, ammonoids are used by
geologist
A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, alt ...
s and
paleontologist
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of foss ...
s for
biostratigraphy
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...
. They are excellent
index fossils, and it is often possible to link the rock layer in which they are found to specific
geologic time periods.
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
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
conditions sufficiently to lower the local solubility of minerals dissolved in the seawater, notably
phosphates and
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonat ...
s. The resulting spontaneous concentric precipitation of minerals around a fossil, a
concretion
A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...
, is responsible for the outstanding preservation of many ammonite fossils.
When ammonites are found in
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay part ...
s, their original
mother-of-pearl
Nacre ( , ), also known as mother of pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer; it is also the material of which pearls are composed. It is strong, resilient, and iridescent.
Nacre is ...
coating is often preserved. This type of preservation is found in ammonites such as ''
Hoplites'' from the Cretaceous
Gault clay
The Gault Formation is a geological formation of stiff blue clay deposited in a calm, fairly deep-water marine environment during the Lower Cretaceous Period (Upper and Middle Albian). It is well exposed in the coastal cliffs at Copt Point in Fol ...
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
concretion
A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...
s.
Other fossils, such as many found in
Madagascar
Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...
and
Alberta
Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest Ter ...
, 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; the outer shell (composed of
aragonite) has been lost during the fossilization process. Only in these internal-mould specimens can the suture lines be observed; in life, the sutures would have been hidden by the outer shell.
The ammonoids as a group continued through several major
extinction events, although often only a few species survived. Each time, however, this handful of species diversified into a multitude of forms. Ammonite fossils became less abundant during the latter part of the
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...
, and although they seemingly survived the
Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden extinction event, mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million y ...
, all known
Paleocene ammonite lineages are restricted to the
Paleocene
The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...
epoch (65–61 Ma).
Evolutionary history
Goniatites, which were a dominant component of Early and Middle Permian faunas, became rare in the Late Permian, and no goniatite is thought to have crossed into the Triassic.
Ceratitida originated during the Middle Permian, likely from the
Daraelitidae, and radiated in the Late Permian. In the aftermath of the
Permian–Triassic extinction event, Ceratitids represent the dominant group of Triassic ammonites.
Ammonites were devastated by the
end-Triassic extinction, with only a handful of genera belonging to the family
Psiloceratidae of the suborder
Phylloceratina surviving and becoming ancestral to all later Jurassic and Cretaceous ammonites. Ammonites explosively diversified during the Early Jurassic, with the orders Psiloceratina, Ammonitina, Lytoceratina, Haploceratina, Perisphinctina and Ancyloceratina all appearing during the Jurassic.
Heteromorph ammonites (ammonites with open or non-spiral coiling) of the order
Ancyloceratina became common during the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
period.
At least 57 species of ammonites, which were widespread and belonged to six superfamilies, were extant during the last 500,000 years of the Cretaceous, indicating that ammonites remained highly diverse until the very end of their existence. All ammonites were wiped out during or shortly after the
K-Pg extinction event, caused by the
Chicxulub impact. It has been suggested that
ocean acidification generated by the impact played a key role in their extinction, as the larvae of ammonites were likely small and
planktonic, and would have been heavily affected.
Nautiloids, exemplified by modern
nautilus
The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina.
It comprises six living species ...
es, are conversely thought to have had a reproductive strategy in which eggs were laid in smaller batches many times during the lifespan, and on the sea floor well away from any direct effects of such a
bolide strike, and thus survived. Many ammonite species were filter feeders, so they might have been particularly susceptible to marine faunal turnovers and climatic change.
Cultural significance
In medieval Europe, fossilised ammonites were thought to be
petrified coiled
snakes, and were called "snakestones" or, more commonly in medieval England, "serpentstones". They were considered to be evidence for the actions of saints, such as
Hilda of Whitby
Hilda (or Hild) of Whitby (c. 614 – 680) was a Christian saint and the founding abbess of the monastery at Whitby, which was chosen as the venue for the Synod of Whitby in 664. An important figure in the Christianisation of Anglo-Saxon ...
, a myth referenced in Sir
Walter Scott's ''
Marmion'',
and
Saint Patrick, and were held to have
healing
With physical trauma or disease suffered by an organism, healing involves the repairing of damaged tissue(s), organs and the biological system as a whole and resumption of (normal) functioning. Medicine includes the process by which the cells ...
or
oracular powers. Traders would occasionally carve the head of a snake onto the empty, wide end of the ammonite fossil, and then sell them as petrified snakes. In other cases, the snake's head would be simply painted on.
Ammonites from the
Gandaki River in
Nepal
Nepal (; ne, नेपाल ), formerly the Federal Democratic Republic of Nepal ( ne,
सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is ma ...
are known as
saligram
A shaligrama shila ( deva, शालिग्राम शिला, Śāligrāma-śilā), also rendered as shaligram, is a particular variety of stone collected from riverbed or banks of the Kali Gandaki, a tributary of the Gandaki River in N ...
s, and are believed by
Hindus to be a concrete manifestation of
Vishnu
Vishnu ( ; , ), also known as Narayana and Hari, is one of the principal deities of Hinduism. He is the supreme being within Vaishnavism, one of the major traditions within contemporary Hinduism.
Vishnu is known as "The Preserver" within ...
.
See also
*
Ammolite – a gemstone formed from fossil ammonite shells
*
Belemnoidea
*
Coleoidea
*
Geologic time scale
The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geoch ...
*
Nautiloid
References
Further reading
*
*
*
*
Walker, Cyril and
Ward, David. ''Fossils''. Dorling, Kindersley Limited, London, 2002.
A Broad Brush History of the Cephalopodaby Dr. Neale Monks, from The Cephalopod Page.
By Dr. Neale Monks, from The Cephalopod Page. Essay about the life span of Ammonites.
Deeply Buried Sediments Tell Story of Sudden Mass Extinction
External links
goniat.org, a palaezoic ammonoid databaseTaxonConcept's data on cretaceous ammonites*
ttp://www.tonmo.com/science/fossils/fossilsjump.php tonmo.com: The octopus news magazine online Cephalopod fossil articles.
* William R. Wahl
Mosasaur Bite Marks on an Ammonite. Preservation of an Aborted Attack?
{{Authority control
Mollusc subclasses
Mesozoic cephalopods
Paleozoic cephalopods
Devonian first appearances
Maastrichtian extinctions
Fossil taxa described in 1884
Cephalopods described in 1884
Taxa named by Karl Alfred Ritter von Zittel