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Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living
coleoid Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whose ...
s (i.e., octopuses, squid and cuttlefish) than they are to shelled
nautiloid Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded specie ...
s 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 in ...
'' 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 mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...
. Ammonites are excellent
index fossils 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 ...
, 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
planispiral Planispiral is a condition in which a tubicolous shell is coiled in a single horizontal plane and the diameter increases away from the axis of coiling. This term is usually used for describing gastropod shells. Many cephalopod A cephalopod is ...
s, 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") 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 geologic period and 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 Jurassic constitutes the middle period of ...
and Cretaceous ammonoids, but extends back all the way to the
Permian The Permian ( ) is a geologic period and System (stratigraphy), 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 ...
. 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'' 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 The phragmocone is the chambered portion of the shell of a cephalopod. It is divided by septa into camerae. In most nautiloids and ammonoids, the phragmocone is a long, straight, curved, or coiled structure, in which the camarae are linked by a ...
to the body or living chamber. This distinguishes them from living nautiloides (''Nautilus'' and ''
Allonautilus The genus ''Allonautilus'' contains two species of nautiluses, which have a significantly different morphology from those placed in the sister taxon '' Nautilus''. ''Allonautilus'' is now thought to be a descendant of ''Nautilus'', rendering t ...
'') and typical
Nautilida The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, ''Nautilus'' and ''Allonauti ...
, 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 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 Agoniatitida, also known as the Anarcestida, is the ancestral order within the cephalopod subclass Ammonoidea originating from bactritoid nautiloids, that lived in what would become Africa, Asia, Australia, Europe, and North America during t ...
, Lower Devonian – Middle Devonian * Clymeniida, Upper Devonian * Goniatitida, Middle Devonian – Upper Permian *
Prolecanitida Prolecanitida is an order of extinct ammonoid cephalopods with discoidal to thinly lenticular shells with goniatitic or ceratitic sutures and which retained the simple retrochoanitic siphuncle with backward extending septal necks. As typical fo ...
, Upper Devonian – Upper Triassic * Ceratitida, Upper Permian – Upper Triassic *
Ammonitida Ammonitida is an order of ammonoid cephalopods that lived from the Jurassic through Paleocene time periods, commonly with intricate ammonitic sutures. Ammonitida is divided into four suborders, the Phylloceratina, Lytoceratina, Ancyloceratin ...
, Lower Jurassic – Lower Paleocene In some classifications, these are left as suborders, included in only three orders: Goniatitida, Ceratitida and
Ammonitida Ammonitida is an order of ammonoid cephalopods that lived from the Jurassic through Paleocene time periods, commonly with intricate ammonitic sutures. Ammonitida is divided into four suborders, the Phylloceratina, Lytoceratina, Ancyloceratin ...
.


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 The Phyllocertina comprise a suborder of ammonoid cephalopods, belonging to the Ammonitida, whose range extends from the Lower Triassic to the Upper Cretaceous. Shells of the Phylloceratina are generally smooth with small to large umbilici and c ...
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 life is found. Many of them (such as ''
Oxynoticeras ''Oxynoticeras'' is an extinct genus of ammonite from the Early Jurassic of Europe and North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the ...
'') 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 crucia ...
. 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 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 The phragmocone is the chambered portion of the shell of a cephalopod. It is divided by septa into camerae. In most nautiloids and ammonoids, the phragmocone is a long, straight, curved, or coiled structure, in which the camarae are linked by a ...
. 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, 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 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 or ...
'' (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 cur ...
'' (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 In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane- ...
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 ...
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 in ...
'' 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 ''Bifericeras'' is a Lower Jurassic ammonite belonging to the family Eoderoceratidae, and sometimes placed in the subfamily Xipherceratinae. Whorls are strongly depressed, but still evolute in coiling. The early growth state is prolonged, and sm ...
'' from the early part of the
Jurassic The Jurassic ( ) is a geologic period and 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 Jurassic constitutes the middle period of ...
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 entirel ...
. 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'' and ''
Bostrychoceras ''Bostrychoceras'' is a genus of heteromorph ammonite from the family Nostoceratidae. Fossils have been found in Late Cretaceous sediments in Europe and North America North America is a continent in the Northern Hemisphere and alm ...
''). 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 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 Cretace ...
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, Swi ...
). These rocks are usually accumulated at great depths. The modern ''Nautilus'' lacks any calcitic plate for closing its shell, and only one extinct
nautiloid Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded specie ...
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 hydropon ...
for food storage. They are unlikely to have dwelt in fresh or brackish water. Many ammonites were likely filter feeders, 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'' 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'' 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 Cretace ...
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, althoug ...
s and paleontologists for biostratigraphy. They are excellent
index fossils 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 ...
, 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 Motility is the ability of an organism to move independently, using metabolic energy. Definitions Motility, the ability of an organism to move independently, using metabolic energy, can be contrasted with sessility, the state of organisms th ...
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 change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
conditions sufficiently to lower the local solubility of minerals dissolved in the seawater, notably
phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...
s 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 carbonate ...
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 par ...
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 Hoplites ( ) ( grc, ὁπλίτης : hoplítēs) were citizen-soldiers of Ancient Greek city-states who were primarily armed with spears and shields. Hoplite soldiers used the phalanx formation to be effective in war with fewer soldiers. The f ...
'' 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 ''Baculites'' ("walking stick rock") is an extinct genus of cephalopods with a nearly straight shell, included in the heteromorph ammonites. The genus, which lived worldwide throughout most of the Late Cretaceous, and which briefly survived the ...
'', '' Placenticeras'', ''
Scaphites ''Scaphites'' is a genus of heteromorph ammonites belonging to the Scaphitidae family. They were a widespread genus that thrived during the Late Cretaceous period. Morphology ''Scaphites'' generally have a chambered, boat-shaped shell. The ini ...
'', '' 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 Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral co ...
) 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 The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...
era, are preserved only as internal molds; the outer shell (composed of
aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...
) 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 Cretace ...
, 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 mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...
, 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 Psiloceratidae is an extinct family of cephalopods belonging to the ammonite subclass. Description The Psiloceratidae are evolute, smooth or with blunt primary ribbing. The venter is rounded and generally smooth, in some feebly keeled. Sutures a ...
of the suborder
Phylloceratina The Phyllocertina comprise a suborder of ammonoid cephalopods, belonging to the Ammonitida, whose range extends from the Lower Triassic to the Upper Cretaceous. Shells of the Phylloceratina are generally smooth with small to large umbilici and c ...
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 The Ancyloceratina were a diverse suborder of ammonite most closely related to the ammonites of order Lytoceratina. They evolved during the Late Jurassic but were not very common until the Cretaceous period, when they rapidly diversified and be ...
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 The Chicxulub crater () is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore near the community of Chicxulub, after which it is named. It was formed slightly over 66 million years ago when a large a ...
. It has been suggested that
ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
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.
Nautiloid Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded specie ...
s, 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 in ...
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 A bolide is normally taken to mean an exceptionally bright meteor, but the term is subject to more than one definition, according to context. It may refer to any large crater-forming body, or to one that explodes in the atmosphere. It can be a ...
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 Engla ...
, a myth referenced in Sir
Walter Scott Sir Walter Scott, 1st Baronet (15 August 1771 – 21 September 1832), was a Scottish novelist, poet, playwright and historian. Many of his works remain classics of European and Scottish literature, notably the novels '' Ivanhoe'', '' Rob Roy ...
'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 The Gandaki River, also known as the Narayani and the Gandak, is one of the major rivers in Nepal and a left bank tributary of the Ganges in India. Its total catchment area amounts to , most of it in Nepal. In the Nepal Himalayas, it is notab ...
in
Nepal Nepal (; ne, :ne:नेपाल, नेपाल ), formerly the Federal Democratic Republic of Nepal ( ne, सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in S ...
are known as saligrams, and are believed by
Hindu Hindus (; ) are people who religiously adhere to Hinduism. Jeffery D. Long (2007), A Vision for Hinduism, IB Tauris, , pages 35–37 Historically, the term has also been used as a geographical, cultural, and later religious identifier for ...
s 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" withi ...
.


See also

*
Ammolite Ammolite is an opal-like organic gemstone found primarily along the eastern slopes of the Rocky Mountains of North America. It is made of the fossilized shells of ammonites, which in turn are composed primarily of aragonite, the same mineral co ...
– a gemstone formed from fossil ammonite shells *
Belemnoidea Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten Cephalopo ...
*
Coleoidea Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whos ...
*
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 geochr ...
*
Nautiloid Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded specie ...


References


Further reading

* * * * Walker, Cyril and Ward, David. ''Fossils''. Dorling, Kindersley Limited, London, 2002.
A Broad Brush History of the Cephalopoda
by 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 database





TaxonConcept'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