The mollusc shell is a calcareous exoskeleton which encloses and protects the soft parts of an animal in the phylum Mollusca, which includes snails, tusk shells, several other classes. Not all shelled; the ancestral mollusc is thought to have had a shell, but this has subsequently been lost or reduced on some families, such as the squid and some smaller groups such as the caudofoveata and solenogastres, the derived Xenoturbella. Today, over 100,000 living species bear a shell. Malacology, the scientific study of molluscs as living organisms, has a branch devoted to the study of shells, this is called conchology—although these terms used to be, to a minor extent still are, used interchangeably by scientists. Within some species of molluscs, there is a wide degree of variation in the exact shape, pattern and color of the shell. A mollusc shell is formed and maintained by a part of the anatomy called the mantle. Any injuries to or abnormal conditions of the mantle are reflected in the shape and form and color of the shell.
When the animal encounters harsh conditions that limit its food supply, or otherwise cause it to become dormant for a while, the mantle ceases to produce the shell substance. When conditions improve again and the mantle resumes its task, a "growth line" is produced; the mantle edge secretes a shell. The organic constituent is made up of polysaccharides and glycoproteins; this organic framework controls the formation of calcium carbonate crystals, dictates when and where crystals start and stop growing, how fast they expand. The shell formation requires certain biological machinery; the shell is deposited within a small compartment, the extrapallial space, sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell, where growth occurs. This caps off the extrapallial space, bounded on its other surfaces by the existing shell and the mantle; the periostracum acts as a framework from which the outer layer of carbonate can be suspended, but in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur.
The accumulation of ions is driven by ion pumps packed within the calcifying epithelium. Calcium ions are obtained from the organism's environment through the gills and epithelium, transported by the haemolymph to the calcifying epithelium, stored as granules within or in-between cells ready to be dissolved and pumped into the extrapallial space when they are required; the organic matrix forms the scaffold that directs crystallization, the deposition and rate of crystals is controlled by hormones produced by the mollusc. Because the extrapallial space is supersaturated, the matrix could be thought of as impeding, rather than encouraging, carbonate deposition. Nucleation is endoepithelial in Neopilina and Nautilus, but exoepithelial in the bivalves and gastropods; the formation of the shell involves a number of genes and transcription factors. On the whole, the transcription factors and signalling genes are conserved, but the proteins in the secretome are derived and evolving. Engrailed serves to demark the edge of the shell field.
In gastropod embryos, Hox1 is expressed. Perlucin increases the rate at which calcium carbonate precipitates to form a shell when in saturated seawater. Perlucin operates in association with Perlustrin, a smaller relative of lustrin A, a protein responsible for the elasticity of organic layers that makes nacre so resistant to cracking. Lustrin A bears remarkable structural similarity to the proteins involved in mineralization in diatoms – though diatoms use silica, not calcite, to form their tests! The shell-secreting area is differentiated early in embryonic development. An area of the ectoderm thickens invaginates to become a "shell gland"; the shape of this gland is tied to the form of the adult shell. The gland subsequently evaginates in molluscs. Whilst invaginated, a periostracum - which will form a scaffold for the developing shell - is formed around the opening of the invagination, allowing the deposition of the shell when the gland
Trilobites are a group of extinct marine arachnomorph arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods; the first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period, they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except the Proetids died out. Trilobites disappeared in the mass extinction at the end of the Permian about 252 million years ago; the trilobites were among the most successful of all early animals, existing in oceans for over 300 million years. By the time trilobites first appeared in the fossil record, they were highly diversified and geographically dispersed; because trilobites had wide diversity and an fossilized exoskeleton, they left an extensive fossil record, with some 50,000 known species spanning Paleozoic time. The study of these fossils has facilitated important contributions to biostratigraphy, evolutionary biology, plate tectonics.
Trilobites are placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha, although several alternative taxonomies are found in the literature. Trilobites had many lifestyles. Most lifestyles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism; some trilobites are thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food. The earliest trilobites known from the fossil record are redlichiids and ptychopariid bigotinids dated to some 540 to 520 million years ago. Contenders for the earliest trilobites include Fritzaspis spp.. Hupetina antiqua and Serrania gordaensis. All trilobites are thought to have originated in present-day Siberia, with subsequent distribution and radiation from this location. All Olenellina lack facial sutures, this is thought to represent the original state; the earliest sutured trilobite found so far, occurs at the same time as the earliest Olenellina, suggesting the trilobites origin lies before the start of the Atdabanian, but without leaving fossils.
Other groups show secondary lost facial sutures, such as all some Phacopina. Another common feature of the Olenellina suggests this suborder to be the ancestral trilobite stock: early protaspid stages have not been found because these were not calcified, this is supposed to represent the original state. Earlier trilobites could shed more light on the origin of trilobites. Three specimens of a trilobite from Morocco, Megistaspis hammondi, dated 478 million years old contain fossilized soft parts. Early trilobites show all the features of the trilobite group as a whole. Morphological similarities between trilobites and early arthropod-like creatures such as Spriggina and other "trilobitomorphs" of the Ediacaran period of the Precambrian are ambiguous enough to make a detailed analysis of their ancestry complex. Morphological similarities between early trilobites and other Cambrian arthropods make analysis of ancestral relationships difficult as well; that trilobites share a common ancestor with other arthropods before the Ediacaran-Cambrian boundary is still reasonable to assume.
Evidence suggests that significant diversification had occurred before trilobites were preserved in the fossil record, allowing for the "sudden" appearance of diverse trilobite groups with complex derived characteristics. For such a long-lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished and surviving groups diversified to fill ecological niches with comparable or unique adaptations. Trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn-out decline in the Devonian, culminating in the final extinction of the last few survivors at the end of the Permian period. Principal evolutionary trends from primitive morphologies, such as exemplified by Eoredlichia, include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium, development of extreme spinosity in certain groups. Changes included narrowing of the thorax and increasing or decreasing numbers of thoracic segments.
Specific changes to the cephalon are noted. Several morphologies appeared independently within different major taxa. Effacement, the loss of surface detail in the cephalon, pygidium, or the thoracic furrows, is a common evolutionary trend. Notable examples of this were the orders Agnostida and Asaphida, the suborder Illaenina of the Corynexochida. Effacement is believed to be an indication of either a pelagic one. Effacement poses a problem for taxonomists since the loss of details can make the determination of phylogenetic relationships difficult. Phylogenetic biogeographic analysis of Early Cambrian Olenellidae and Redlichiidae
Anatomy is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of living things, it is an old science. Anatomy is inherently tied to developmental biology, comparative anatomy, evolutionary biology, phylogeny, as these are the processes by which anatomy is generated over immediate and long timescales. Anatomy and physiology, which study the structure and function of organisms and their parts, make a natural pair of related disciplines, they are studied together. Human anatomy is one of the essential basic sciences; the discipline of anatomy is divided into microscopic anatomy. Macroscopic anatomy, or gross anatomy, is the examination of an animal's body parts using unaided eyesight. Gross anatomy includes the branch of superficial anatomy. Microscopic anatomy involves the use of optical instruments in the study of the tissues of various structures, known as histology, in the study of cells.
The history of anatomy is characterized by a progressive understanding of the functions of the organs and structures of the human body. Methods have improved advancing from the examination of animals by dissection of carcasses and cadavers to 20th century medical imaging techniques including X-ray and magnetic resonance imaging. Derived from the Greek ἀνατομή anatomē "dissection", anatomy is the scientific study of the structure of organisms including their systems and tissues, it includes the appearance and position of the various parts, the materials from which they are composed, their locations and their relationships with other parts. Anatomy is quite distinct from physiology and biochemistry, which deal with the functions of those parts and the chemical processes involved. For example, an anatomist is concerned with the shape, position, blood supply and innervation of an organ such as the liver; the discipline of anatomy can be subdivided into a number of branches including gross or macroscopic anatomy and microscopic anatomy.
Gross anatomy is the study of structures large enough to be seen with the naked eye, includes superficial anatomy or surface anatomy, the study by sight of the external body features. Microscopic anatomy is the study of structures on a microscopic scale, along with histology, embryology. Anatomy can be studied using both invasive and non-invasive methods with the goal of obtaining information about the structure and organization of organs and systems. Methods used include dissection, in which a body is opened and its organs studied, endoscopy, in which a video camera-equipped instrument is inserted through a small incision in the body wall and used to explore the internal organs and other structures. Angiography using X-rays or magnetic resonance angiography are methods to visualize blood vessels; the term "anatomy" is taken to refer to human anatomy. However the same structures and tissues are found throughout the rest of the animal kingdom and the term includes the anatomy of other animals.
The term zootomy is sometimes used to refer to non-human animals. The structure and tissues of plants are of a dissimilar nature and they are studied in plant anatomy; the kingdom Animalia contains multicellular organisms that are motile. Most animals have bodies differentiated into separate tissues and these animals are known as eumetazoans, they have an internal digestive chamber, with two openings. Metazoans do not include the sponges. Unlike plant cells, animal cells have neither chloroplasts. Vacuoles, when present, are much smaller than those in the plant cell; the body tissues are composed of numerous types of cell, including those found in muscles and skin. Each has a cell membrane formed of phospholipids, cytoplasm and a nucleus. All of the different cells of an animal are derived from the embryonic germ layers; those simpler invertebrates which are formed from two germ layers of ectoderm and endoderm are called diploblastic and the more developed animals whose structures and organs are formed from three germ layers are called triploblastic.
All of a triploblastic animal's tissues and organs are derived from the three germ layers of the embryo, the ectoderm and endoderm. Animal tissues can be grouped into four basic types: connective, epithelial and nervous tissue. Connective tissues are fibrous and made up of cells scattered among inorganic material called the extracellular matrix. Connective tissue holds them in place; the main types are loose connective tissue, adipose tissue, fibrous connective tissue and bone. The extracellular matrix contains proteins, the chief and most abundant of, collagen. Collagen plays a major part in maintaining tissues; the matrix can be modified to form a skeleton to protect the body. An exoskeleton is a thickened, rigid cuticle, stiffened by mineralization, as in crustaceans or by the cross-linkin
Chitin n, a long-chain polymer of N-acetylglucosamine, is a derivative of glucose. It is a primary component of cell walls in fungi, the exoskeletons of arthropods, such as crustaceans and insects, the radulae of molluscs, cephalopod beaks, the scales of fish and lissamphibians; the structure of chitin is comparable to another polysaccharide - cellulose, forming crystalline nanofibrils or whiskers. In terms of function, it may be compared to the protein keratin. Chitin has proved useful for several medicinal and biotechnological purposes; the English word "chitin" comes from the French word chitine, derived in 1821 from the Greek word χιτών, meaning covering. A similar word, "chiton", refers to a marine animal with a protective shell; the structure of chitin was determined by Albert Hofmann in 1929. Chitin is a modified polysaccharide; these units form covalent β--linkages. Therefore, chitin may be described as cellulose with one hydroxyl group on each monomer replaced with an acetyl amine group.
This allows for increased hydrogen bonding between adjacent polymers, giving the chitin-polymer matrix increased strength. In its pure, unmodified form, chitin is translucent, pliable and quite tough. In most arthropods, however, it is modified, occurring as a component of composite materials, such as in sclerotin, a tanned proteinaceous matrix, which forms much of the exoskeleton of insects. Combined with calcium carbonate, as in the shells of crustaceans and molluscs, chitin produces a much stronger composite; this composite material is much harder and stiffer than pure chitin, is tougher and less brittle than pure calcium carbonate. Another difference between pure and composite forms can be seen by comparing the flexible body wall of a caterpillar to the stiff, light elytron of a beetle. In butterfly wing scales, chitin is organized into stacks of gyroids constructed of chitin photonic crystals that produce various iridescent colors serving phenotypic signaling and communication for mating and foraging.
The elaborate chitin gyroid construction in butterfly wings creates a model of optical devices having potential for innovations in biomimicry. Scarab beetles in the genus Cyphochilus utilize chitin to form thin scales that diffusely reflect white light; these scales are networks of randomly ordered filaments of chitin with diameters on the scale of hundreds of nanometres, which serve to scatter light. The multiple scattering of light is thought to play a role in the unusual whiteness of the scales. In addition, some social wasps, such as Protopolybia chartergoides, orally secrete material containing predominantly chitin to reinforce the outer nest envelopes, composed of paper. Chitosan is produced commercially by deacetylation of chitin. Nanofibrils have been made using chitosan. Chitin-producing organisms like protozoa, fungi and nematodes are pathogens in other species. Humans and other mammals have chitinase-like proteins that can degrade chitin. Chitin is sensed in the lungs or gastrointestinal tract where it can activate the innate immune system through eosinophils or macrophages, as well as an adaptive immune response through T helper cells.
Keratinocytes in skin can react to chitin or chitin fragments. According to in vitro studies, chitin is sensed by receptors, such as FIBCD1, KLRB1, REG3G, Toll-like receptor 2, CLEC7A, mannose receptors; the immune response can sometimes clear the chitin and its associated organism, but sometimes the immune response is pathological and becomes an allergy. Plants have receptors that can cause a response to chitin, namely chitin elicitor receptor kinase 1 and chitin elicitor-binding protein; the first chitin receptor was cloned in 2006. When the receptors are activated by chitin, genes related to plant defense are expressed, jasmonate hormones are activated, which in turn activate systematic defenses. Commensal fungi have ways to interact with the host immune response that as of 2016 were not well understood; some pathogens produce chitin-binding proteins. Zymoseptoria tritici is an example of a fungal pathogen. Chitin was present in the exoskeletons of Cambrian arthropods such as trilobites; the oldest preserved chitin dates to the Oligocene, about 25 million years ago, consisting of a scorpion encased in amber.
Chitin is a good inducer of plant defense mechanisms for controlling diseases. It has been assessed as a fertilizer that can improve overall crop yields. Chitin is used in industry in many processes. Examples of the potential uses of chemically modified chitin in food processing include the formation of edible films and as an additive to thicken and stabilize foods. Processes to size and strengthen paper employ chitosan. How chitin interacts with the immune system of plants and animals has been an active area of research, including the identity of key receptors with which chitin interacts, whether the size of chitin particles is relevant to the kind of immune response triggered, mechanisms by which immune systems respond. Chitin and chitosan have bee
Papillifera papillaris known as Papillifera bidens, is a species of small, air-breathing land snail with a clausilium, a terrestrial pulmonate gastropod mollusk in the family Clausiliidae, the door snails. This is a Mediterranean species. In Britain this species is now sometimes called the "Cliveden snail", as in 2004 a small colony was found to have been living on the estate at Cliveden House, a large stately home in Buckinghamshire, England. Individuals of the species had been living on an Italian balustrade, imported to Britain in the late 19th century, have survived at the estate for over a century before they were discovered there. Other introduced. There is a complicated nomenclatural problem with the name of this species; some argued. See further discussion under "Nomenclature"; the ICZN opinion, number 2176, preserved the name Turbo bidens Linnaeus, 1758, indicated implicitly that the name Helix papillaris Müller, 1774 was a junior synonym of the same species. However, at this time the meaning of the name Turbo bidens was not fixed with a valid type specimen designation.
In 2009 Kadolsky reviewed the nomenclatural history of the name Turbo bidens and concluded that a neotype designation proposed by Falkner et al. was invalid because it was not based on an existing specimen but on a figure of Papillifera papillaris published by Gualtierus, which did not agree with Linnaeus' description of Turbo bidens, which Linnaeus did not quote. Kadolsky argued that Linnaeus' brief description was consistent with a figure in Gualtierius' work that Linnaeus quoted, so Kadolsky fixed the meaning of the nominal species Turbo bidens Linnaeus, 1758 with the designation of a neotype; this neotype is a specimen from Florence of the clausiliid species hitherto known as Cochlodina incisa. However, the malacologist Hartmut Nordsieck and others did not accept Kadolsky's interpretation. One reason for this opinion was Linnaeus' description of the shell suture of Turbo bidens as "subcrenata"; this does not apply to Cochlodina incisa, except for minute crenellations which hardly deserve the name, but Gualtierius' figure does show these crenellations.
Kadolsky argued that Linnaeus described his species accordingly. Nordsieck and others instead argued that Linnaeus accidentally referred to the wrong figure, but that his verbal description was an accurate description of the Papillifera species. Kadolsky's neotype designation for Turbo bidens claims to fix the meaning of this name conclusively. In this case the valid name for the Papillifera species would be Papillifera papillaris. Others did not accept that the designation of a neotype was valid, in which case the correct name is Papillifera bidens; the issue was raised with the ICZN and their ruling was not to set aside Kadolsky's neotype. The shells of Papillifera papillaris are coiled sinistrally and, like other clausilids high-spired, with 10–11 whorls; the width of the shell is 3.2–3.8 mm, the height of the shell is 12–15 mm. The genus name Papillifera means "bearing papules", in other words having pimples, a reference to the small white shell structures along the suture line; the papules are noticeable.
In most of its range, this species lives in rocky limestone habitats, can be found near the seashore. The native range of this species is Mediterranean; this species has been introduced and has become established throughout the Mediterranean region, including Malta, Gibraltar, the south coast of France, Great Britain, Montenegro, Greece, Libya, Tunisia and Morocco. At least some of these introductions appear to have been accidental, on imported stonework, may in some cases date back to the Roman occupation of these areas, but the process is continuing: in 2009–2010, Papillifera papillaris imported on Italian limestone blocks were found to have survived overwinter in a stonemason's yard near Stuttgart, Germany. This snail was accidentally introduced to southern England, more than once, became established there. In 2004, the species was found in Buckinghamshire, southeastern England, in the crevices of a travertine marble and brick balustrade; this balustrade was constructed in Italy in about 1816, had stood in the grounds of the Villa Borghese, in Rome.
In the late 19th century the balustrade was taken from there, was installed in the formal gardens of the country house Cliveden in 1896. These small snails shelter in the many nooks and crannies of the travertine marble stonework; the snails at Cliveden were noticed by a specialist volunteer, cleaning the stonework and statuary. The snails have spread from the balustrade to a red brick terrace and a stone fountain, but no further than that. Although this is an introduced species, it is not an invasive species. Subsequent to the publicity surrounding this find, it was pointed out that the same species had been recorded in 1993 from Brownsea Island, Dorset in southwest England. At Brownsea Island, as at Cliveden, the snails live on stonework and statuary imported from Italy a century or more earlier. There are indications of a Dorset occurrence of this snail from the Brownsea I
The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, its base, are somewhat in flux; the period was established by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells; as a result, our understanding of the Cambrian biology surpasses that of some periods. The Cambrian marked a profound change in life on Earth. Complex, multicellular organisms became more common in the millions of years preceding the Cambrian, but it was not until this period that mineralized—hence fossilized—organisms became common; the rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia; the seas were warm, polar ice was absent for much of the period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified; the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult since the occurrence of similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia and Newfoundland, in the western USA.
The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, in Spain. The Cambrian Period was followed by the Ordovician Period; the Cambrian is divided into ten ages. Only three series and six stages are named and have a GSSP; because the international stratigraphic subdivision is not yet complete, many local subdivisions are still used. In some of these subdivisions the Cambrian is divided into three series with locally differing names – the Early Cambrian, Middle Cambrian and Furongian. Rocks of these epochs are referred to as belonging to Upper Cambrian. Trilobite zones allow biostratigraphic correlation in the Cambrian; each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance: *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.
The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian was held to represent the first appearance of complex life, represented by trilobites; the recognition of small shelly fossils before the first trilobites, Ediacara biota earlier, led to calls for a more defined base to the Cambrian period. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, to be correlated worldwide by the earliest appearance of Treptichnus pedum. Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, it is the T. pedum ichnofossil assemblage, now formally used to correlate the base of the Cambrian. This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate.
A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, to the disappearance of distinctive Ediacaran fossils. There are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which w
Ammonoids are an extinct group of marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs referred to as ammonites, are more related to living coleoids than they are to shelled nautiloids such as the living Nautilus species; the earliest ammonites appear during the Devonian, the last species died out in the Cretaceous–Paleogene extinction event. Ammonites are excellent index fossils, it is possible to link the rock layer in which a particular species or genus is found to specific geologic time periods, their fossil shells take the form of planispirals, although there were some helically spiraled and nonspiraled forms. The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble coiled rams' horns. Pliny the Elder called fossils of these animals ammonis cornua because the Egyptian god Ammon was depicted wearing ram's horns; the name of an ammonite genus ends in -ceras, Greek for "horn".
Ammonites 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 joint the outer shell wall, in general by their siphuncles. Ammonoid septa characteristically have bulges and indentations and are to varying degrees convex from the front, distinguishing them from nautiloid septa which are simple concave dish-shaped structures; the topology of the septa around the rim, results in the various suture patterns found. Three major types of suture patterns are found in the Ammonoidea: Goniatitic - numerous undivided lobes and saddles; this pattern is characteristic of the Paleozoic ammonoids. Ceratitic - lobes have subdivided tips, giving them a saw-toothed appearance, 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. 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 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 and typical Nautilida, in which the siphuncle runs through the center of each chamber. However the earliest nautiloids from the Late Cambrian and Ordovician had ventral siphuncles like ammonites, although proportionally larger and more internally structured; the word "siphuncle" comes from the New Latin siphunculus, meaning "little siphon". Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Devonian and became extinct at the close of the Cretaceous 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.
While nearly all nautiloids show curving sutures, the ammonoid suture line is variably folded, forming saddles and lobes. 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 - Upper CretaceousIn some classifications, these are left as suborders, included in only three orders: Goniatitida and Ammonitida; the Treatise on Invertebrate Paleontology divides the Ammonoidea, regarded as an order, into eight suborders, the Anarcestina, Clymeniina and Prolecanitina from the Paleozoic. In subsequent taxonomies, these are sometimes regarded as orders within the subclass Ammonoidea; because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are 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 lived in the open water of ancient seas, rather than at the sea bottom, because their fossils are found in rocks laid down under conditions where no bottom-dwelling life is found. Many of them are thought to have been good swimmers, with flattened, discus-shaped, streamlined shells, although some ammonoids were less effective swimmers and were 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, they may have avoided predation by squirting ink, much like modern cephalopods. 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 ab