95-431: See text Clydonautilaceae The Clydonautiloidea are a superfamily within the nautiloid order Nautilida characterized by smooth, generally globular, shells with nearly straight sutures , in early forms, but developing highly differentiated sutures in some later forms. Where known, the siphuncle tends to be central to subcentral. The Clydonautiloidea, name based on Hyatt's Clydonautilidae of 1900, are more or less
190-513: A Baltic coast Ordovician genus, in prior times it was employed as a general name given to all straight-shelled nautiloids that lived from the Ordovician to the Triassic periods (but were most common in the early Paleozoic era). Nautiloids are first known from the late Cambrian Fengshan Formation of northeastern China , where they seem to have been quite diverse (at the time this was
285-474: A grade group that is thought to have given rise to orthoceratoids, ammonoids and coleoids, and are defined by the exclusion of those descendent groups. Both ammonoids and coleoids have traditionally been assumed to have descended from bactritids , which in turn arose from straight-shelled orthoceratoids . The ammonoids appeared early in the Devonian period (some 400 million years ago) and became abundant in
380-411: A flat plane. The most fundamental difference in spiral form is how strongly successive whorls expand and overlap their predecessors. This can be inferred by the size of the umbilicus, the sunken-in inner part of the coil, exposing older and smaller whorls. Evolute shells have very little overlap, a large umbilicus, and many exposed whorls. Involute shells have strong overlap, a small umbilicus, and only
475-526: 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. The smallest ammonoid was Maximites from the Upper Carboniferous . Adult specimens reached only 10 mm (0.39 in) in shell diameter. Few of the ammonites occurring in the lower and middle part of the Jurassic period reached
570-524: 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 , and although they seemingly survived the Cretaceous–Paleogene extinction event , all known Paleocene ammonite lineages are restricted to
665-426: A growth spurt, the rear of the mantle secretes a new septum, adding another chamber to the series of shell chambers. At the same time, shell material is added around the shell opening ( aperture ), enlarging the body chamber and providing more room for the growing animal. Sutures (or suture lines) appear where each septum contacts the wall of the outer shell. In life, they are visible as a series of narrow wavy lines on
760-400: A head with two simple lens-free eyes and arms (or tentacles). They have a smooth shell over a large body chamber, which is divided into subchambers filled with an inert gas (similar to the composition of atmospheric air, but with more nitrogen and less oxygen ) making the animal neutrally buoyant in the water. As many as 90 tentacles are arranged in two circles around the mouth. The animal
855-405: A large external shell, divided into a narrowing chambered region (the phragmocone ) and a broad, open body chamber occupied by the animal in life. The outer wall of the shell, also known as the conch, defines its overall shape and texture. The chambers ( camerae ) of the phragmocone are separated from each other by thin curved walls ( septa ), which formed during growth spurts of the animal. During
950-488: A narrower definition of Nautiloidea ( Nautiloidea sensu stricto ), as a singular subclass including only those cephalopods which are closer to living nautiluses than they are to either ammonoids or coleoids. Nautiloids are among the group of animals known as cephalopods , an advanced class of mollusks which also includes ammonoids , belemnites and modern coleoids such as octopus and squid. Other mollusks include gastropods , scaphopods and bivalves . Traditionally,
1045-430: A process which may have been connected with controlling buoyancy . The nature of the siphuncle and its position within the shell are important in classifying nautiloids and can help distinguish them from ammonoids. The siphuncle is on the shell periphery in most ammonoids whereas it runs through the center of the chambers in some nautiloids, including living nautiluses. The subclass Nautiloidea, in its broader definition,
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#17327877270631140-411: 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. The chambered part of
1235-558: 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
1330-480: A single nautiloid suborder, the Nautilina , continued throughout the Mesozoic , where they co-existed quite happily with their more specialised ammonoid cousins. Most of these forms differed only slightly from the modern nautilus. They had a brief resurgence in the early Tertiary (perhaps filling the niches vacated by the ammonoids in the end Cretaceous extinction ), and maintained a worldwide distribution up until
1425-650: A size exceeding 23 cm (9.1 in) in diameter. Much larger forms are found in the later rocks of the upper part of the Jurassic and the lower part of the Cretaceous, such as Titanites from the Portland Stone of Jurassic of southern England, which is often 53 cm (1.74 ft) in diameter, and Parapuzosia seppenradensis of the Cretaceous period of Germany, which is one of the largest-known ammonites, sometimes reaching 2 m (6.6 ft) in diameter. The largest-documented North American ammonite
1520-417: A smooth shell. The shells are formed of aragonite, although the cameral deposits may consist of primary calcite. The coloration of the shell of the modern nautilus is quite prominent, and, although somewhat rarely, the shell coloration has been known to be preserved in fossil nautiloids. They often show color patterns only on the dorsal side, suggesting that the living animals swam horizontally. Much of what
1615-551: A subclass of its own, Bactritoidea . Recently some workers in the field have come to recognize Dissidocerida as a distinct order, along with Pseudorthocerida, both previously included in Orthocerida as subtaxa. Cladistic approaches are rare in nautiloid systematics. Many nautiloid orders (not to mention the group as a whole) are not monophyletic clades , but rather paraphyletic grades . This means that they include some descendant taxa while excluding others. For example,
1710-575: A suture like that of Clydonautilus , but with a more highly developed median saddle and double-pointed annular lobes. The Siberionautilidae , named by Popov in 1951 for the Upper Triassic Siberionautilus , have an involute, finely ribbed, globular shell with flattened flanks that converge toward a rounded venter and a highly differentiated goniatitic suture. The origin of the Clydonautiloidea (Liroceratidae)
1805-488: A warm shallow sea rich in marine life). However, although four orders have been proposed from the 131 species named, there is no certainty that all of these are valid, and indeed it is likely that these taxa are seriously oversplit. Most of these early forms died out, but a single family, the Ellesmeroceratidae , survived to the early Ordovician , where it ultimately gave rise to all subsequent cephalopods. In
1900-465: Is Baculites , which has a nearly straight shell convergent with the older orthocone nautiloids. Still other species' shells are coiled helically (in two dimensions), similar in appearance to some gastropods (e.g., Turrilites and Bostrychoceras ). Some species' shells are even initially uncoiled, then partially coiled, and finally straight at maturity (as in Australiceras ). Perhaps
1995-506: Is Parapuzosia bradyi from the Cretaceous, with specimens measuring 137 cm (4.5 ft) in diameter. Starting from the mid-Devonian, ammonoids were extremely abundant, especially as ammonites during the Mesozoic era. Many genera evolved and ran their course quickly, becoming extinct in a few million years. Due to their rapid evolution and widespread distribution, ammonoids are used by geologists and paleontologists for biostratigraphy . They are excellent index fossils , and it
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#17327877270632090-626: 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
2185-402: Is distinguished from other cephalopods by two main characteristics: the septa are smoothly concave in the forward direction, producing external sutures which are generally simple and smooth. The siphuncle is supported by septal necks which point to the rear (i.e. retrosiphonate) throughout the ontogeny of the animal. Modern nautiluses have deeply coiled shells which are involute, meaning that
2280-411: Is from κέρας ( kéras ) meaning "horn". 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 . Ammonoid septa characteristically have bulges and indentations and are to varying degrees convex when seen from
2375-540: Is from the mid to Upper Triassic. The Liroceratidae , named by Miller and Youngquist in 1949, are the ancestral family, which gave rise to the other four, and which has the longest duration, from the Upper Devonian to the Upper Triassic. The Liroceratidae are characterized by generally smooth shells with broadly rounded, depressed whorls, occluded umbilicus, slightly sinuous sutures, and a siphuncle that
2470-574: Is known about the extinct nautiloids is based on what we know about modern nautiluses , such as the chambered nautilus , which is found in the southwest Pacific Ocean from Samoa to the Philippines , and in the Indian Ocean off the coast of Australia . It is not usually found in waters less than 100 meters (328 feet) deep and may be found as far down as 500 to 700 meters (1,640 to 2,300 feet). Nautili are free swimming animals that possess
2565-516: Is most similar to coiled early nautiloids such as the Tarphycerida and Oncocerida. However, these orders diverged from coleoid ancestors in the early Ordovician at the latest, while genetic divergence estimates suggest that Nautilida diverged in the Silurian or Devonian. A more recent phylogenetic study by Lindgren et al. (2004), which supports the monophyly of cephalopods, does not bear on
2660-422: 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
2755-467: 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
2850-633: Is often preserved. This type of preservation is found in ammonites such as Hoplites from the Cretaceous Gault clay of Folkestone in Kent, England. The Cretaceous Pierre Shale formation of the United States and Canada is well known for the abundant ammonite fauna it yields, including Baculites , Placenticeras , Scaphites , Hoploscaphites and Jeletzkytes , as well as many uncoiled forms. Many of these also have much or all of
2945-426: Is predatory, and has jaws which are horny and beak-like, allowing it to feed on crustaceans . Empty nautilus shells may drift a considerable distance and have been reported from Japan , India and Africa . Undoubtedy the same applies to the shells of fossil nautiloids, the gas inside the shell keeping it buoyant for some time after the animal's death, allowing the empty shell to be carried some distance from where
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3040-589: Is the Treatise on Invertebrate Paleontology Part K by Teichert et al. 1964, though new information has rendered this volume outdated and in need of revision. Treatise Part K was based on previous classification schemes by Flower & Kummel (1950) and the Russian Osnovy Paleontologii Vol. 5 (1962) textbook. Other comprehensive taxonomic schemes have been devised by Wade (1988), Teichert (1988), and Shevyrev (2006). Wade (1988) divided
3135-538: Is thought to be because the female required a larger body size for egg production. A good example of this sexual variation is found in Bifericeras from the early part of the Jurassic period of Europe . 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
3230-657: Is unknown for sure, but is generally surmised as being from the Rutoceratidae, late in the Devonian. The Liroceratidae were the only clydonautilaceans living at the end of the Permian, when a significant number of nautilid genera perished, but enough survived to carry the family, and by inference the superfamily into the Triassic. Already reduced in number, although suddenly diverse, none of the Clydonautiloidea escaped
3325-650: Is usually more or less central. The Ephippioceratidae , named by Miller and Youngquist in 1949, which have a range from the Lower Carboniferous (Miss) to the Lower Permian, are closely similar to the Liroceratidae, but have deep ventral and dorsal saddles in the suture. The Gonionautilidae , named by Kummel in 1950 to contain the Upper Triassic genus Gonionautilus , have a smooth, involute, compressed shell with narrow flattened venter and
3420-591: The Carboniferous and Permian . The massive extinctions at the end of the Permian were less damaging to nautiloids than to other taxa and a few groups survived into the early Mesozoic , including pseudorthocerids , bactritids , nautilids and possibly orthocerids . The last straight-shelled forms were long thought to have disappeared at the end of the Triassic , but a possible orthocerid has been found in Cretaceous rocks. Apart from this exception, only
3515-482: The Mesozoic era , before their extinction at the end of the Cretaceous. Some workers apply the name Nautiloidea to a more exclusive group, called Nautiloidea sensu stricto . This taxon consists only of those orders that are clearly related to the modern nautilus to the exclusion of other modern cephalopods. In this restricted definition, membership is somewhat variable between authors, but it usually includes Tarphycerida, Oncocerida, and Nautilida. All nautiloids have
3610-536: The Neo-Latin siphunculus , meaning "little siphon". 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 (66 Mya). The classification of ammonoids is based in part on the ornamentation and structure of the septa comprising their shells' gas chambers. The Ammonoidea can be divided into six orders, listed here starting with
3705-621: The Ordovician period in the Baltic coast and parts of the United States contain a variety of nautiloid fossils, and specimens such as Discitoceras and Rayonnoceras may be found in the limestones of the Carboniferous period in Ireland . The marine rocks of the Jurassic period in Britain often yield specimens of Cenoceras , and nautiloids such as Eutrephoceras are also found in
3800-712: The Paleocene epoch (65–61 Ma). 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
3895-638: The Solnhofen Limestone , their soft-part record is surprisingly sparse. Beyond a tentative ink sac and possible digestive organs, no soft parts were known until 2021. When neutron imaging was used on a fossil found in 1998, part of the musculature became visible and showed they were able to retract themselves into the shell for protection, and that the retractor muscles and hyponome that work together to enable jet propulsion in nautilus worked independently in ammonites. The reproductive organs show possible traces of spermatophores, which would support
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3990-433: The buoyancy 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. One feature found in shells of
4085-653: 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. 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
4180-530: The Clydonautilidae, Gonionautilidae, and Siberonautilidae, all with prominent lobes and saddles in the suture. The families are described as: The Clydonautilidae , named by Hyatt in 1900, were the first to be defined and provide the name for the superfamily. Clydonautilids are characterized by generally smooth, involute, globular to compressed shells with a very small to occluded (hidden) umbilicus and sutures with prominent lobes and saddles. Their range
4275-484: The Devonian period through those of the Cretaceous period. Calcified aptychi only occur in ammonites from the Mesozoic 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
4370-483: The Early and Middle Ordovician the nautiloids underwent an evolutionary radiation. Some eight new orders appeared at this time, covering a great diversity of shell types and structure, and ecological lifestyles. Nautiloids remained at the height of their range of adaptations and variety of forms throughout the Ordovician, Silurian , and Devonian periods, with various straight, curved and coiled shell forms coexisting at
4465-678: The Liroceratina of Shimansky 1962. The Clydonautiloidea, which began in the Late Devonian, combine five families, three of which are restricted to the Triassic. Principal and forming the root stock are the Late Devonian to Upper Triassic Liroceratidae, from which Shimankiy derived his suborder. Derived from the Liroceratidae in the Early Mississippian are this Mississippian to mid Permian Ephippioceratidae. The three Triassic families, derived from Triassic liroceratids, are
4560-519: The Mesozoic in the Alps ). These rocks are usually accumulated at great depths. The modern Nautilus lacks any calcitic plate for closing its shell, and only one extinct nautiloid genus is known to have borne anything similar. Nautilus does, however, have a leathery head shield (the hood) which it uses to cover the opening when it retreats inside. There are many forms of aptychus, varying in shape and
4655-521: The Palcephalopod/Neocephalopod question, since the only cephalopods included were Nautilus and coleoids. For an in-process revision of Treatise Part K, King & Evans (2019) reclassified nautiloids sensu lato into five subclasses. Major groups were primarily defined by variation in their muscle attachment types. Other traits referenced during this reclassification include protoconch morphology, connecting ring structure, and
4750-505: The Pierre Shale formation of the Cretaceous period in the north-central United States. Specimens of the Ordovician nautiloid Endoceras have been recorded measuring up to 5.7 meters (19 feet) in shell length, and there is a description of a specimen estimated to have reached 9.1 meters (30 feet), although that specimen is reported as destroyed. These large nautiloids would have been formidable predators of other marine animals at
4845-440: The ammonite shell is called a phragmocone . It contains a series of progressively larger chambers, called camerae (sing. camera) that are divided by thin walls called septa (sing. septum). Only the last and largest chamber, the body chamber , was occupied by the living animal at any given moment. As it grew, it added newer and larger chambers to the open end of the coil. Where the outer whorl of an ammonite shell largely covers
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#17327877270634940-456: 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 (or ventral) saddle, when present, lies directly on
5035-456: The ancestors of subsequent stocks; Orthoceratoidea, which unites different primarily orthoconic orders (including the ancestors for Bacritida and Ammonoidea); and Nautilitoidea, which includes the first coiled cephalopods, Tarphycerida, as well as Nautilida, which includes the recent Nautilus . Another order, Bactritida , which is derived from Orthocerida , is sometimes included with Nautiloidea, sometimes with Ammonoidea , and sometimes placed in
5130-426: The animal lived before finally sinking to the seafloor. Nautili propel themselves by jet propulsion, expelling water from an elongated funnel called the hyponome , which can be pointed in different directions to control their movement. Unlike the belemnites and other cephalopods, modern nautili do not have an ink sac, and there is no evidence to suggest that the extinct forms possessed one either. Furthermore, unlike
5225-474: 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
5320-408: The cephalopods). Palcephalopoda is meant to correspond to groups which are closer to living nautilus, while Neocephalopoda is meant to correspond to groups closer to living coleoids. One issue which this scheme is the necessity of establishing a firm ancestry for nautilus, to contextualize which cephalopods are closer to which of the two living end members. On the basis of morphological traits, Nautilida
5415-419: The delicate balance of local redox conditions sufficiently to lower the local solubility of minerals dissolved in the seawater, notably phosphates and carbonates . The resulting spontaneous concentric precipitation of minerals around a fossil, a concretion , is responsible for the outstanding preservation of many ammonite fossils. When ammonites are found in clays , their original mother-of-pearl coating
5510-405: The development of OMZs , preventing nautiloids from retreating into deeper water, are also cited as other potential causes of extinction. A consensus on nautiloid classification has traditionally been elusive and subject to change, as different workers emphasize different fundamental traits when reconstructing evolutionary events. The largest and most widely cited publication on nautiloid taxonomy
5605-423: The extent of cameral and endosiphuncular deposits. While most previous studies referred to subclasses with the suffix '-oidea', these authors instead opted for the suffix '-ia', to prevent confusion between group levels. For example, Nautiloidea sensu stricto was renamed to Nautilia, to differentiate it from the informal broader definition of "nautiloid". In addition, they used the unsimplified names for orders, with
5700-408: The extinct ammonoids (ammonites) and living coleoids (such as squid , octopus , and kin). While ammonoids and coleoids are monophyletic clades with exclusive ancestor-descendant relationships, this is not the case for nautiloids. Instead, nautiloids are a paraphyletic grade of various early-diverging cephalopod lineages, including the ancestors of ammonoids and coleoids. Some authors prefer
5795-430: The extinct ammonoids , the modern nautilus lacks an aptychus , a biomineralized plate which is proposed to act as an operculum which closes the shell to protect the body. However, aptychus-like plates are known from some extinct nautiloids, and they may be homologous to the fleshy hood of a modern nautilus. Nautiloids are often found as fossils in early Palaeozoic rocks (less so in more recent strata). The rocks of
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#17327877270635890-466: The first heteromorph ammonoid fossils belongs to the genus Rhabdoceras. The three other heteromorphic genera were Hannaoceras, Cochloceras and Choristoceras. All of them went extinct at the end of Triassic. In the Jurassic an uncoiled shell was found in the Spiroceratoidea, but by the end of Cretaceous the only heteromorph ammonites remaining belonged to the suborder Ancyloceratina. One example
5985-420: The first saddle and lobe pair past the external region as the suture line extends up the side of the shell. The lateral saddle and lobe are usually larger than the ventral saddle and lobe. 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. In many cases the distinction between
6080-418: The fleshy tube of the siphuncle are structures made of aragonite (a polymorph of calcium carbonate – which during fossilisation is often recrystallized to calcite, a more stable form of calcium carbonate [CaCO 3 ]): septal necks and connecting rings. Some of the earlier nautiloids deposited calcium carbonate in the empty chambers (called cameral deposits ) or within the siphuncle ( endosiphuncular deposits ),
6175-433: 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. The septal curvature in nautiloids and ammonoids also differ in that the septa curves towards the opening in nautiloids, and away from the opening in ammоnoids. While nearly all nautiloids show gently curving sutures,
6270-503: The hypothesis that the microconchs were males. 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 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
6365-651: The larger and more recent whorls overlap and obscure older whorls. The shells of fossil nautiloids may be either straight (i.e., orthoconic as in Orthoceras and Rayonnoceras ), curved (as in Cyrtoceras ) coiled (as in Cenoceras ), or rarely a helical coil (as in Lorieroceras ). Some species' shells—especially in the late Paleozoic and early Mesozoic—are ornamented with spines and ribs, but most have
6460-511: The largest and most recent whorls are exposed. Shell structure can be broken down further by the width of the shell, with implications for hydrodynamic efficiency. Major shell forms include: Ammonites vary greatly in the ornamentation (surface relief) of their shells. Some may be smooth and relatively featureless, except for growth lines, resembling that of the modern Nautilus . In others, various patterns of spiral ridges, ribs, nodes, or spines are presented. This type of complex ornamentation of
6555-510: The lateral and umbilical regions are unclear; new umbilical features can develop from subdivisions of other umbilical features, or from subdivisions of lateral features. Lobes and saddles which are so far towards the center of the whorl that they are covered up by succeeding whorls are labelled internal (or dorsal) lobes and saddles. Three major types of suture patterns are found in the Ammonoidea: The siphuncle in most ammonoids
6650-434: The lower midline of the shell. As a result, it is often called the median saddle. On suture diagrams the median saddle is supplied with an arrow which points towards the aperture. The median saddle is edged by fairly small external (or ventral) lobes. The earliest ammonoids lacked a median saddle and instead had a single midline ventral lobe, which in later forms is split into two or more components. The lateral region involves
6745-513: The main predatory animals. Early in their evolution, nautiloids developed an extraordinary diversity of shell shapes, including coiled morphologies and giant straight-shelled forms ( orthocones ). No orthoconic and only a handful of coiled species, the nautiluses , survive to the present day. In a broad sense, "nautiloid" refers to a major cephalopod subclass or collection of subclasses ( Nautiloidea sensu lato ). Nautiloids are typically considered one of three main groups of cephalopods, along with
6840-547: The middle of the Cenozoic Era. With the global cooling of the Miocene and Pliocene , their geographic distribution shrank and these hardy and long-lived animals declined in diversity again. Today there are only six living species, all belonging to two genera, Nautilus (the pearly nautilus), and Allonautilus . The recent decrease in the once worldwide distribution of nautiloids is now believed to have been caused by
6935-432: The modern Nautilus is the variation in the shape and size of the shell according to the sex of the animal, the shell of the male being slightly smaller and wider than that of the female. This sexual dimorphism is thought to be an explanation for the variation in size of certain ammonite shells of the same species, the larger shell (the macroconch ) being female, and the smaller shell (the microconch ) being male. This
7030-447: The most common classification of the cephalopods has been a four-fold division (by Bather, 1888), into the orthoceratoids , nautiloids, ammonoids , and coleoids . This article is about nautiloids in that broad sense, sometimes called Nautiloidea sensu lato . Cladistically speaking, nautiloids are a paraphyletic assemblage united by shared primitive ( plesiomorphic ) features not found in derived cephalopods. In other words, they are
7125-485: 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. Nipponites occurs in rocks of the upper part of the Cretaceous in Japan and the United States. Some ammonites have been found in association with
7220-575: The most primitive and going to the more derived: In some classifications, these are left as suborders, included in only three orders: Goniatitida , Ceratitida and Ammonitida . 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;
7315-781: 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. In general, they appear to have inhabited the upper 250 meters of the water column. 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 . They may have avoided predation by squirting ink , much like modern cephalopods; ink
7410-478: The order Ammonitida , the only remaining group of ammonoids from the Jurassic up until their extinction. 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 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
7505-767: The order level (although various isolated families also originated during this diversification event): Plectronocerida Yanhecerida Ellesmerocerida ( paraphyletic to Endoceratoidea, Multiceratoidea, and Orthoceratoidea) Endocerida Bisonocerida Cyrtocerinida Tarphycerida (possibly paraphyletic to Nautilida ) Oncocerida (paraphyletic to Ascocerida and Discosorida) Ascocerida Discosorida Riocerida (possibly paraphyletic to later orthoceratoids) Dissidocerida (paraphyletic to later orthoceratoids) Lituitida Actinocerida Pseudorthocerida Orthocerida (paraphyletic to Bactritida , Ammonoidea , and Coleoidea ) Ammonoidea Ammonoids are extinct spiral shelled cephalopods comprising
7600-478: The original shell, as well as the complete body chamber, still intact. Many Pierre Shale ammonites, and indeed many ammonites throughout earth history, are found inside concretions . Other fossils, such as many found in Madagascar and Alberta , Canada display iridescence . These iridescent ammonites are often of gem quality ( ammolite ) when polished. In no case would this iridescence have been visible during
7695-400: The outer surface of the shell. Like their underlying septa, the sutures of the nautiloids are simple in shape, being either straight or slightly curved. This is different from the "zigzag" sutures of the goniatites and the highly complex sutures of the ammonites. The septa are perforated by the siphuncle , a fleshy tube which runs through each of the internal chambers of the shell. Surrounding
7790-455: The paraphyletic order Orthocerida includes numerous orthocerids stretching through the Paleozoic, but it excludes colloids, despite colloids having a well-established ancestry among the orthocerids. Interpretations by Engeser (1996–1998) suggests that nautiloids, and indeed cephalopods in general, should be split into two main clades: Palcephalopoda (including all the nautiloids except Orthocerida and Ascocerida) and Neocephalopoda (the rest of
7885-461: The preceding whorls, the specimen is said to be involute (e.g., Anahoplites ). Where it does not cover those preceding, the specimen is said to be evolute (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
7980-494: 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"). The majority of ammonite species feature planispiral shells, tightly coiled in
8075-581: The same time. Several of the early orders became extinct over that interval, but others rose to prominence. Nautiloids began to decline in the Devonian, perhaps due to competition with their descendants and relatives the Ammonoids and Coleoids , with only the Nautilida holding their own (and indeed increasing in diversity). Their shells became increasingly tightly coiled, while both numbers and variety of non-nautilid species continued to decrease throughout
8170-519: 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 ( d. 79 AD near Pompeii) called fossils of these animals ammonis cornua (" 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
8265-479: 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. Although ammonites do occur in exceptional lagerstatten such as
8360-415: The shell is especially evident in the later ammonites of the Cretaceous. Ammonoids with a shell shape diverging from the typical planispiral form are known as heteromorphs , instead forming a conch with detached whorls (open coiling) or non-planispiral coiling. These types of shells evolved four times in ammonoids, with the first forms appearing already in the Devonian period. In late Norian age in Triassic
8455-469: The smaller extinction at the end of the Triassic. Nautiloidea 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 species rich, with over 2,500 recorded species. They flourished during the early Paleozoic era, when they constituted
8550-593: The spread of pinnipeds . From the Oligocene onward, the appearance of pinnipeds in the geological record of a region coincides with the disappearance of nautiloids from that region. As a result, nautiloids are now limited to their current distribution in the tropical Indo-Pacific Ocean, where pinnipeds are absent. The genus Aturia seem to have temporarily survive regions where pinnipeds were present through adaptations to fast and agile swimming, but eventually went extinct as well. Predation by short-snouted whales and
8645-432: The subclass Ammonoidea . They 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 earliest ammonoids appeared during the Devonian , with the last species vanishing during or soon after the Cretaceous–Paleogene extinction event . They are often called ammonites , which is most frequently used for members of
8740-447: The subclass Nautiloidea ( sensu lato ) into 6 superorders, incorporating orders that are phylogenetically related. They are: Three of these superorders were established for orders of uncertain placement: Endocerida, Actinocerida, and Discosorida. The other three unite related orders which share a common ancestor and form a branch of the nautiloid taxonomic tree: Plectronoceratoidea, which consists mostly of small Cambrian forms that include
8835-444: The suffix '-atida' rather than the common simplified form, '-ida'. Traditional nautiloid classification schemes emphasize certain character traits over others, potentially involving personal bias as to which traits are worth emphasizing according to different authors. This issue may be resolved by sampling all morphological traits equally through bayesian phylogenetic inference . The first cephalopod-focused paper to use this technique
8930-402: The time they lived. In some localities, such as Scandinavia and Morocco , the fossils of orthoconic nautiloids accumulated in such large numbers that they form limestones composed of nonspecific assemblages known as cephalopod beds , cephalopod limestones , nautiloid limestones , or Orthoceras limestones in the geological literature. Although the term Orthoceras now only refers to
9025-412: Was published by Pohle et al. (2022). They recovered several previously hypothesized groups, though many orders were determined to be paraphyletic. The study was focused on early cephalopod diversification in the Late Cambrian and Ordovician, and did not discuss in detail the origin of post-Ordovician groups. The following is a simplified version of their cladogram , showing early cephalopod relationships to
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