The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk . Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites , and the living nautiluses , cuttlefish , and Spirula . In the case of the cuttlefish, the siphuncle is indistinct and connects all the small chambers of that animal's highly modified shell; in the other cephalopods it is thread-like and passes through small openings in the septa (walls) dividing the camerae (chambers). Some older studies have used the term siphon for the siphuncle, though this naming convention is uncommon in modern studies to prevent confusion with a mollusc organ of the same name .
25-536: The Piloceratidae are a compressed, rapidly expanding, cyrtoconic brevicones with holochoanitic ventral siphuncles and simple endocones. Most likely evolved from Clitendoceras , a narrow, slightly endogastric genus intermediate in form between straight shelled Proendoceras and the bulkier Piloceratidae. Found in shallow carbonate marine sediments of Demingian through the Cassinian age, (essentially Arenigian, = early Middle -Upper Canadian) . Pilocerids split off from
50-773: A superorder instead. Rousseau Flower rejected this separation on the grounds that endocerids were no more diverse or complex than any other order. He considered them to be simply another order within the Nautiloidea. Flower (1958) divided the Endocerida into two suborders, the Proterocamerocerina and the Endocerina. As he defined the two suborders, Proterocamerocerina included the Proterocameroceratidae , Manchuroceratidae , and Emmonsoceratidae , while Endocerina included
75-671: A dozen cephalopod orders that appeared in the Lower Ordovician. They reached their greatest diversity during the Lower to Mid-Ordovician, but were already in decline by the middle of this period with most genera becoming extinct by the end of the Sandbian (late Ordovician), while some rare hangers on lasted into the Silurian . In any case, the endocerid lineage became completely extinct relatively early on in cephalopod history. Endocerids evolved from
100-482: A very thick and porous inner calcitic layer. This more strongly-mineralized form is known as a calciosiphonate connecting ring. Connecting rings are strongly variable in morphology, from narrow homogenous tubes to bulbous, segmented cavities. Some are infolded, sending lobes or blades of calcite into the siphuncle. Connecting rings are typically continuous with the septal necks, and are difficult to distinguish without close examination. However, their developmental origin
125-421: Is a passive process. Most energy is expended through the absorption of water from the chamber. Removing water from the chambers of the shell reduces the overall density of the shell, and thus the shell behaves as a flotation device comparable to the swim bladder in bony fish . Typically, cephalopods maintain a density close to that of sea water , allowing them to keep a stable buoyancy with minimal effort. In
150-700: Is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle . Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silurian . Their shells were variable in form. Some were straight ( orthoconic ), others curved (cyrtoconic); some were long (longiconic), others short (breviconic). Some long-shelled forms like Endoceras attained shell lengths close to 6 metres (20 ft). The related Cameroceras
175-435: Is anecdotally reported to have reached lengths approaching 9 metres (30 ft), but these claims are problematic. The overwhelming majority of endocerids and nautiloids in general are much smaller, usually less than a meter long when fully grown. Endocerids had a relatively small body chamber as well as a proportionally large siphuncle , which in some genera reached nearly half the shell diameter. This suggests that much of
200-426: Is assumed to have worked in the same general way as in living nautiluses . The siphuncle itself is only rarely preserved, but its shape can be inferred from hardened structures which lie around it. Many fossils show the holes where the siphuncle passes through each septum. Around these holes, the rim of the septum is bent into a stout aragonitic tube known as a septal neck (or siphuncle notch). In each chamber of
225-501: Is debated. Endocerids may have been the apex predators of the Ordovician, probably living close to the sea floor, and preying on trilobites , molluscs, brachiopods and other bottom-dwelling organisms. They were probably not active nektonic swimmers, but rather crawled over the floor of epicontinental seas or lay there in ambush. Although there is study that supports filter feeding ecology, according to hydrostatic properties, it
250-415: Is not likely and still supports benthic predators. Endocerids laid relatively large eggs, and hatched at a relatively large body size. It is likely that endocerids were demersal after hatching, as large eggs would make an easy target for predators in the pelagic zone. Endocerids may have migrated from their habitat in the open ocean to shallower water to lay their eggs. Endocerids were among some half
275-501: Is wholly separate from the shell and septa, and they utilize calcite rather than aragonite as a biomineralized reinforcement. Biomineralized structures which develop within the siphuncle are known as endosiphuncular deposits (or simply siphonal deposits). These may include horizontal partitions ( diaphragms ), stacked conical structures ( endocones ), longitudinal rods, and various other concretions. Endosiphuncular deposits are typically thin structures which may be homologous to parts of
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#1732881356215300-731: The Piloceratidae and Endoceratidae . Endocerid classification since then has relied on a dichotomy between short-shelled forms with complex endocones and long-shelled forms with simple endocones. Endocerid relationships have been difficult to establish both within the order and relative to other nautiloids. Their generally orthoconic shell shape and dorsomyarian muscle scars are similar to the subclass Orthoceratoidea , which are ancestral to ammonoids (ammonites) and coleoids (squid, octopus, etc.). However, their nautilosiphonate connecting ring structure and lack of cameral deposits are more similar to living nautilus and their proposed ancestors,
325-541: The Proterocameroceratidae very shortly after their inception and are the first family which the proterocamerocerids gave rise to. With the exception of Humeoceras, found in the middle Silurian , pilocerid genera are limited to the Lower Ordovician (Canadian in North America). The general shape of the pilocerid shell precludes an ambush predator lying in wait on the sea floor, or a stealthful hunter drifting through
350-483: The Orthocerida and Actinocerida . Endocerids reached enormous body sizes. The largest confirmed specimen, belonging to Endoceras giganteum , is 3 metres (9.8 ft) long as preserved, but is missing a substantial portion of its aboral end. The reconstructed length of the shell is nearly 6 metres (20 ft). An alleged endocerid specimen 30 feet (9.1 m) long is unconfirmed. The mode of life of endocerids
375-474: The earlier ellesmerocerids , most likely from a genus similar to Pachendoceras . This ellesmerocerid gave rise to Proendoceras , the earliest representative of the Proterocameroceratidae and hence of the Endocerida. Endocerids evolved from ellesmerocerids by reduction of siphuncle diaphragms and the development of endocones. In the early part of the mid-Lower Ordovician, the Endocerida quickly diversified into many different families. In true endocerids, there
400-442: The geologic past, many cephalopods grew to an enormous size (perhaps approaching ten meters in length) thanks to this. Generally, the siphuncle is unable to provide a way to change the density of shell rapidly and thus cause the animal to rise or sink at will; rather, the animal must swim up or down as required. Cephalopods with a wider siphuncle have a higher rate of metabolic activity. The siphuncle of fossilised cephalopods
425-420: The septae or connecting rings. In most fossil nautiluses, the siphuncle runs more or less through the center of each chamber, but in ammonites and belemnites it usually runs along the ventral edge of the shell. In some fossil straight shelled nautiloids, cylindrical calcareous growths ("siphuncular deposits") around the siphuncle can be seen towards the apex of the shell. These were apparently counterweights for
450-407: The shell grows. To perform this task, the cephalopod increases the saltiness of the blood in the siphuncle, and the water moves from the more dilute chamber into the blood through osmosis . At the same time gasses, mostly nitrogen , oxygen , and carbon dioxide , diffuse from the blood in the siphuncle into the emptying chamber. This is not a form of active pumping: the gas moving into the chamber
475-410: The shell, the siphuncle is encased by a tubular structure known as a connecting ring . In living nautiluses, the connecting ring is a simple, thin-walled cylinder, with organic or thinly calcitic layers secreted from the tissues of the siphuncle. This fragile and poorly-mineralized form is known as a nautilosiphonate morphology. Many extinct cephalopods have a much more prominent connecting ring, with
500-523: The soft body at the other end of the shell, and allowed the nautilus to swim in a horizontal position. Without these deposits, the apex of the buoyant shell would have pointed upwards and the heavier body downwards, making horizontal swimming difficult. The siphuncle of the Endocerida also contained much of the organisms' body organs. Endocerida † Cyrtendoceratidae † Endoceratidae † Proterocameroceratidae † Yorkoceratidae Endocerida
525-685: The strongly curved Piloceras , Bisonoceras with its hooked bison horn shape, nearly straight Allopiloceras , Dartonoceras , and internally complex Cassinoceras . Ecologically the Piloceratidae were replaced in the Middle Ordovician by the Cyrtendoceradae, which may have evolved early from the Endoceratidae rather than being direct pilocerid descendants. Siphuncle The siphuncle is used primarily in emptying water from new chambers as
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#1732881356215550-412: The subclass Multiceratoidea . Some studies have re-established Endoceratoidea to clarify that endocerids occupy a unique subclass of nautiloids. Restudy of piloceratid-like families with complex endocones has suggested that Endocerida in its broadest form is polyphyletic , with piloceratid-like and proterocameroceratid-like members having independent origins from ellesmerocerids. In light of this issue,
575-495: The visceral mass may have been housed within the siphuncle itself rather than just in the body chamber as with other nautiloids (Teichert, 1964). Endocerids are primarily distinguished by the presence of conical calcareous deposits, known as endocones, found in the more apical portion of the siphuncle. They are thought to act as a counterweight for the animal’s body. The chambers ( camerae ) of endocerids are always free of organic deposits, unlike orthoceratoid cephalopod orders such as
600-445: The water. Rather, they probably crawled over the sea floor, head down with the shell off the bottom, searching for prey. Nothing is known of their soft part anatomy, other than what can be surmised from the fact they are cephalopods. They likely possessed tentacles of some sort, but were they numerous like those of modern Nautilus , limited to eight or ten as with octopus or squid, or of some other arrangement. Pilocerid genera include
625-515: Was a trend of overall increasing size, eventually resulting in massive orthoconic genera such as Endoceras and Cameroceras . In another lineage (now known as Bisonocerida ), the siphuncle grew more complex, resulting in genera such as Chihlioceras and Allotrioceras . Citing its diversity, Curt Teichert (1964) placed the Endocerida in its own subclass called the Endoceroidea or Endoceratoidea (which some Russian paleontologists ranked as
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