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91-735: Some scientists have argued that the Jehol Biota evolved directly from the preceding Daohugou Biota without any strongly defined division. However, the absolute dating of the Daohugou beds has been the subject of divergent opinion: in 2006, Wang et al. found an overall similarity between the fossil animals found in the Daohugou Beds and the "Jehol Biota" from the Yixian Formation. Several other research teams, including Liu et al. , have attempted to disprove this reasoning by using Zircon U-Pb dating on

182-410: A bivalve is composed of calcium carbonate , and consists of two, usually similar, parts called valves . These valves are for feeding and for disposal of waste. These are joined together along one edge (the hinge line ) by a flexible ligament that, usually in conjunction with interlocking "teeth" on each of the valves, forms the hinge . This arrangement allows the shell to be opened and closed without

273-443: A calcified exoskeleton consisting of a hinged pair of half- shells known as valves . As a group, bivalves have no head and lack some typical molluscan organs such as the radula and the odontophore . Their gills have evolved into ctenidia , specialised organs for feeding and breathing. Common bivalves include clams , oysters , cockles , mussels , scallops , and numerous other families that live in saltwater, as well as

364-420: A different way, scraping detritus from the seabed, and this may be the original mode of feeding used by all bivalves before the gills became adapted for filter feeding. These primitive bivalves hold on to the bottom with a pair of tentacles at the edge of the mouth, each of which has a single palp , or flap. The tentacles are covered in mucus , which traps the food, and cilia, which transport the particles back to

455-528: A feature shared with two other major groups of marine invertebrates, the bryozoans and the phoronids . Some brachiopod shells are made of calcium phosphate but most are calcium carbonate in the form of the biomineral calcite , whereas bivalve shells are always composed entirely of calcium carbonate, often in the form of the biomineral aragonite . The Cambrian explosion took place around 540 to 520 million years ago (Mya). In this geologically brief period, most major animal phyla diverged including some of

546-460: A hinged shell in two parts. However, brachiopods evolved from a very different ancestral line, and the resemblance to bivalves only arose because they occupy similar ecological niches . The differences between the two groups are due to their separate ancestral origins. Different initial structures have been adapted to solve the same problems, a case of convergent evolution . In modern times, brachiopods are not as common as bivalves. Both groups have

637-420: A number of families that live in freshwater. Majority of the class are benthic filter feeders that bury themselves in sediment, where they are relatively safe from predation . Others lie on the sea floor or attach themselves to rocks or other hard surfaces. Some bivalves, such as scallops and file shells , can swim . Shipworms bore into wood, clay, or stone and live inside these substances. The shell of

728-499: A pair of nephridia . Each of these consists of a long, looped, glandular tube, which opens into the pericardium , and a bladder to store urine. They also have pericardial glands either line the auricles of the heart or attach to the pericardium, and serve as extra filtration organs. Metabolic waste is voided from the bladders through a nephridiopore near the front of the upper part of the mantle cavity and excreted. The sexes are usually separate in bivalves but some hermaphroditism

819-483: A partial skeleton with soft tissue impressions. Darwinopterus D. modularis Liaoning A wukongopterid named after Charles Darwin . The type species, D. modularis was the first known pterosaur to display features of both long-tailed rhamphorhynchoids and short-tailed pterodactyloids , and was described as a transitional fossil between the two groups. Darwinopterus specimens have also been reported to show several differences between males and females, with

910-551: A partial skull and lower jaw. Cascocauda C. rong Hebei Daohugou bed One specimen A long-tailed batrachognathine anurognathid known from a complete skeleton of a juvenile with extensive preservation of pycnofibres and wing membranes. Changchengopterus C. pani Hebei A pterodactyliform known only from a single specimen of a young juvenile, measuring 475 millimeters (18.7 in) in wingspan. Daohugoupterus D. delicatus Inner Mongolia One specimen A relatively basal pterosaur known from

1001-484: A relatively small dispersal potential before settling out. The common mussel ( Mytilus edulis ) produces 10 times as many eggs that hatch into larvae and soon need to feed to survive and grow. They can disperse more widely as they remain planktonic for a much longer time. Freshwater bivalves have different lifecycle. Sperm is drawn into a female's gills with the inhalant water and internal fertilization takes place. The eggs hatch into glochidia larvae that develop within

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1092-644: A resting state, even when they are permanently submerged. In oysters, for example, their behaviour follows very strict circatidal and circadian rhythms according to the relative positions of the moon and sun. During neap tides, they exhibit much longer closing periods than during spring tides. Although many non-sessile bivalves use their muscular foot to move around, or to dig, members of the freshwater family Sphaeriidae are exceptional in that these small clams climb about quite nimbly on weeds using their long and flexible foot. The European fingernail clam ( Sphaerium corneum ), for example, climbs around on water weeds at

1183-424: A shadow falling over the animal. Bivalves have an open circulatory system that bathes the organs in blood ( hemolymph ). The heart has three chambers: two auricles receiving blood from the gills, and a single ventricle . The ventricle is muscular and pumps hemolymph into the aorta , and then to the rest of the body. Some bivalves have a single aorta, but most also have a second, usually smaller, aorta serving

1274-436: A shell consisting of two valves, but the organization of the shell is quite different in the two groups. In brachiopods, the two valves are positioned on the dorsal and ventral surfaces of the body, while in bivalves, the valves are on the left and right sides of the body, and are, in most cases, mirror images of one other. Brachiopods have a lophophore , a coiled, rigid cartilaginous internal apparatus adapted for filter feeding,

1365-424: A steady pull. Paired pedal protractor and retractor muscles operate the animal's foot. The sedentary habits of the bivalves have meant that in general the nervous system is less complex than in most other molluscs. The animals have no brain ; the nervous system consists of a nerve network and a series of paired ganglia . In all but the most primitive bivalves, two cerebropleural ganglia are on either side of

1456-619: A strange skull morphology, at first believed to come from the Early Cretaceous . Liaoxitriton L. daohugouensis A little-known cryptobranchoid. Pangerpeton P. sinensis A cryptobranchoid characterized by its short trunk (only 14 presacrals) and short and wide head, giving a fat body shape, from which the genus name was derived ("Pang" means fat in Chinese). Archaeoistiodactylus A. linglongtaensis Liaoning A monofenestratan known from an incomplete skeleton with

1547-432: Is composed of two calcareous valves held together by a ligament. The valves are made of either calcite , as is the case in oysters, or both calcite and aragonite . Sometimes, the aragonite forms an inner, nacreous layer, as is the case in the order Pteriida . In other taxa , alternate layers of calcite and aragonite are laid down. The ligament and byssus, if calcified, are composed of aragonite. The outermost layer of

1638-557: Is known. The gonads either open into the nephridia or through a separate pore into a chamber over the gills. The ripe gonads of males and females release sperm and eggs into the water column . Spawning may take place continually or be triggered by environmental factors such as day length, water temperature, or the presence of sperm in the water. Some species are "dribble spawners", releasing gametes during protracted period that can extend for weeks. Others are mass spawners and release their gametes in batches or all at once. Fertilization

1729-610: Is particularly noteworthy for the very high diversity of fossils and the very large numbers of individuals of each species that have been recovered. The Jehol Biota has produced fossils of plant macro- and microfossils , including angiosperms (the earliest known), charophytes and dinocysts , snails ( gastropods ), clams ( bivalves ), superabundant aquatic arthropods called conchostracans , ostracods , shrimps , insects , spiders , fish , frogs and salamanders ( amphibians ), turtles , choristoderes , lizards ( squamates ), pterosaurs , and dinosaurs including feathered dinosaurs ,

1820-408: Is secreted by the mantle edge, and the valves themselves thicken as more material is secreted from the general mantle surface. Calcareous matter comes from both its diet and the surrounding seawater. Concentric rings on the exterior of a valve are commonly used to age bivalves. For some groups, a more precise method for determining the age of a shell is by cutting a cross section through it and examining

1911-439: Is that the land animals and plants were washed into the lakes very gently, or were already in the lakes when they died. They do not show the damage seen in fossils formed by large floods. Secondly, volcanic ash is commonly inter-bedded with lake sediments, and ashfalls seem to have quickly buried the fossilized organisms, creating anoxic conditions around them and preventing scavenging. Zhonghe Zhou et al. (2003) noted that, for

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2002-472: Is the ventral or underside region. The anterior or front of the shell is where the byssus (when present) and foot are located, and the posterior of the shell is where the siphons are located. With the hinge uppermost and with the anterior edge of the animal towards the viewer's left, the valve facing the viewer is the left valve and the opposing valve the right. Many bivalves such as clams, which appear upright, are evolutionarily lying on their side. The shell

2093-432: Is usually external. Typically, a short stage lasts a few hours or days before the eggs hatch into trochophore larvae. These later develop into veliger larvae which settle on the seabed and undergo metamorphosis into adults. In some species, such as those in the genus Lasaea , females draw water containing sperm in through their inhalant siphons and fertilization takes place inside the female. These species then brood

2184-661: The Early Cretaceous , the Jehol Biota includes a mixture of advanced and ancient species, and also of species found only in the Jehol and others found all around the world. It is possible that northeast Asia was isolated for part of the Jurassic by the Turgai Sea which separated Europe from Asia at the time. The Jehol Biota includes many species that were previously known only from the Late Jurassic or earlier. These "relict" species include

2275-595: The Early Silurian , the gills were becoming adapted for filter feeding, and during the Devonian and Carboniferous periods, siphons first appeared, which, with the newly developed muscular foot, allowed the animals to bury themselves deep in the sediment. By the middle of the Paleozoic , around 400 Mya, the brachiopods were among the most abundant filter feeders in the ocean, and over 12,000 fossil species are recognized. By

2366-573: The Huajiying Formation . Daohugou Biota The Tiaojishan Formation is a geological formation in Hebei and Liaoning , People's Republic of China , dating to the middle- late Jurassic period ( Bathonian - Oxfordian stages). It is known for its exceptionally preserved fossils, including those of plants , insects and vertebrates. It is made up mainly of pyroclastic rock interspersed with basic volcanic and sedimentary rocks . Previously,

2457-721: The Permian–Triassic extinction event 250 Mya, bivalves were undergoing a huge radiation of diversity. The bivalves were hard hit by this event, but re-established themselves and thrived during the Triassic period that followed. In contrast, the brachiopods lost 95% of their species diversity . The ability of some bivalves to burrow and thus avoid predators may have been a major factor in their success. Other new adaptations within various families allowed species to occupy previously unused evolutionary niches. These included increasing relative buoyancy in soft sediments by developing spines on

2548-571: The Veneridae , with more than 680 species and the Tellinidae and Lucinidae , each with over 500 species. The freshwater bivalves include seven families, the largest of which are the Unionidae , with about 700 species. The taxonomic term Bivalvia was first used by Linnaeus in the 10th edition of his Systema Naturae in 1758 to refer to animals having shells composed of two valves . More recently,

2639-479: The compsognathid dinosaur Sinosauropteryx and the anurognathid pterosaur Dendrorhynchoides . It also has the earliest and most primitive known members of groups that spread all around the world by the Late Cretaceous , including neoceratopsians , therizinosaurs , tyrannosaurs , and oviraptorids . Northeastern Asia may have been the center of diversification of these dinosaur groups. The Jehol Biota

2730-591: The intertidal and sublittoral zones of the oceans. A sandy sea beach may superficially appear to be devoid of life, but often a very large number of bivalves and other invertebrates are living beneath the surface of the sand. On a large beach in South Wales , careful sampling produced an estimate of 1.44 million cockles ( Cerastoderma edule ) per acre of beach. Bivalves inhabit the tropics, as well as temperate and boreal waters. A number of species can survive and even flourish in extreme conditions. They are abundant in

2821-517: The oesophagus . The cerebral ganglia control the sensory organs, while the pleural ganglia supply nerves to the mantle cavity. The pedal ganglia, which control the foot, are at its base, and the visceral ganglia, which can be quite large in swimming bivalves, are under the posterior adductor muscle. These ganglia are both connected to the cerebropleural ganglia by nerve fibres . Bivalves with long siphons may also have siphonal ganglia to control them. The sensory organs of bivalves are largely located on

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2912-400: The " crystalline style " projected into the stomach from an associated sac. Cilia in the sac cause the style to rotate, winding in a stream of food-containing mucus from the mouth, and churning the stomach contents. This constant motion propels food particles into a sorting region at the rear of the stomach, which distributes smaller particles into the digestive glands, and heavier particles into

3003-454: The Arctic, about 140 species being known from that zone. The Antarctic scallop, Adamussium colbecki , lives under the sea ice at the other end of the globe, where the subzero temperatures mean that growth rates are very slow. The giant mussel, Bathymodiolus thermophilus , and the giant white clam, Calyptogena magnifica , both live clustered around hydrothermal vents at abyssal depths in

3094-566: The Binggou, Haizhou and Upper Volcanic formations). Chiappe et al. argued in 1999 that the lower beds of the Yixian were best subdivided into a separate formation, the Chaomidianzi Formation, with a type locality at the village of Sihetun, approximately 25 km south of Beipiao City. However, this classification has fallen out of favor, and the Chaomidianzi Formation is disused as a synonym of

3185-479: The Jehol Biota is represented by the Huajiying Formation . The Yixian and Jiufotang Formations are considered Lagerstätte , meaning that they have exceptionally good conditions for fossil preservation. The fossils are numerous, but also very well preserved – often including articulated skeletons, soft tissues, colour patterns, stomach contents, and twigs with leaves and flowers still attached. Zhonghe Zhou et al. . (2003) deduced two things from this. The first

3276-547: The Jehol Biota: The name "Jehol Biota" was first published by Gu (1962), but was in use by geologists and paleontologists by 1959. This term replaced the former "Jehol Fauna", which Amadeus William Grabau (1923) defined as the fossil assemblage typified by numerous fossils of the conchostracan Eosestheria , the mayfly Ephemeropsis , and the Teleost fish Lycoptera . Thus it was sometimes called "EEL". The Jehol group

3367-459: The Jianshangou Bed of the Yixian Formation. In 2008, Ji et al. argued that these traditional definitions of the Jehol Biota arbitrarily excluded earlier fossil beds that clearly represent the first evolutionary stages of the later faunas, even though lower beds also had representatives of Ephemeropsis and Lycoptera . They argued that the boundaries of the biota should rather be set based on

3458-457: The Pacific Ocean. They have chemosymbiotic bacteria in their gills that oxidise hydrogen sulphide , and the molluscs absorb nutrients synthesized by these bacteria. Some species are found in the hadal zone , like Vesicomya sergeevi, which occurs at depths of 7600–9530 meters. The saddle oyster, Enigmonia aenigmatica , is a marine species that could be considered amphibious . It lives above

3549-498: The Romans, and mariculture has more recently become an important source of bivalves for food. Modern knowledge of molluscan reproductive cycles has led to the development of hatcheries and new culture techniques. A better understanding of the potential hazards of eating raw or undercooked shellfish has led to improved storage and processing. Pearl oysters (the common name of two very different families in salt water and fresh water) are

3640-651: The Tiaojishan Formation was grouped together with the underlying Haifanggou Formation (also known as the Jiulongshan Formation) as a single " Lanqi Formation ." The Tiaojishan Formation forms a key part of the Yanliao Biota assemblage, alongside the Haifanggou Formation. Using Argon–argon dating , Wang and colleagues in 2005 dated part of the Tiaojishan Formation to about 160 million years ago,

3731-467: The air, can gape the shell slightly and gas exchange can take place. Oysters, including the Pacific oyster ( Magallana gigas ), are recognized as having varying metabolic responses to environmental stress, with changes in respiration rate being frequently observed. Most bivalves are filter feeders , using their gills to capture particulate food such as phytoplankton from the water. Protobranchs feed in

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3822-536: The beginning of the Oxfordian stage , the first stage of the Upper Jurassic epoch. In 2006, a study by Liu and colleagues used U-Pb zircon dating to conclude that the Tiaojishan Formation correlates with the Daohugou Beds, and the complete chronological range of this shared biota dates to between 168 and 164/152 Ma ago. A subsequent study, published in 2008, refined the age range of the formation further, finding that

3913-568: The class was known as Pelecypoda, meaning " axe -foot" (based on the shape of the foot of the animal when extended). The name "bivalve" is derived from the Latin bis , meaning 'two', and valvae , meaning 'leaves of a door'. ("Leaf" is an older word for the main, movable part of a door. We normally consider this the door itself.) Paired shells have evolved independently several times among animals that are not bivalves; other animals with paired valves include certain gastropods (small sea snails in

4004-526: The distinctive large-scale sequences of volcanism which produced the strata, with the upper boundary set at the Shahai and Fuxin formations and the lower boundary at the Zhangjiakou Formation . Along with this sedimentary correlation, they noted that the best index fossils to identify the biota are Peipiaosteus and Lycoptera . Under this definition, the earliest stage of the Jehol Biota is represented by

4095-1301: The earliest known gliding mammal ( Volaticotherium ) have been discovered in these rocks.The tuffaceous composition of some rock layers show that this was a volcanic area, occasionally experiencing heavy ashfalls from eruptions. The landscape then was dominated by mountain streams and deep lakes surrounded by forests of gymnosperm trees. The forests of the Yanliao biota grew in a humid, warm - temperate climate and were dominated by gymnosperm trees. There were ginkgopsids like Ginkoites , Ginkgo , Baiera , Czekanowskia , and Phoenicopsis . There were also conifers like Pityophyllum , Rhipidiocladus , Elatocladus , Schizolepis , and Podozamites . Also, Lycopsids like Lycopodites and Sellaginellities , horsetails ( Sphenopsida ) like Equisetum , cycads like Anomozamites , and ferns ( Filicopsida ) like Todites and Coniopteris . Yanliaomyzon Y. occisor Liaoning A lamprey . Beiyanerpeton B. jianpingensis Liaoning A salamandroid known from an almost complete and articulated skeleton exposed in ventral view. Chunerpeton C. tianyiensis A cryptobranchoid measuring 18 centimeters in length. Jeholotriton J. paradoxus A cryptobranchoid with

4186-436: The edges of lakes and ponds; this enables the clam to find the best position for filter feeding. The thick shell and rounded shape of bivalves make them awkward for potential predators to tackle. Nevertheless, a number of different creatures include them in their diet. Many species of demersal fish feed on them including the common carp ( Cyprinus carpio ), which is being used in the upper Mississippi River to try to control

4277-402: The family Juliidae ), members of the phylum Brachiopoda and the minute crustaceans known as ostracods and conchostracans . Bivalves have bilaterally symmetrical and laterally flattened bodies, with a blade-shaped foot, vestigial head and no radula . At the dorsal or back region of the shell is the hinge point or line, which contain the umbo and beak and the lower, curved margin

4368-421: The female's shell. Later they are released and attach themselves parasitically to the gills or fins of a fish host. After several weeks they drop off their host, undergo metamorphosis and develop into adults on the substrate . Some of the species in the freshwater mussel family, Unionidae , commonly known as pocketbook mussels, have evolved an unusual reproductive strategy. The female's mantle protrudes from

4459-434: The first creatures with mineralized skeletons. Brachiopods and bivalves made their appearance at this time, and left their fossilized remains behind in the rocks. Possible early bivalves include Pojetaia and Fordilla ; these probably lie in the stem rather than crown group. Watsonella and Anabarella are perceived to be (earlier) close relatives of these taxa. Only five genera of supposed Cambrian "bivalves" exist,

4550-434: The fish's gills, where they attach and trigger a tissue response that forms a small cyst around each larva. The larvae then feed by breaking down and digesting the tissue of the fish within the cysts. After a few weeks they release themselves from the cysts and fall to the stream bed as juvenile molluscs. Brachiopods are shelled marine organisms that superficially resemble bivalves in that they are of similar size and have

4641-467: The gills are also much longer than those in more primitive bivalves, and are folded over to create a groove through which food can be transported. The structure of the gills varies considerably, and can serve as a useful means for classifying bivalves into groups. A few bivalves, such as the granular poromya ( Poromya granulata ), are carnivorous , eating much larger prey than the tiny microalgae consumed by other bivalves. Muscles draw water in through

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4732-577: The high tide mark in the tropical Indo-Pacific on the underside of mangrove leaves, on mangrove branches, and on sea walls in the splash zone . Some freshwater bivalves have very restricted ranges. For example, the Ouachita creekshell mussel, Villosa arkansasensis , is known only from the streams of the Ouachita Mountains in Arkansas and Oklahoma, and like several other freshwater mussel species from

4823-434: The hind parts of the animal. The hemolymph usually lacks any respiratory pigment. In the carnivorous genus Poromya , the hemolymph has red amoebocytes containing a haemoglobin pigment. The paired gills are located posteriorly and consist of hollow tube-like filaments with thin walls for gas exchange . The respiratory demands of bivalves are low, due to their relative inactivity. Some freshwater species, when exposed to

4914-399: The incremental growth bands. The shipworms , in the family Teredinidae have greatly elongated bodies, but their shell valves are much reduced and restricted to the anterior end of the body, where they function as scraping organs that permit the animal to dig tunnels through wood. The main muscular system in bivalves is the posterior and anterior adductor muscles. These muscles connect

5005-521: The inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey. Many bivalves have no eyes, but a few members of the Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea have simple eyes on the margin of the mantle. These consist of a pit of photosensory cells and a lens . Scallops have more complex eyes with a lens, a two-layered retina , and a concave mirror. All bivalves have light-sensitive cells that can detect

5096-421: The inhalant siphon which is modified into a cowl-shaped organ, sucking in prey. The siphon can be retracted quickly and inverted, bringing the prey within reach of the mouth. The gut is modified so that large food particles can be digested. The unusual genus, Entovalva , is endosymbiotic , being found only in the oesophagus of sea cucumbers . It has mantle folds that completely surround its small valves. When

5187-443: The intestine. Waste material is consolidated in the rectum and voided as pellets into the exhalent water stream through an anal pore. Feeding and digestion are synchronized with diurnal and tidal cycles. Carnivorous bivalves generally have reduced crystalline styles and the stomach has thick, muscular walls, extensive cuticular linings and diminished sorting areas and gastric chamber sections. The excretory organs of bivalves are

5278-489: The invasive zebra mussel ( Dreissena polymorpha ). Birds such as the Eurasian oystercatcher ( Haematopus ostralegus ) have specially adapted beaks which can pry open their shells. The herring gull ( Larus argentatus ) sometimes drops heavy shells onto rocks in order to crack them open. Sea otters feed on a variety of bivalve species and have been observed to use stones balanced on their chests as anvils on which to crack open

5369-454: The largest mammals known from the Mesozoic , and a great diversity of birds including the earliest advanced birds. The forests around the lakes were dominated by conifers including members of the podocarp , pine , araucaria , and cypress families. There were also ginkgos , czekanowskialeans , bennettitaleans , ephedra , horsetails , ferns , and mosses . The leaves and needles of

5460-485: The largest living bivalve is the giant clam Tridacna gigas , which can grow to a length of 1,200 mm (47 in) and a weight of more than 200 kg (441 lb). The largest known extinct bivalve is a species of Platyceramus whose fossils measure up to 3,000 mm (118 in) in length. In his 2010 treatise, Compendium of Bivalves , Markus Huber gives the total number of living bivalve species as about 9,200 combined in 106 families. Huber states that

5551-644: The lower boundary of the Tiaojishan was formed 165 Ma ago, and the upper boundary somewhere between 156 and 153 Ma ago. Based on the plant life present in the Tiaojishan Formation, Wang Yongdong and colleagues determined that the climate in Liaoning during the mid Jurassic would have been subtropical to temperate , warm and humid . Beautifully preserved fossils of dinosaurs , pterosaurs , salamanders , insects , arachnids and other invertebrates , conifers , ginkgoes , cycads , horsetails , and ferns , and even

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5642-520: The males having distinctive crests on their heads. They are known to have laid their eggs on the ground, and may have also not shown that much for parental care. Bivalve And see text Bivalvia ( / b aɪ ˈ v æ l v i ə / ) or bivalves , in previous centuries referred to as the Lamellibranchiata and Pelecypoda , is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by

5733-418: The mantle lobes secrete the valves, and the mantle crest secretes the whole hinge mechanism consisting of ligament , byssus threads (where present), and teeth . The posterior mantle edge may have two elongated extensions known as siphons , through one of which water is inhaled, and the other expelled. The siphons retract into a cavity, known as the pallial sinus . The shell grows larger when more material

5824-452: The mantle though the valves. In sedentary or recumbent bivalves that lie on one valve, such as the oysters and scallops, the anterior adductor muscle has been lost and the posterior muscle is positioned centrally. In species that can swim by flapping their valves, a single, central adductor muscle occurs. These muscles are composed of two types of muscle fibres, striated muscle bundles for fast actions and smooth muscle bundles for maintaining

5915-626: The most common source of natural pearls . The shells of bivalves are used in craftwork, and the manufacture of jewellery and buttons. Bivalves have also been used in the biocontrol of pollution. Bivalves appear in the fossil record first in the early Cambrian more than 500 million years ago. The total number of known living species is about 9,200. These species are placed within 1,260 genera and 106 families. Marine bivalves (including brackish water and estuarine species) represent about 8,000 species, combined in four subclasses and 99 families with 1,100 genera. The largest recent marine families are

6006-422: The number of 20,000 living species, often encountered in literature, could not be verified and presents the following table to illustrate the known diversity: The bivalves are a highly successful class of invertebrates found in aquatic habitats throughout the world. Most are infaunal and live buried in sediment on the seabed, or in the sediment in freshwater habitats. A large number of bivalve species are found in

6097-499: The others being Tuarangia , Camya and Arhouriella and potentially Buluniella . Bivalve fossils can be formed when the sediment in which the shells are buried hardens into rock. Often, the impression made by the valves remains as the fossil rather than the valves. During the Early Ordovician , a great increase in the diversity of bivalve species occurred, and the dysodont, heterodont, and taxodont dentitions evolved. By

6188-400: The palps. These then sort the particles, rejecting those that are unsuitable or too large to digest, and conveying others to the mouth. In more advanced bivalves, water is drawn into the shell from the posterior ventral surface of the animal, passes upwards through the gills, and doubles back to be expelled just above the intake. There may be two elongated, retractable siphons reaching up to

6279-519: The period is before the larva first feeds, the larger the egg and yolk need to be. The reproductive cost of producing these energy-rich eggs is high and they are usually smaller in number. For example, the Baltic tellin ( Macoma balthica ) produces few, high-energy eggs. The larvae hatching out of these rely on the energy reserves and do not feed. After about four days, they become D-stage larvae, when they first develop hinged, D-shaped valves. These larvae have

6370-409: The posterior mantle margins. The organs are usually mechanoreceptors or chemoreceptors , in some cases located on short tentacles . The osphradium is a patch of sensory cells located below the posterior adductor muscle that may serve to taste the water or measure its turbidity . Statocysts within the organism help the bivalve to sense and correct its orientation. In the order Anomalodesmata ,

6461-516: The prominence of modern bivalves over brachiopods seems due to chance disparities in their response to extinction events . The adult maximum size of living species of bivalve ranges from 0.52 mm (0.02 in) in Condylonucula maya , a nut clam, to a length of 1,532 millimetres (60.3 in) in Kuphus polythalamia , an elongated, burrowing shipworm. However, the species generally regarded as

6552-496: The sea cucumber sucks in sediment, the bivalve allows the water to pass over its gills and extracts fine organic particles. To prevent itself from being swept away, it attaches itself with byssal threads to the host 's throat. The sea cucumber is unharmed. The digestive tract of typical bivalves consists of an oesophagus , stomach , and intestine . Protobranch stomachs have a mere sac attached to them while filter-feeding bivalves have elongated rod of solidified mucus referred to as

6643-419: The seabed, one each for the inhalant and exhalant streams of water. The gills of filter-feeding bivalves are known as ctenidia and have become highly modified to increase their ability to capture food. For example, the cilia on the gills, which originally served to remove unwanted sediment, have become adapted to capture food particles, and transport them in a steady stream of mucus to the mouth. The filaments of

6734-469: The sediment remains damp even when the tide is out. When buried in the sediment, burrowing bivalves are protected from the pounding of waves, desiccation, and overheating during low tide, and variations in salinity caused by rainwater. They are also out of the reach of many predators. Their general strategy is to extend their siphons to the surface for feeding and respiration during high tide, but to descend to greater depths or keep their shell tightly shut when

6825-435: The shell and develops into an imitation small fish, complete with fish-like markings and false eyes. This decoy moves in the current and attracts the attention of real fish. Some fish see the decoy as prey, while others see a conspecific . They approach for a closer look and the mussel releases huge numbers of larvae from its gills, dousing the inquisitive fish with its tiny, parasitic young. These glochidia larvae are drawn into

6916-468: The shell is the periostracum , a thin layer composed of horny conchiolin . The periostracum is secreted by the outer mantle and is easily abraded. The outer surface of the valves is often sculpted, with clams often having concentric striations, scallops having radial ribs and oysters a latticework of irregular markings. In all molluscs, the mantle forms a thin membrane that covers the animal's body and extends out from it in flaps or lobes. In bivalves,

7007-571: The shell, gaining the ability to swim, and in a few cases, adopting predatory habits. For a long time, bivalves were thought to be better adapted to aquatic life than brachiopods were, outcompeting and relegating them to minor niches in later ages. These two taxa appeared in textbooks as an example of replacement by competition. Evidence given for this included the fact that bivalves needed less food to subsist because of their energetically efficient ligament-muscle system for opening and closing valves. All this has been broadly disproven, though; rather,

7098-420: The shells and open them more easily when they could tackle them from different angles. Octopuses either pull bivalves apart by force, or they bore a hole into the shell and insert a digestive fluid before sucking out the liquified contents. Certain carnivorous gastropod snails such as whelks ( Buccinidae ) and murex snails ( Muricidae ) feed on bivalves by boring into their shells. A dog whelk ( Nucella ) drills

7189-400: The shells with their pincers and starfish use their water vascular system to force the valves apart and then insert part of their stomach between the valves to digest the bivalve's body. It has been found experimentally that both crabs and starfish preferred molluscs that are attached by byssus threads to ones that are cemented to the substrate. This was probably because they could manipulate

7280-736: The shells. The Pacific walrus ( Odobenus rosmarus divergens ) is one of the main predators feeding on bivalves in Arctic waters. Shellfish have formed part of the human diet since prehistoric times, a fact evidenced by the remains of mollusc shells found in ancient middens. Examinations of these deposits in Peru has provided a means of dating long past El Niño events because of the disruption these caused to bivalve shell growth. Further changes in shell development due to environmental stress has also been suggested to cause increased mortality in oysters due to reduced shell strength. Invertebrate predators include crustaceans, starfish and octopuses. Crustaceans crack

7371-577: The southeastern US, it is in danger of extinction. In contrast, a few species of freshwater bivalves, including the golden mussel ( Limnoperna fortunei ), are dramatically increasing their ranges. The golden mussel has spread from Southeast Asia to Argentina, where it has become an invasive species . Another well-travelled freshwater bivalve, the zebra mussel ( Dreissena polymorpha ) originated in southeastern Russia, and has been accidentally introduced to inland waterways in North America and Europe, where

7462-416: The species damages water installations and disrupts local ecosystems . Most bivalves adopt a sedentary or even sessile lifestyle, often spending their whole lives in the area in which they first settled as juveniles. The majority of bivalves are infaunal, living under the seabed, buried in soft substrates such as sand, silt, mud, gravel, or coral fragments. Many of these live in the intertidal zone where

7553-590: The tide goes out. They use their muscular foot to dig into the substrate. To do this, the animal relaxes its adductor muscles and opens its shell wide to anchor itself in position while it extends its foot downwards into the substrate. Then it dilates the tip of its foot, retracts the adductor muscles to close the shell, shortens its foot and draws itself downwards. This series of actions is repeated to dig deeper. Other bivalves, such as mussels , attach themselves to hard surfaces using tough byssus threads made of collagen and elastin proteins. Some species, including

7644-414: The trees show adaptations to a dry season, but some of the ferns and mosses are types that grow in very wet habitats. It is possible that the latter avoided dry conditions by growing very close to bodies of water. Archaefructus has been described as the earliest known flowering plant ( Angiosperm ), and it is reconstructed as an aquatic plant. Gu (1983 and 1995) defined the following species as typifying

7735-558: The true oysters, the jewel boxes , the jingle shells , the thorny oysters and the kitten's paws , cement themselves to stones, rock or larger dead shells. In oysters, the lower valve may be almost flat while the upper valve develops layer upon layer of thin horny material reinforced with calcium carbonate. Oysters sometimes occur in dense beds in the neritic zone and, like most bivalves, are filter feeders. Bivalves filter large amounts of water to feed and breathe but they are not permanently open. They regularly shut their valves to enter

7826-406: The two halves detaching. The shell is typically bilaterally symmetrical , with the hinge lying in the sagittal plane . Adult shell sizes of bivalves vary from fractions of a millimetre to over a metre in length, but the majority of species do not exceed 10 cm (4 in). Bivalves have long been a part of the diet of coastal and riparian human populations. Oysters were cultured in ponds by

7917-402: The two valves and contract to close the shell. The valves are also joined dorsally by the hinge ligament , which is an extension of the periostracum. The ligament is responsible for opening the shell, and works against the adductor muscles when the animal opens and closes. Retractor muscles connect the mantle to the edge of the shell, along a line known as the pallial line . These muscles pull

8008-479: The volcanic rocks overlying and underlying salamander-bearing layers (salamanders are often used as index fossils ). Liu et al. found that the Daohugou beds formed between 164 and 158 million years ago, in the Middle to Late Jurassic . Later, Ji et al. argued that the key indicator of the Jehol biota are the index fossil fishes Peipiaosteus and Lycoptera . Under this definition, the earliest evolutionary stage of

8099-417: The young inside their mantle cavity, eventually releasing them into the water column as veliger larvae or as crawl-away juveniles. Most of the bivalve larvae that hatch from eggs in the water column feed on diatoms or other phytoplankton. In temperate regions, about 25% of species are lecithotrophic , depending on nutrients stored in the yolk of the egg where the main energy source is lipids . The longer

8190-843: Was defined by Gu (1962 and 1983) as a group of geological formations including the Jehol Coal-bearing Beds, the Jehol Oil Shale Beds, and the Jehol Volcanic Rocks. By now the group includes, in ascending order, the Yixian Formation (including the Jingangshan, Tuhulu, Jianchang, Lower Volcanic and Volcanic Rock formations), the Jiufotang Formation (including the Shahai Formation) and the Fuxin Formation (including

8281-418: Was not entirely isolated, however, because it also includes animals which were known from all around the world at the same time, including discoglossid frogs , paramacellodid lizards , multituberculate mammals , enantiornithine birds, ctenochasmatid pterosaurs , iguanodontian ornithopods , titanosauriform sauropods , nodosaurid ankylosaurs , and dromaeosaurid theropods . The Jehol Biota

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