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91-463: Fish include a high diversity of unusual chondrichthyans (cartilaginous fish) and one of the oldest known lampreys , along with other vertebrates . Invertebrates include numerous fossils of crustaceans , worms , cephalopods , and sea sponges , which are concentrated in different parts of the bay basin. Shelled cephalopods are abundant, and the Bear Gulch Limestone also preserves some of

182-1054: A dagger (†); groups of uncertain placement are labelled with a question mark (?) and dashed lines (- - - - -). Jawless fishes (118 species: hagfish , lampreys ) [REDACTED] † Thelodonti , † Conodonta , † Anaspida [REDACTED] [REDACTED] [REDACTED] † Galeaspida [REDACTED] † Osteostraci [REDACTED] † Placodermi [REDACTED] † Acanthodii [REDACTED]  (>1,100 species: sharks , rays , chimaeras ) [REDACTED]  (2 species: coelacanths ) [REDACTED] Dipnoi (6 species: lungfish ) [REDACTED] Tetrapoda (>38,000 species, not considered fish: amphibians, reptiles, birds, mammals) [REDACTED]  (14 species: bichirs , reedfish ) [REDACTED]  (27 species: sturgeons , paddlefish ) [REDACTED] Ginglymodi (7 species: gars , alligator gars ) [REDACTED] Halecomorphi (2 species: bowfin , eyetail bowfin ) [REDACTED]  (>32,000 species) [REDACTED] Fishes (without tetrapods) are

273-407: A faunal assemblage , rather than an individual species — this allows greater precision as the time span in which all of the species in the assemblage existed together is narrower than the time spans of any of the members. Furthermore, if only one species is present in a sample, it can mean either that (1) the strata were formed in the known fossil range of that organism; or (2) that the fossil range of

364-579: A paraphyletic group and for this reason, the class Pisces seen in older reference works is no longer used in formal classifications. Traditional classification divides fish into three extant classes (Agnatha, Chondrichthyes, and Osteichthyes), and with extinct forms sometimes classified within those groups, sometimes as their own classes. Fish account for more than half of vertebrate species. As of 2016, there are over 32,000 described species of bony fish, over 1,100 species of cartilaginous fish, and over 100 hagfish and lampreys. A third of these fall within

455-616: A transgressive sequence in a narrow saltwater seaway, known as the Central Montana Trough or Big Snowy Trough. This seaway flowed into the Williston Basin , a shallow inland sea further east. The Central Montana Trough would have also been linked to fully marine basins on the western coastline of Laurussia , but this connection may have been broken by the time of the Bear Gulch Limestone's deposition. Many distinct limestone lenses (localized sediment packages) are developed in

546-404: A cause of death. One possible explanation for rapid asphyxiation and burial places blame on freshwater runoff during storms in the wet season. Heavy rainfall would bring a cascade of organic-rich marginal sediments into the central basin. As the sediments sink, they quickly absorb oxygen from the seawater, killing and burying basin organisms in only a matter of hours. The ecosystem represented by

637-415: A different section. Fossils within these strata are useful because sediments of the same age can look completely different, due to local variations in the sedimentary environment . For example, one section might have been made up of clays and marls , while another has more chalky limestones . However, if the fossil species recorded are similar, the two sediments are likely to have been laid down around

728-748: A geological instant. The Bear Gulch Limestone is commonly considered to be part of the Heath Formation , the youngest formation in the Big Snowy Group of central Montana . Some authors instead consider the Bear Gulch Limestone to be an early member of the Tyler Formation , a patchy but widespread unit of Carboniferous limestone and terrestrial sediments. Most of the Heath Formation is represented by black shales and marls , indicative of brackish and salty littoral environments. It developed along

819-436: A more spherical lens . Their retinas generally have both rods and cones (for scotopic and photopic vision ); many species have colour vision , often with three types of cone. Teleosts can see polarized light ; some such as cyprinids have a fourth type of cone that detects ultraviolet . Amongst jawless fish , the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. Hearing too

910-754: A mostly terrestrial clade of vertebrates that have dominated the top trophic levels in both aquatic and terrestrial ecosystems since the Late Paleozoic , evolved from lobe-finned fish during the Carboniferous , developing air-breathing lungs homologous to swim bladders. Despite the cladistic lineage, tetrapods are usually not considered to be fish, making "fish" a paraphyletic group. Fish have been an important natural resource for humans since prehistoric times, especially as food . Commercial and subsistence fishers harvest fish in wild fisheries or farm them in ponds or in breeding cages in

1001-399: A particular span of geologic time or environment, and can be used to identify and date the containing rocks. To be practical, index fossils must have a limited vertical time range, wide geographic distribution, and rapid evolutionary trends. Rock formations separated by great distances but containing the same index fossil species are thereby known to have both formed during the limited time that

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1092-419: A protective bony cover or operculum . They are able to oxygenate their gills using muscles in the head. Some 400 species of fish in 50 families can breathe air, enabling them to live in oxygen-poor water or to emerge on to land. The ability of fish to do this is potentially limited by their single-loop circulation, as oxygenated blood from their air-breathing organ will mix with deoxygenated blood returning to

1183-429: A quantum radical pair mechanism . Biostratigraphy Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. The primary objective of biostratigraphy is correlation , demonstrating that a particular horizon in one geological section represents the same period of time as another horizon at

1274-441: A sense of touch and of hearing . Blind cave fish navigate almost entirely through the sensations from their lateral line system. Some fish, such as catfish and sharks, have the ampullae of Lorenzini , electroreceptors that detect weak electric currents on the order of millivolt. Vision is an important sensory system in fish. Fish eyes are similar to those of terrestrial vertebrates like birds and mammals, but have

1365-449: A sequence of sediments: dense, massive ( homogenous ) beds followed by laminae with graded bedding , which may be bioturbated in the upper part of the layer. These small sequences, sometimes termed microturbidites, draw comparisons to the Bouma sequences of deep marine turbidite deposits. The pale shale layers of the central basin were likely slowly accumulated during the dry season, while

1456-421: A single " synziphosurine " ( Anderella ). Trilobites , though common in adjacent formations, are practically absent in the Bear Gulch Limestone. Fish A fish ( pl. : fish or fishes ) is an aquatic , anamniotic , gill -bearing vertebrate animal with swimming fins and a hard skull , but lacking limbs with digits . Fish can be grouped into the more basal jawless fish and

1547-485: A tail fin, jaws, skin covered with scales , and lays eggs. Each criterion has exceptions, creating a wide diversity in body shape and way of life. For example, some fast-swimming fish are warm-blooded, while some slow-swimming fish have abandoned streamlining in favour of other body shapes. Fish species are roughly divided equally between freshwater and marine (oceanic) ecosystems; there are some 15,200 freshwater species and around 14,800 marine species. Coral reefs in

1638-679: A third of the Bear Gulch fish diversity. In terms of specimen count, they are the most abundant fish fossils in the Bear Gulch Limestone, representing approximately 80% of all fossil vertebrates. However, they are more restricted than chondrichthyans in terms of ecomorphology . Most have a small and generalized fusiform (smoothly tapering) body shape, with only a few deviations towards more elongated or deep-bodied forms. The most abundant actinopterygians include Wendyichthys and three closely related unnamed species (code names "Yogo", "Fub", and "Cop"). The relative abundance of actinopterygians drops off in

1729-585: A true "land fish" as this worm-like catfish strictly lives among waterlogged leaf litter . Cavefish of multiple families live in underground lakes , underground rivers or aquifers . Like other animals, fish suffer from parasitism . Some species use cleaner fish to remove external parasites. The best known of these are the bluestreak cleaner wrasses of coral reefs in the Indian and Pacific oceans. These small fish maintain cleaning stations where other fish congregate and perform specific movements to attract

1820-422: A typical fish is adapted for efficient swimming by alternately contracting paired sets of muscles on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the tail fin, force is applied to the water, moving the fish forward. The other fins act as control surfaces like an aircraft's flaps, enabling the fish to steer in any direction. Since body tissue

1911-404: A unique association of three or more taxa within it. Abundance biozones are strata in which the abundance of a particular taxon or group of taxa is significantly greater than in the adjacent part of the section. Index fossils (also known as guide fossils , indicator fossils , or dating fossils ) are the fossilized remains or traces of particular plants or animals that are characteristic of

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2002-513: A warm environment, death and burial had to have been very rapid for the vast majority of articulated skeletons. Rare disarticulated fragments may correspond to large or buoyant carcasses which rise to the water surface to gradually decay and fall apart in a "bloat and float" taphonomic process. Fossils are dispersed throughout the Bear Gulch lens, rather than concentrated in specific fossil-rich beds (which would be expected if organisms were killed by algal blooms ). The cyclically deposited seabed of

2093-803: A wider range of salinity tolerance ( Acanthodes , gastropods , filamentous algae) are fairly common. The Bear Gulch Limestone is a konservat lagerstätte , meaning that its fossils are uniquely well-preserved, including soft tissue details which offer rare insights into the biology of Carboniferous organisms. The fine-grained sediments common in the formation allow for fossilized structures to retain fine resolution, as seen in equivalent plattenkalk-based lagerstätten throughout geological history. Most fossils are isolated and flattened into very thin films between sheet-like layers. Arborispongia assemblages, cephalopod shells, and large vertebrate bones occasionally project through several thin layers, approaching three-dimensional preservation. Vertebrates typically come in

2184-465: Is an important sensory system in fish. Fish sense sound using their lateral lines and otoliths in their ears, inside their heads. Some can detect sound through the swim bladder. Some fish, including salmon, are capable of magnetoreception ; when the axis of a magnetic field is changed around a circular tank of young fish, they reorient themselves in line with the field. The mechanism of fish magnetoreception remains unknown; experiments in birds imply

2275-431: Is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a swim bladder that allows them to adjust their buoyancy by increasing or decreasing the amount of gas it contains. The scales of fish provide protection from predators at the cost of adding stiffness and weight. Fish scales are often highly reflective; this silvering provides camouflage in

2366-402: Is renowned for its unusual and ecologically diverse chondrichthyans , the group of cartilaginous fish containing modern sharks , rays , and chimaeras . Over 80 species of chondrichthyans are known (as of 2012), representing close to 60% of the fish diversity (by species count) in the bay. This is a far cry from modern ecosystems, where chondrichthyans comprise only around 3% of fish species. In

2457-465: Is the biggest part of the brain; it is small in hagfish and lampreys , but very large in mormyrids , processing their electrical sense . The brain stem or myelencephalon controls some muscles and body organs, and governs respiration and osmoregulation . The lateral line system is a network of sensors in the skin which detects gentle currents and vibrations, and senses the motion of nearby fish, whether predators or prey. This can be considered both

2548-567: The Cambrian as small filter feeders ; they continued to evolve through the Paleozoic , diversifying into many forms. The earliest fish with dedicated respiratory gills and paired fins , the ostracoderms , had heavy bony plates that served as protective exoskeletons against invertebrate predators . The first fish with jaws , the placoderms, appeared in the Silurian and greatly diversified during

2639-514: The Devonian , the "Age of Fishes". Bony fish, distinguished by the presence of swim bladders and later ossified endoskeletons , emerged as the dominant group of fish after the end-Devonian extinction wiped out the apex placoderms. Bony fish are further divided into the lobe-finned and ray-finned fish . About 96% of all living fish species today are teleosts , a crown group of ray-finned fish that can protrude their jaws . The tetrapods ,

2730-606: The Gnathostomata or (for bony fish) Osteichthyes , also contains the clade of tetrapods (four-limbed vertebrates, mostly terrestrial), which are usually not considered fish. Some tetrapods, such as cetaceans and ichthyosaurs , have secondarily acquired a fish-like body shape through convergent evolution . Fishes of the World comments that "it is increasingly widely accepted that tetrapods, including ourselves, are simply modified bony fishes, and so we are comfortable with using

2821-709: The Indo-Pacific constitute the center of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests , especially the Amazon , Congo , and Mekong basins. More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth. Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon ) to

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2912-561: The Silurian , with giant armoured placoderms such as Dunkleosteus . Jawed fish, too, appeared during the Silurian: the cartilaginous Chondrichthyes and the bony Osteichthyes . During the Devonian , fish diversity greatly increased, including among the placoderms, lobe-finned fishes, and early sharks, earning the Devonian the epithet "the age of fishes". Fishes are a paraphyletic group, since any clade containing all fish, such as

3003-469: The abyssal and even hadal depths of the deepest oceans (e.g., cusk-eels and snailfish ), although none have been found in the deepest 25% of the ocean. The deepest living fish in the ocean so far found is a cusk-eel, Abyssobrotula galatheae , recorded at the bottom of the Puerto Rico Trench at 8,370 m (27,460 ft). In terms of temperature, Jonah's icefish live in cold waters of

3094-407: The equator , on the boundary between the arid subtropics and tropical equatorial region. Cyclical deposition supports a climate model arguing that the overall climate was warm and monsoonal , with pronounced rainy and dry seasons. During the quiet dry season , sedimentation was low and the basin would have been influenced by northeasterly trade winds , tidal currents, and evaporation . Early in

3185-400: The intertidal zone , are facultative air breathers, able to breathe air when out of water, as may occur daily at low tide , and to use their gills when in water. Some coastal fish like rockskippers and mudskippers choose to leave the water to feed in habitats temporarily exposed to the air. Some catfish absorb air through their digestive tracts. The digestive system consists of a tube,

3276-404: The rainy season , warmer temperatures and reduced trade winds would have isolated the basin and increased its overall salinity . As the rainy season progressed, heavy rainfall enhances the bay's horizontal salinity gradient, from the freshwater-influenced upper bay to the marine-influenced lower bay. Storms would also produce a shallow layer of freshwater, washing sediment and organic material from

3367-409: The rhizodont Strepsodus , which have very different habitat preferences. Coelacanths, particularly fossils of the genus Caridosuctor , are abundant in the central basin and its margins, but are practically absent in the upper bay. Strepsodus , on the other hand, is moderately common in the upper bay and very rare elsewhere. Neither are common in the sheltered eastern reef habitat. Acanthodes ,

3458-425: The stout infantfish . Swimming performance varies from fish such as tuna, salmon , and jacks that can cover 10–20 body-lengths per second to species such as eels and rays that swim no more than 0.5 body-lengths per second. A typical fish is cold-blooded , has a streamlined body for rapid swimming, extracts oxygen from water using gills, has two sets of paired fins, one or two dorsal fins, an anal fin and

3549-854: The Bear Gulch Limestone was vibrant and well-represented by fossils, with many described and undescribed species of vertebrates and invertebrates among their ranks. Animals show a wide range of body types and show preferences to certain areas of the bay, indicating that the bay offered a wide range of habitats and niches to be filled. Many indicators of paleoecology and paleobiology have been preserved, from associations between encrusting shelled organisms and algae, to gut contents and other feeding traces, and even some animals fossilized while mating. Fish are diverse and abundant, with thousands of specimens representing approximately 150 species (as of 2015). Many of these species are undescribed and unnamed, only mentioned through "code names"; unnamed species are not included in paleobiota lists here. The Bear Gulch Limestone

3640-426: The Bear Gulch Limestone, chondrichthyans possess a wide range of ecologies and body types, including eel -like forms (" Thrinacoselache "), deep-bodied durophages ( petalodontiforms ), small active swimmers (most holocephalans ), ray-like benthic forms ( Squatinactis ) and more typical shark-like predators ( symmoriiforms and elasmobranchs ). Chondrichthyan diversity and abundance patterns are correlated, with

3731-683: The Heath Formation. They overlap each other in an east-to-west sequence which extends over a distance of 160 km in the Central Montana Trough. The only exposed portions of the sequence are found at Potter Creek Dome , a small uplifted area northeast of the Big Snowy Mountains . The last few limestone lenses form a large portion of the Upper Heath Formation, which is sometimes termed the Bear Gulch Member in recognition of

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3822-594: The Southern Ocean, including under the Filchner–Ronne Ice Shelf at a latitude of 79°S, while desert pupfish live in desert springs, streams, and marshes, sometimes highly saline, with water temperatures as high as 36 C. A few fish live mostly on land or lay their eggs on land near water. Mudskippers feed and interact with one another on mudflats and go underwater to hide in their burrows. A single undescribed species of Phreatobius has been called

3913-641: The area were succeeded by freshwater lake sediments of the Cameron Creek Formation , the oldest unit of the early Pennsylvanian-age Amsden Group . A wide variety of biostratigraphic evidence places the Heath Formation in the upper part of the Chesterian stage, near the end of the Mississippian subperiod (early Carboniferous period). The upper Chesterian is the North American regional equivalent to

4004-556: The attention of the cleaners. Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, Etroplus maculatus , the cleaner, and the much larger E. suratensis . Fish occupy many trophic levels in freshwater and marine food webs . Fish at the higher levels are predatory , and a substantial part of their prey consists of other fish. In addition, mammals such as dolphins and seals feed on fish, alongside birds such as gannets and cormorants . The body of

4095-436: The basin margins down into deeper areas. Fine-grained lithographic limestone ( plattenkalk ) is predominant in the Bear Gulch lens, though clay to silt -sized siliclastic material also forms a significant portion of rock in some areas. Several facies (sediment associations) are developed in different parts of the bay. The thick and fossiliferous central basin facies develop along the main northwest-to-southeast axis of

4186-440: The bay's lifespan was indicated by a sequence of layers with leaf fragments, limestone conglomerates, marls , and finally fully freshwater sediments. When the bay was in its heyday, the south edge of the central basin axis saw the development of the filamentous algal facies. These are mostly dark, organic-rich limestones and shales, similar to the central basin facies though with more siliciclastic silt and fewer microturbidites. As

4277-409: The bay, which is delimited by small dewatering microfaults . Well-preserved fish and other nektonic (free-swimming) animals are common, but algae and typical benthic (seabed-living) animals are very rare. Central basin sedimentation was cyclical, with thin beds of pale shale interbedded between thicker layers of dark, organic-rich limestone with a distinct oily smell. Each dark layer contains

4368-487: The best early fossils of coleoid cephalopods. Fossils of typical benthic (seabed-attached) organisms are rare, and those that do occur are restricted to reef-like sponge patches in eastern exposures. This indicates that the conditions of the bay responsible for the Bear Gulch Limestone were at odds with other marine areas nearby. Some Bear Gulch fossils were preserved so rapidly and efficiently that mating behaviors, internal organs, coloration patterns, gut content, and even

4459-550: The body, and produce a concentrated urine. The reverse happens in freshwater fish : they tend to gain water osmotically, and produce a dilute urine. Some fish have kidneys able to operate in both freshwater and saltwater. Fish have small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. However, some fish have relatively large brains, notably mormyrids and sharks , which have brains about as large for their body weight as birds and marsupials . At

4550-629: The calm and shallow northwest rim of the basin, Arborispongia reefs, large stromatolites , and plant debris can be observed. Salt crystal casts are abundant, indicative of hypersalinity and high rates of evaporation in this area. As the bay was filled in by sediments, similar conditions extended into the center of the basin. This results in deposition of the "shallow" or "marly" facies, a collection of very pale micrites with very little organic or siliciclastic material. They preserve various algal laminations , diverse Arborispongia reefs, and local concentrations of chert and gypsum nodules . The end of

4641-488: The central basin transitions into the lightly colored laminated sediments of the Arborispongia - productid facies. They are characterized by reefs and patches of the arborescent (branching) sponge Arborispongia , which is commonly covered with spiny productid brachiopods . Marine benthic organisms, such as algae, bivalves , bryozoans , and crinoids , make up a significant portion of the ecosystem in these few areas. At

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4732-427: The central basin, though fossiliferous, is deprived of benthic invertebrates. Some authors have suggested that the deepest waters in the basin were too salty or oxygen-poor for most life. However, bottom-dwelling fish are common in the central basin, suggesting that anoxia (low oxygen) was not a persistent quality of bottom waters. Nevertheless, many fish fossils are found with distended gills, favoring asphyxiation as

4823-474: The central basin. This may have been a consequence of acidity in the pore fluids of sediments, with less acidic fluids favoring the preservation of calcareous fossils and more acidic fluids favoring phosphatized fossils. There is some uncertainty over how this exceptional preservation was achieved. Most fossils are complete and undecomposed animals, with no signs of disturbance from scavengers or strong currents. To prevent decomposition of fragile soft tissue in

4914-463: The conclusion that fossils then indicated a series of chronological events, establishing layers of rock strata as some type of unit, later termed biozone . From here on, scientists began relating the changes in strata and biozones to different geological eras, establishing boundaries and time periods within major faunal changes. By the late 18th century the Cambrian and Carboniferous periods were internationally recognized due to these findings. During

5005-455: The darker layers correspond to settling bay margin sediments washed into the basin during the rainy season. Sedimentary slumps along the central basin axis have been used to reconstruct flow within the basin, indicating that it deepened and drained to the northeast. The Bear Gulch Limestone reached a thickness of 30 meters at its eastern outlet, and total basin depth was approximately 40 meters. At its eastern outlet and nearby sheltered alcoves,

5096-445: The diet of some Bear Gulch animals, and phosphatized muscles have been found in very rare situations. Soft invertebrates are indicated by a variety of molds, casts , and organic discolorations on rocks. In the central basin, fossils of invertebrates with calcareous shells are mostly dissolved, leaving only molds in the surrounding rock. On the other hand, invertebrates with chitinous or phosphate -rich shells become more common in

5187-413: The duration of periods. Since a large change in fauna was required to make early stratigraphers create a new period, most of the periods we recognize today are terminated by a major extinction event or faunal turnover. A stage is a major subdivision of strata, each systematically following the other each bearing a unique assemblage of fossils. Therefore, stages can be defined as a group of strata containing

5278-635: The early 20th century, advancements in technology gave scientists the ability to study radioactive decay . Using this methodology, scientists were able to establish geological time, the boundaries of the different eras ( Paleozoic , Mesozoic , Cenozoic ), as well as Periods ( Cambrian , Ordovician , Silurian ) through the isotopes found within fossils via radioactive decay. Current 21st century uses of biostratigraphy involve interpretations of age for rock layers, which are primarily used by oil and gas industries for drilling workflows and resource allocations. Fossil assemblages were traditionally used to designate

5369-486: The eastern edge of exposures has produced a high number of Heteropetalus fossils. Regardless, each habitat observes significant species overlap with other habitats, and "common" chondrichthyans (>3 specimens) are not abundant to the point that they outnumber more rare species (<3 specimens), in terms of total specimen count. Actinopterygians (ray-finned bony fish) are also quite diverse, with about 50 described and undescribed species (as of 2012) making up more than

5460-511: The exact root is unknown; some authorities reconstruct a Proto-Indo-European root * peysk- , attested only in Italic , Celtic , and Germanic . About 530 million years ago during the Cambrian explosion , fishlike animals with a notochord and eyes at the front of the body, such as Haikouichthys , appear in the fossil record . During the late Cambrian , other jawless forms such as conodonts appear. Jawed vertebrates appear in

5551-525: The first geologists to recognize that rock layers correlate to the Law of Superposition . With advancements in science and technology, by the 18th century it began to be accepted that fossils were remains left by species that had become extinct, but were then preserved within the rock record. The method was well-established before Charles Darwin explained the mechanism behind it— evolution . Scientists William Smith , George Cuvier , and Alexandre Brongniart came to

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5642-434: The form of complete skeletons, and their fossils often preserve internal and external organic pigments . Skin pigments can indicate color patterns and the extent of fins and other external structures, while internal pigments are used to outline the liver , spleen , eyes, and other organs. In rare cases, even the structure of fragile blood vessels can be recognized from preserved blood pigments. Uncommon gut contents indicate

5733-405: The front of the brain are the olfactory lobes , a pair of structures that receive and process signals from the nostrils via the two olfactory nerves . Fish that hunt primarily by smell, such as hagfish and sharks, have very large olfactory lobes. Behind these is the telencephalon , which in fish deals mostly with olfaction. Together these structures form the forebrain. Connecting the forebrain to

5824-412: The gills flows in the opposite direction to the water, resulting in efficient countercurrent exchange . The gills push the oxygen-poor water out through openings in the sides of the pharynx. Cartilaginous fish have multiple gill openings: sharks usually have five, sometimes six or seven pairs; they often have to swim to oxygenate their gills. Bony fish have a single gill opening on each side, hidden beneath

5915-532: The gills. Oxygen-rich blood then flows without further pumping, unlike in mammals, to the body tissues. Finally, oxygen-depleted blood returns to the heart. Fish exchange gases using gills on either side of the pharynx . Gills consist of comblike structures called filaments. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide . Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. Capillary blood in

6006-702: The global Serpukhovian stage, as well as the lower part of the European Namurian stage. The Bear Gulch Limestone in particular is correlated with ammonoid zone E2b (middle Arnsbergian) of the Namurian, not far from the top of the Serpukhovian. Strata from zone E2b in England and the Czech Republic have been dated to around 324 million years ago. During the time of deposition, the area was about 10-12 degrees north of

6097-447: The gut, leading from the mouth to the anus. The mouth of most fishes contains teeth to grip prey, bite off or scrape plant material, or crush the food. An esophagus carries food to the stomach where it may be stored and partially digested. A sphincter, the pylorus, releases food to the intestine at intervals. Many fish have finger-shaped pouches, pyloric caeca , around the pylorus, of doubtful function. The pancreas secretes enzymes into

6188-452: The heart from the rest of the body. Lungfish, bichirs, ropefish, bowfins, snakefish, and the African knifefish have evolved to reduce such mixing, and to reduce oxygen loss from the gills to oxygen-poor water. Bichirs and lungfish have tetrapod-like paired lungs, requiring them to surface to gulp air, and making them obligate air breathers. Many other fish, including inhabitants of rock pools and

6279-413: The highest number of species and fossils found in the central basin facies and the lowest in the marginal facies. Large predatory chondrichthyans such as Listracanthus and symmoriiforms (except Falcatus ) make up a higher portion of fossils in the hypersaline upper bay, while Falcatus and holocephalans are the most common fossils in the marginal and central basin facies. A sheltered reef habitat at

6370-455: The intestine to digest the food; other enzymes are secreted directly by the intestine itself. The liver produces bile which helps to break up fat into an emulsion which can be absorbed in the intestine. Most fish release their nitrogenous wastes as ammonia . This may be excreted through the gills or filtered by the kidneys . Salt is excreted by the rectal gland. Saltwater fish tend to lose water by osmosis ; their kidneys return water to

6461-481: The known stratigraphic and geographic range of occurrence of a single taxon. Concurrent range biozone includes the concurrent, coincident, or overlapping part of the range of two specified taxa. Interval biozones include the strata between two specific biostratigraphic surfaces and can be based on lowest or highest occurrences. Lineage biozones are strata containing species representing a specific segment of an evolutionary lineage. Assemblage biozones are strata that contain

6552-419: The midbrain is the diencephalon ; it works with hormones and homeostasis . The pineal body is just above the diencephalon; it detects light, maintains circadian rhythms, and controls color changes. The midbrain contains the two optic lobes . These are very large in species that hunt by sight, such as rainbow trout and cichlids . The hindbrain controls swimming and balance.The single-lobed cerebellum

6643-476: The more common jawed fish , the latter including all living cartilaginous and bony fish , as well as the extinct placoderms and acanthodians . Most fish are cold-blooded , their body temperature varying with the surrounding water, though some large active swimmers like white shark and tuna can hold a higher core temperature . Many fish can communicate acoustically with each other, such as during courtship displays . The earliest fish appeared during

6734-663: The most fundamental unit of measurement. The thickness and range of these zones can be a few meters, up to hundreds of meters. They can also range from local to worldwide, as the extent of which they can reach in the horizontal plane relies on tectonic plates and tectonic activity. Two of the tectonic processes that run the risk of changing these zones' ranges are metamorphic folding and subduction . Furthermore, biostratigraphic units are divided into six principal kinds of biozones: Taxon range biozone , Concurrent range biozone, Interval biozone, Lineage biozone, Assemblage biozone, and Abundance biozone . The Taxon range biozone represents

6825-554: The most well-exposed and fossiliferous lens in the sequence. This lens, the Bear Gulch Limestone, was also one of the last in the sequence, only succeeded by the Surenough Beds immediately west of it. The Bear Gulch Limestone can be observed in numerous outcrops, spread out over an area of more than 50 km2 in Fergus County, Montana . The creation of limestone lenses in the Heath Formation has been linked to tectonic activity extending

6916-492: The name indicates, strands of filamentous algae are abundant. Further beyond the main basin axis, the rocks are even more silty, not clearly bedded, and have a very dark coloration. These sediments, the marginal facies, have a high content of peloids , plant debris, and other organic material. They likely correspond to shallow, brackish areas with an influx of freshwater. Fossils of fully saltwater taxa are rare and poorly preserved relative to other environments, though organisms with

7007-443: The nine largest families; from largest to smallest, these are Cyprinidae , Gobiidae , Cichlidae , Characidae , Loricariidae , Balitoridae , Serranidae , Labridae , and Scorpaenidae . About 64 families are monotypic , containing only one species. Fish range in size from the huge 16-metre (52 ft) whale shark to some tiny teleosts only 8-millimetre (0.3 in) long, such as the cyprinid Paedocypris progenetica and

7098-496: The ocean. Fish are caught for recreation , or raised by fishkeepers as ornaments for private and public exhibition in aquaria and garden ponds . Fish have had a role in human culture through the ages, serving as deities , religious symbols, and as the subjects of art, books and movies. The word fish is inherited from Proto-Germanic , and is related to German Fisch , the Latin piscis and Old Irish īasc , though

7189-516: The only acanthodian known from the Bear Gulch Limestone, has a similar distribution pattern to Strepsodus : common in the upper bay and nowhere else. Lampreys , represented by Hardistiella , are too rare to estimate their habitat preference. Arthropods are diverse, the most abundant and speciose being various early hoplocarids , relatives of modern stomatopods ( mantis shrimps ). Other arthropods include an assortment of typical Carboniferous crustaceans , rare xiphosurans (horseshoe crabs), and

7280-402: The open ocean. Because the water all around is the same colour, reflecting an image of the water offers near-invisibility. Fish have a closed-loop circulatory system . The heart pumps the blood in a single loop throughout the body; for comparison, the mammal heart has two loops, one for the lungs to pick up oxygen, one for the body to deliver the oxygen. In fish, the heart pumps blood through

7371-461: The organism was incompletely known, and the strata extend the known fossil range. For instance, the presence of the trace fossil Treptichnus pedum was used to define the base of the Cambrian period, but it has since been found in older strata. If the fossil is easy to preserve and easy to identify, more precise time estimating of the stratigraphic layers is possible. The concept of faunal succession

7462-430: The overlapping range of fossils. They represent the time between the appearance of species chosen at the base of the zone and the appearance of other species chosen at the base of the next succeeding zone. Oppel's zones are named after a particular distinctive fossil species, called an index fossil. Index fossils are one of the species from the assemblage of species that characterize the zone. Biostratigraphy uses zones for

7553-419: The pattern of blood vessels could be observed in fossils. This exceptional preservation may be a consequence of the prevailing warm monsoonal climate, as storms could frequently and rapidly cover the seabed with oxygen-poor organic runoff from shallower areas. Although up to 40 meters of sediment are found in the Bear Gulch Limestone, biostratigraphic data suggests that the lens was emplaced in only 1000 years,

7644-443: The same major fossil assemblages. French palaeontologist Alcide d'Orbigny is credited for the invention of this concept. He named stages after geographic localities with particularly good sections of rock strata that bear the characteristic fossils on which the stages are based. In 1856 German palaeontologist Albert Oppel introduced the concept of zone (also known as biozones or Oppel zone). A zone includes strata characterized by

7735-404: The same time. Ideally these fossils are used to help identify biozones , as they make up the basic biostratigraphy units, and define geological time periods based upon the fossil species found within each section. Basic concepts of biostratigraphic principles were introduced centuries ago, going as far back as the early 1800s. A Danish scientist and bishop by the name of Nicolas Steno was one of

7826-440: The seaway by excavating bays out of the surrounding land. As old bays are filled in and buried by sediment, faulting and seismic events form new bays in a long eastward to westward succession. It may have taken a mere 1000 years for the bay responsible for the Bear Gulch Limestone lens to fill in completely, after only 25,000 years for the entire bay formation sequence to run its course across Montana. The final limestone deposits in

7917-832: The species lived. Index fossils were originally used to define and identify geologic units, then became a basis for defining geologic periods , and then for faunal stages and zones. Ammonites , graptolites , archeocyathids , inoceramids , and trilobites are groups of animals from which many species have been identified as index fossils that are widely used in biostratigraphy. Species of microfossils such as acritarchs , chitinozoans , conodonts , dinoflagellate cysts, ostracods , pollen , spores and foraminiferans are also frequently used. Different fossils work well for sediments of different ages; trilobites, for example, are particularly useful for sediments of Cambrian age. A long series of ammonite and inoceramid species are particularly useful for correlating environmental events around

8008-415: The taxon Osteichthyes as a clade, which now includes all tetrapods". The biodiversity of extant fish is unevenly distributed among the various groups; teleosts , bony fishes able to protrude their jaws , make up 96% of fish species. The cladogram shows the evolutionary relationships of all groups of living fishes (with their respective diversity ) and the tetrapods. Extinct groups are marked with

8099-408: The upper bay, though they are still fairly common. Schools of freshly hatched larvae have been found in isolated reef habitats near the central basin. Non-actinopterygian and non-chondrichthyan fish make up a fairly small component of the overall diversity, though their fossils are still common in some habitats. Preserved sarcopterygians (lobe-finned fish) include actinistians ( coelacanths ) and

8190-635: The world during the super-greenhouse of the Late Cretaceous . To work well, the fossils used must be widespread geographically, so that they can be found in many different places. They must also be short-lived as a species, so that the period of time during which they could be incorporated in the sediment is relatively narrow. The longer lived the species, the poorer the stratigraphic precision, so fossils that evolve rapidly, such as ammonites, are favored over forms that evolve much more slowly, like nautiloids . Often biostratigraphic correlations are based on

8281-619: Was theorized at the beginning of the 19th century by William Smith . When Smith was studying rock strata, he began to recognize that rock outcrops contained a unique collection of fossils. The idea that these distant rock outcrops contained similar fossils allowed for Smith to order rock formations throughout England. With Smith's work on these rock outcrops and mapping around England, he began to notice some beds of rock may contain mostly similar species, however there were also subtle differences within or between these fossil groups. This difference in assemblages that appeared identical at first, lead to

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