Absolute dating is the process of determining an age on a specified chronology in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy. Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events. In archaeology, absolute dating is based on the physical and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates. Techniques include tree rings in timbers, radiocarbon dating of wood or bones, trapped-charge dating methods such as thermoluminescence dating of glazed ceramics. Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year.
In historical geology, the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from young to systems such as uranium–lead dating that allow acquisition of absolute ages for some of the oldest rocks on earth. Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes. Particular isotopes are suitable for different applications due to the types of atoms present in the mineral or other material and its approximate age. For example, techniques based on isotopes with half lives in the thousands of years, such as carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments. One of the most used and well-known absolute dating techniques is carbon-14 dating, used to date organic remains.
This is a radiometric technique. Cosmic radiation entering the earth’s atmosphere produces carbon-14, plants take in carbon-14 as they fix carbon dioxide. Carbon-14 moves up the food chain as predators eat other animals. With death, the uptake of carbon-14 stops, it takes 5,730 years for half the carbon-14 to change to nitrogen. After another 5,730 years only one-quarter of the original carbon-14 will remain. After yet another 5,730 years only one-eighth will be left. By measuring the carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact; the short half-life of carbon-14, 5,730 years, makes dating reliable only up to about 50,000 years. The technique cannot pinpoint the date of an archeological site better than historic records, but is effective for precise dates when calibrated with other dating techniques such as tree-ring dating. An additional problem with carbon-14 dates from archeological sites is known as the "old wood" problem.
It is possible in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. Thus dating that particular tree does not indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating; the development of accelerator mass spectrometry dating, which allows a date to be obtained from a small sample, has been useful in this regard. Other radiometric dating techniques are available for earlier periods. One of the most used is potassium–argon dating. Potassium-40 is a radioactive isotope of potassium that decays into argon-40; the half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated.
Argon, a noble gas, is not incorporated into such samples except when produced in situ through radioactive decay. The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K–Ar dating was used to calibrate the geomagnetic polarity time scale. Thermoluminescence testing dates items to the last time they were heated; this technique is based on the principle. This process frees electrons within minerals. Heating an item to 500 degrees Celsius or higher releases the trapped electrons, producing light; this light can be measured to determine the last time. Radiation levels do not remain constant over time. Fluctuating levels can skew results – for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger.
Because of these and other factors, Thermoluminescence is at the most about 15% accurate. It cannot be used to date a site on its own. However, it can be used to confirm the antiquity of an item. Optically stimulated luminescence dating constrains the time at which sediment was last exposed to light. During sediment transport, exposure to s
Hypoxia refers to low oxygen conditions. 20.9% of the gas in the atmosphere is oxygen. The partial pressure of oxygen in the atmosphere is 20.9% of the total barometric pressure. In water however, oxygen levels are much lower 1%, fluctuate locally depending on the presence of photosynthetic organisms and relative distance to the surface. Atmospheric hypoxia occurs at high altitudes. Total atmospheric pressure decreases as altitude increases, causing a lower partial pressure of oxygen, defined as hypobaric hypoxia. Oxygen remains at 20.9% of the total gas mixture, differing from hypoxic hypoxia, where the percentage of oxygen in the air is decreased. This is common, for example, in the sealed burrows of some subterranean animals, such as blesmols. Atmospheric hypoxia is the basis of altitude training, a standard part of training for elite athletes. Several companies mimic hypoxia using normobaric artificial atmosphere. Oxygen depletion is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system.
Dissolved oxygen is expressed as a percentage of the oxygen that would dissolve in the water at the prevailing temperature and salinity. An aquatic system lacking dissolved oxygen is termed reducing, or anoxic. Most fish cannot live below 30% saturation. Hypoxia leads to impaired reproduction of remaining fish via endocrine disruption. A "healthy" aquatic environment should experience less than 80%; the exaerobic zone is found at the boundary of hypoxic zones. Hypoxia can occur throughout the water column and at high altitudes as well as near sediments on the bottom, it extends throughout 20-50% of the water column, but depending on the water depth and location of pycnoclines. It can occur in 10-80% of the water column. For example, in a 10-meter water column, it can reach up to 2 meters below the surface. In a 20-meter water column, it can extend up to 8 meters below the surface. Oxygen depletion can result from a number of natural factors, but is most a concern as a consequence of pollution and eutrophication in which plant nutrients enter a river, lake, or ocean, phytoplankton blooms are encouraged.
While phytoplankton, through photosynthesis, will raise DO saturation during daylight hours, the dense population of a bloom reduces DO saturation during the night by respiration. When phytoplankton cells die, they sink towards the bottom and are decomposed by bacteria, a process that further reduces DO in the water column. If oxygen depletion progresses to hypoxia, fish kills can occur and invertebrates like worms and clams on the bottom may be killed as well. Hypoxia may occur in the absence of pollutants. In estuaries, for example, because freshwater flowing from a river into the sea is less dense than salt water, stratification in the water column can result. Vertical mixing between the water bodies is therefore reduced, restricting the supply of oxygen from the surface waters to the more saline bottom waters; the oxygen concentration in the bottom layer may become low enough for hypoxia to occur. Areas prone to this include shallow waters of semi-enclosed water bodies such as the Waddenzee or the Gulf of Mexico, where land run-off is substantial.
In these areas a so-called "dead zone" can be created. Low dissolved oxygen conditions are seasonal, as is the case in Hood Canal and areas of Puget Sound, in Washington State; the World Resources Institute has identified 375 hypoxic coastal zones around the world, concentrated in coastal areas in Western Europe, the Eastern and Southern coasts of the US, East Asia in Japan. Hypoxia may be the explanation for periodic phenomena such as the Mobile Bay jubilee, where aquatic life rushes to the shallows trying to escape oxygen-depleted water. Recent widespread shellfish kills near the coasts of Oregon and Washington are blamed on cyclic dead zone ecology. Scientists have determined that high concentrations of minerals dumped into bodies of water causes significant growth of phytoplankton blooms; as these blooms are broken down by bacteria, such as Phanerochaete chrysosprium, oxygen is depleted by the enzymes of these organisms. Phytoplankton are made up of lignin and cellulose, which are broken down by enzymes present in organisms such as P. chrysosprium, known as white-rot.
However, the breakdown of cellulose does not deplete oxygen concentration in water, whereas breakdown of lignin does. This breakdown of lignin includes an oxidative mechanism, requires the presence of dissolved oxygen to take place by enzymes like ligninperoxidase. Other fungi such as brown-rot, soft-rot, blue stain fungi are necessary in lignin transformation; as this oxidation takes place, CO2 is formed in its place Ligninperoxidase serves as the most import enzyme because it is best at breaking down lignin in these organisms. LiP disrupts C-C bonds and C-O bonds within Lignin’s three-dimensional structure, causing it to break down. LiP consists of ten alpha helices, two Ca2+ structural ions, as well as a heme group called a tetrapyrrol ring. Oxygen serves an important role in the catalytic cycle of LiP to form a double bond on the Fe2+ ion in the tetrapyrrol ring. Without the presence of diatomic oxygen in the water, this breakdown cannot
The Carnian is the lowermost stage of the Upper Triassic series. It lasted from 237 to 227 million years ago; the Carnian is followed by the Norian. Its boundaries are not characterized by major extinctions or biotic turnovers, but a climatic event occurred during the Carnian and seems to be associated with important extinctions or biotic radiations; the Carnian was named in 1869 by Mojsisovics. It is unclear if it was named after the Carnic Alps or after the Austrian region of Carinthia or after the Carnia historical region in northwestern Italy; the name, was first used referring to a part of the Hallstatt Limestone cropping out in Austria. The base of the Carnian stage is defined as the place in the stratigraphic record where the ammonite species Daxatina canadensis first appears; the global reference profile for the base is located at the Stuores-Wiesen near Badia in the Val Badia in the region of South Tyrol, Italy. The top of the Carnian is at the bases of the ammonite biozones of Klamathites macrolobatus or Stikinoceras kerri and the conodont biozones of Metapolygnathus communisti or Metapolygnathus primitius.
There is no established, standard usage for the Carnian subdivisions, while in some regional stratigraphies a two-substage subdivision is common: Julian Tuvalianothers prefer a three-substage organization of the stage as follows: Cordevolian Julian Tuvalian In the Tethys domain, the Carnian stage contains six ammonite biozones: zone of Anatropites spinosus zone of Tropites subbullatus zone of Tropites dilleri zone of Austrotrachyceras austriacum zone of Trachyceras The paleogeography of the Carnian was the same as for the rest of the Triassic. Most continents were merged into the supercontinent Pangaea, there was a single global ocean, Panthalassa; the global ocean had a western branch at tropical latitudes called Paleo-Tethys. The sediments of Paleo-Tethys now crop out in southeastern Europe, in the Middle East, in the Himalayas, up to the island of Timor; the extreme land-sea distribution led to "mega-monsoons", i.e. an atmospheric monsoon regime more intense than the present one. As for most of the Mesozoic, there were no ice caps.
Climate was arid in the tropics, but an episode of wet tropical climate is documented at least in the Paleo-Tethys. This putative climatic event is called the "Carnian Pluvial Event", its age being between latest early Carnian and the beginning of late Carnian. In the marine realm, the Carnian saw the first abundant occurrences of calcareous nannoplankton, a morphological group including the Coccolithophores. There are a few invertebrates which are characteristic of the Carnian. Among molluscs, the ammonoid genus Trachyceras is exclusive to the lower Carnian; the family Tropitidae and the genus Tropites appear at the base of the upper Carnian. The bivalve genus Halobia, a bottom-dweller of deep sea environments, differentiated from Daonella at the beginning of this age. Scleractinian coral reefs, i.e. reefs with corals of the modern type, became common for the first time in the Carnian. The earliest dinosaur Eoraptor originated during the Carnian, around 230 Ma; the oldest well documented dinosaurian assemblage, in the Ischigualasto Formation of Argentina, is most late Carnian in age.
In this stage the archosaurs became the dominant faunas in the world, evolving into groups such as the phytosaurs, rhynchosaurs and rauisuchians. The first dinosaurs appeared in this stage, though at the time they were small and insignificant, they diversified and would dominate the fauna for the rest of the Mesozoic. On the other hand, the therapsids, which included the ancestors of mammals, decreased in both size and diversity, would remain small until the extinction of the dinosaurs. Conodonts were present in Triassic marine sediments. Paragondolella polygnathiformis appeared at the base of the Carnian stage, is considered a characteristic species. A partial list of Carnian vertebrates is given below. Many Carnian vertebrates are found in Santa Maria Formation rocks of the Paleorrota geopark; the lower Carnian fauna of the San Cassiano Formation has been studied since the 19th century. Fossiliferous localities are many, are distributed in the surroundings of Cortina d'Ampezzo and in the high Badia Valley, near the village of San Cassiano, after which the formation was named.
This fauna is diverse, including ammonoids, bivalves, calcareous sponge, brachiopods, a variety of less common fossils. A collection of this fauna is exposed in a museum in Cortina d'Ampezzo; the Ischigualasto Formation of northwestern Argentina yielded a important vertebrate association, including the oldest dinosaurian assemblage. Brack, P.. Broglio Loriga, C.. C.. Furin, S..
Ammonoids are an extinct group of marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs referred to as ammonites, are more related to living coleoids than they are to shelled nautiloids such as the living Nautilus species; the earliest ammonites appear during the Devonian, the last species died out in the Cretaceous–Paleogene extinction event. Ammonites are excellent index fossils, it is possible to link the rock layer in which a particular species or genus is found to specific geologic time periods, their fossil shells take the form of planispirals, although there were some helically spiraled and nonspiraled forms. The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble coiled rams' horns. Pliny the Elder called fossils of these animals ammonis cornua because the Egyptian god Ammon was depicted wearing ram's horns; the name of an ammonite genus ends in -ceras, Greek for "horn".
Ammonites can be distinguished by their septa, the dividing walls that separate the chambers in the phragmocone, by the nature of their sutures where the septa joint the outer shell wall, in general by their siphuncles. Ammonoid septa characteristically have bulges and indentations and are to varying degrees convex from the front, distinguishing them from nautiloid septa which are simple concave dish-shaped structures; the topology of the septa around the rim, results in the various suture patterns found. Three major types of suture patterns are found in the Ammonoidea: Goniatitic - numerous undivided lobes and saddles; this pattern is characteristic of the Paleozoic ammonoids. Ceratitic - lobes have subdivided tips, giving them a saw-toothed appearance, rounded undivided saddles; this suture pattern is characteristic of Triassic ammonoids and appears again in the Cretaceous "pseudoceratites". Ammonitic - lobes and saddles are much subdivided. Ammonoids of this type are the most important species from a biostratigraphical point of view.
This suture type is characteristic of Jurassic and Cretaceous ammonoids, but extends back all the way to the Permian. The siphuncle in most ammonoids is a narrow tubular structure that runs along the shell's outer rim, known as the venter, connecting the chambers of the phragmocone to the body or living chamber; this distinguishes them from living nautiloides and typical Nautilida, in which the siphuncle runs through the center of each chamber. However the earliest nautiloids from the Late Cambrian and Ordovician had ventral siphuncles like ammonites, although proportionally larger and more internally structured; the word "siphuncle" comes from the New Latin siphunculus, meaning "little siphon". Originating from within the bactritoid nautiloids, the ammonoid cephalopods first appeared in the Devonian and became extinct at the close of the Cretaceous along with the dinosaurs; the classification of ammonoids is based in part on the ornamentation and structure of the septa comprising their shells' gas chambers.
While nearly all nautiloids show curving sutures, the ammonoid suture line is variably folded, forming saddles and lobes. The Ammonoidea can be divided into six orders, listed here starting with the most primitive and going to the more derived: Agoniatitida, Lower Devonian - Middle Devonian Clymeniida, Upper Devonian Goniatitida, Middle Devonian - Upper Permian Prolecanitida, Upper Devonian - Upper Triassic Ceratitida, Upper Permian - Upper Triassic Ammonitida, Lower Jurassic - Upper CretaceousIn some classifications, these are left as suborders, included in only three orders: Goniatitida and Ammonitida; the Treatise on Invertebrate Paleontology divides the Ammonoidea, regarded as an order, into eight suborders, the Anarcestina, Clymeniina and Prolecanitina from the Paleozoic. In subsequent taxonomies, these are sometimes regarded as orders within the subclass Ammonoidea; because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are rarely preserved in any detail.
Nonetheless, much has been worked out by examining ammonoid shells and by using models of these shells in water tanks. Many ammonoids lived in the open water of ancient seas, rather than at the sea bottom, because their fossils are found in rocks laid down under conditions where no bottom-dwelling life is found. Many of them are thought to have been good swimmers, with flattened, discus-shaped, streamlined shells, although some ammonoids were less effective swimmers and were to have been slow-swimming bottom-dwellers. Synchrotron analysis of an aptychophoran ammonite revealed remains of isopod and mollusc larvae in its buccal cavity, indicating at least this kind of ammonite fed on plankton, they may have avoided predation by squirting ink, much like modern cephalopods. The soft body of the creature occupied the largest segments of the shell at the end of the coil; the smaller earlier segments were walled off and the animal could maintain its buoyancy by filling them with gas. Thus, the smaller sections of the coil would have floated ab
The Tethys Ocean called the Tethys Sea or the Neotethys, was an ocean during much of the Mesozoic Era located between the ancient continents of Gondwana and Laurasia, before the opening of the Indian and Atlantic oceans during the Cretaceous Period. The name stems from the mythological Greek sea goddess Tethys and consort of Oceanus, mother of the great rivers and fountains of the world and of the Oceanid sea nymphs; the eastern part of the Tethys Ocean is sometimes referred to as Eastern Tethys. The western part of the Tethys Ocean is called Tethys Sea, Western Tethys Ocean, or Paratethys or Alpine Tethys Ocean; the Black and Aral seas are thought to be its crustal remains, though the Black Sea may, in fact, be a remnant of the older Paleo-Tethys Ocean. The Western Tethys was not a single open ocean, it covered many small plates, Cretaceous island arcs, microcontinents. Many small oceanic basins were separated from each other by continental terranes on the Alboran and Apulian plates; the high sea level in the Mesozoic flooded most of these continental domains.
As theories have improved, scientists have extended the "Tethys" name to refer to three similar oceans that preceded it, separating the continental terranes: in Asia, the Paleo-Tethys, Meso-Tethys, Ceno-Tethy are recognized. Neither Tethys Ocean should be confused with the Rheic Ocean, which existed to the west of them in the Silurian Period. To the north of the Tethys, the then-land mass was called Angaraland and to the south of it, it was called Gondwanaland. From the Ediacaran into the Devonian, the Proto-Tethys Ocean existed and was situated between Baltica and Laurentia to the north and Gondwana to the south. From the Silurian through the Jurassic periods, the Paleo-Tethys Ocean existed between the Hunic terranes and Gondwana. Over a period of 400 million years, continental terranes intermittently separated from Gondwana in the Southern Hemisphere to migrate northward to form Asia in the Northern Hemisphere. About 250 Mya, during the Triassic, a new ocean began forming in the southern end of the Paleo-Tethys Ocean.
A rift formed along the northern continental shelf of Southern Pangaea. Over the next 60 million years, that piece of shelf, known as Cimmeria, traveled north, pushing the floor of the Paleo-Tethys Ocean under the eastern end of northern Pangaea; the Tethys Ocean formed between Cimmeria and Gondwana, directly over where the Paleo-Tethys used to be. During the Jurassic period about 150 Mya, Cimmeria collided with Laurasia and stalled, so the ocean floor behind it buckled under, forming the Tethyan Trench. Water levels rose, the western Tethys shallowly covered significant portions of Europe, forming the first Tethys Sea. Around the same time and Gondwana began drifting apart, opening an extension of the Tethys Sea between them which today is the part of the Atlantic Ocean between the Mediterranean and the Caribbean; as North and South America were still attached to the rest of Laurasia and Gondwana the Tethys Ocean in its widest extension was part of a continuous oceanic belt running around the Earth between about latitude 30°N and the Equator.
Thus, ocean currents at the time around the Early Cretaceous ran differently from the way they do today. Between the Jurassic and the Late Cretaceous, which started about 100 Mya, Gondwana began breaking up, pushing Africa and India north across the Tethys and opening up the Indian Ocean; as these land masses crowded in on the Tethys Ocean from all sides, to as as the Late Miocene, 15 Mya, the ocean continued to shrink, becoming the Tethys Seaway or second Tethys Sea. Throughout the Cenozoic, global sea levels fell hundreds of meters, the connections between the Atlantic and the Tethys closed off in what is now the Middle East. During the Oligocene, large parts of central and eastern Europe were covered by a northern branch of the Tethys Ocean, called the Paratethys; the Paratethys was separated from the Tethys with the formation of the Alps, Dinarides and Elburz mountains during the Alpine orogeny. During the late Miocene, the Paratethys disappeared, became an isolated inland sea. In 1885, the Austrian palaeontologist Melchior Neumayr deduced the existence of the Tethys Ocean from Mesozoic marine sediments and their distribution, calling his concept Zentrales Mittelmeer and described it as a Jurassic seaway, which extended from the Caribbean to the Himalayas.
In 1893, the Austrian geologist Eduard Suess proposed the theory that an ancient and extinct inland sea had once existed between Laurasia and the continents which formed Gondwana II. He named it the Tethys Sea after the Greek sea goddess Tethys, he provided evidence for his theory using fossil records from the Africa. He proposed the concept of Tethys in his four-volume work Das Antlitz der Erde. In the following decades during the 20th century, "mobilist" geologists such as Uhlig and Daque regarded Tethys as a large trough between two supercontinents which lasted from the late Palaeozoic until continental fragments derived from Gondwana obliterated it. After World War II, Tethys was described as a triangular ocean with a wide eastern end. From 1920s to the 1960s, "fixist" geologists, regarded Tethys as a composite trough, which evolved through a series of orogenic cycles, they used the terms'Paleotethys','Mesotethys', and'Neotethys' for the Caledonian and Alpine orogenies, respe
Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia. Modern amphibians are all Lissamphibia, they inhabit a wide variety of habitats, with most species living within terrestrial, arboreal or freshwater aquatic ecosystems. Thus amphibians start out as larvae living in water, but some species have developed behavioural adaptations to bypass this; the young undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely on their skin, they are superficially similar to lizards but, along with mammals and birds, reptiles are amniotes and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are ecological indicators; the earliest amphibians evolved in the Devonian period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land.
They diversified and became dominant during the Carboniferous and Permian periods, but were displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia; the three modern orders of amphibians are Anura and Apoda. The number of known amphibian species is 8,000, of which nearly 90% are frogs; the smallest amphibian in the world is a frog from New Guinea with a length of just 7.7 mm. The largest living amphibian is the 1.8 m Chinese giant salamander, but this is dwarfed by the extinct 9 m Prionosuchus from the middle Permian of Brazil. The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology; the word "amphibian" is derived from the Ancient Greek term ἀμφίβιος, which means "both kinds of life", ἀμφί meaning "of both kinds" and βιος meaning "life". The term was used as a general adjective for animals that could live on land or in water, including seals and otters.
Traditionally, the class Amphibia includes all tetrapod vertebrates. Amphibia in its widest sense was divided into three subclasses, two of which are extinct: Subclass Lepospondyli† Subclass Temnospondyli† Subclass Lissamphibia Salientia: Jurassic to present—6,200 current species in 53 families Caudata: Jurassic to present—652 current species in 9 families Gymnophiona: Jurassic to present—192 current species in 10 families The actual number of species in each group depends on the taxonomic classification followed; the two most common systems are the classification adopted by the website AmphibiaWeb, University of California and the classification by herpetologist Darrel Frost and the American Museum of Natural History, available as the online reference database "Amphibian Species of the World". The numbers of species cited above follows Frost and the total number of known amphibian species as of March 31, 2019 is 8,000, of which nearly 90% are frogs. With the phylogenetic classification, the taxon Labyrinthodontia has been discarded as it is a polyparaphyletic group without unique defining features apart from shared primitive characteristics.
Classification varies according to the preferred phylogeny of the author and whether they use a stem-based or a node-based classification. Traditionally, amphibians as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians and all their descendants is called Lissamphibia; the phylogeny of Paleozoic amphibians is uncertain, Lissamphibia may fall within extinct groups, like the Temnospondyli or the Lepospondyli, in some analyses in the amniotes. This means that advocates of phylogenetic nomenclature have removed a large number of basal Devonian and Carboniferous amphibian-type tetrapod groups that were placed in Amphibia in Linnaean taxonomy, included them elsewhere under cladistic taxonomy. If the common ancestor of amphibians and amniotes is included in Amphibia, it becomes a paraphyletic group. All modern amphibians are included in the subclass Lissamphibia, considered a clade, a group of species that have evolved from a common ancestor.
The three modern orders are Anura and Gymnophiona. It has been suggested that salamanders arose separately from a Temnospondyl-like ancestor, that caecilians are the sister group of the advanced reptiliomorph amphibians, thus of amniotes. Although the fossils of several older proto-frogs with primitive characteristics are known, the oldest "true frog" is Prosalirus bitis, from the Early Jurassic Kayenta Formation of Arizona, it is anatomically similar to modern frogs. The oldest known caecilian is another Early Jurassic species, Eocaecilia micropodia from Arizona; the earliest salamander is Beiyanerpeton jianpingensis from the Late Jurassic of northeastern China. Authorities disagree as to whether Salientia is a superorder that includes the order Anura, or whether
Archosauromorpha is a clade of diapsid reptiles containing all reptiles more related to archosaurs rather than lepidosaurs. Archosauromorphs first appeared during the middle Permian, though they became much more common and diverse during the Triassic period. Although Archosauromorpha was first named in 1946, its membership did not become well-established until the 1980s. Archosauromorpha encompasses four main groups of reptiles: the stocky, herbivorous allokotosaurs and rhynchosaurs, the hugely diverse Archosauriformes, a polyphyletic grouping of various long-necked reptiles including Protorosaurus and Prolacerta. Other groups including pantestudines and the semiaquatic choristoderes have been placed in Archosauromorpha by some authors. Archosauromorpha is one of the most diverse groups of reptiles, but its members can be united by several shared skeletal characteristics; these include laminae on the vertebrae, a posterodorsal process of the premaxilla, a lack of notochordal canals, the loss of the entepicondylar foramen of the humerus.
The term Archosauromorpha was first used by Friedrich von Huene in 1946 to refer to reptiles more related to archosaurs than to lepidosaurs. However, there was little consensus on ancient reptile relationships prior to the late 20th century, so the term Archosauromorpha was used until many years after its creation; the advent of cladistics helped to sort out at least some of the relationships within Reptilia, it became clear that there was a split between the archosaur lineage and the lepidosaur lineage somewhere within the Permian, with certain reptiles closer to archosaurs and others allied with lepidosaurs. Jacques Gauthier reused the term Archosauromorpha for the archosaur lineage at the 1982 annual meeting of the American Society of Zoologists, used it within his 1984 Ph. D. thesis. Archosauromorpha, as formulated by Gauthier, included four main groups of reptiles: Rhynchosauria, "Prolacertiformes", "Trilophosauria", Archosauria. Cladistic analyses created during the 1980s by Gauthier, Michael J. Benton, Susan E. Evans implemented Gauthier's classification scheme within large studies of reptile relations.
Michel Laurin defined Archosauromorpha as the clade containing the most recent common ancestor of Prolacerta, Trilophosaurus and all of its descendants. David Dilkes formulated a more inclusive definition of Archosauromorpha, defining it as the clade containing Protorosaurus and all other saurians that are more related to Protorosaurus than to Lepidosauria. In 2016, Martin Ezcurra named a subgroup of Crocopoda. Crocopoda is defined as all archosauromorphs more related to allokotosaurs, rhynchosaurs, or archosauriforms rather than Protorosaurus or tanystropheids; this group corresponds to Laurin's definition of Archosauromorpha. Since the seminal studies of the 1980s, Archosauromorpha has been found to contain four specific reptile groups, although the definitions and validity of the groups themselves have been questioned; the least controversial group is a monophyletic clade of stocky herbivores. Many rhynchosaurs had modified skulls, with beak-like premaxillary bones and wide heads. Another group of archosauromorphs has traditionally been represented by Trilophosaurus, an unusual iguana-like herbivorous reptile quite different from the rhynchosaurs.
Gauthier used the name "Trilophosauria" for this group, but a 2015 study offered an alternative name. This study found that Azendohsauridae, Triassic reptiles mistaken for "prosauropod" dinosaurs, were in fact close relatives of Trilophosaurus and the rest of Trilophosauridae. Trilophosaurids and azendohsaurids are now united under the group Allokotosauria; these two groups did not survive the end of the Triassic period, but the most famous group of archosauromorphs not only survived, but have continued to diversify and dominate beyond the Triassic-Jurassic extinction. These were the Archosauriformes, a diverse assortment of animals including the famous dinosaurs and pterosaurs. Two subclades of Archosauriformes survive to the present day: the semiaquatic crocodilians and the last of the feathered dinosaurs: birds. Gauthier used the name Archosauria to refer to; the final unambiguous members of Archosauromorpha represent the most controversial group. These were the first archosauromorphs to appear, can be characterized by their long necks, sprawling posture, carnivorous habits.
One name for the group, Protorosauria, is named after Protorosaurus, the oldest archosauromorph known from good remains. Another name, Prolacertiformes, is in reference to Prolacerta. Protorosauria/Prolacertiformes has had a complicated history, many taxa have entered and left the group as paleontologists discover and re-evaluate reptiles of the Triassic. By far the most famous of these are tanystropheids such as Tanystropheus, known for having necks longer than their entire body. Other notable genera include Boreopricea and Macrocnemus, as well as strange gliding reptiles such as Sharovipteryx and Mecistotrachelos. A landmark 1998 study by David