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132-675: The most famous assemblage of organisms are referred to as the Chengjiang biota for the multiple scattered fossil sites in Chengjiang. The age of the Chengjiang Lagerstätte is locally termed Qiongzhusian , a stage correlated to the late Atdabanian Stage in Siberian sequences of the middle of the Early Cambrian . The shales date to ≤ 518  million years ago . The shales also contain

264-700: A ρ w 1 − ρ a ρ w = R D V − ρ a ρ w 1 − ρ a ρ w . {\displaystyle RD_{\mathrm {A} }={{\rho _{\mathrm {s} } \over \rho _{\mathrm {w} }}-{\rho _{\mathrm {a} } \over \rho _{\mathrm {w} }} \over 1-{\rho _{\mathrm {a} } \over \rho _{\mathrm {w} }}}={RD_{\mathrm {V} }-{\rho _{\mathrm {a} } \over \rho _{\mathrm {w} }} \over 1-{\rho _{\mathrm {a} } \over \rho _{\mathrm {w} }}}.} In

396-465: A ) . {\displaystyle F_{\mathrm {w} }=g\left(m_{\mathrm {b} }-\rho _{\mathrm {a} }{\frac {m_{\mathrm {b} }}{\rho _{\mathrm {b} }}}+V\rho _{\mathrm {w} }-V\rho _{\mathrm {a} }\right).} If we subtract the force measured on the empty bottle from this (or tare the balance before making the water measurement) we obtain. F w , n = g V ( ρ w − ρ

528-445: A ) = ρ s − ρ a ρ w − ρ a . {\displaystyle SG_{\mathrm {A} }={\frac {gV(\rho _{\mathrm {s} }-\rho _{\mathrm {a} })}{gV(\rho _{\mathrm {w} }-\rho _{\mathrm {a} })}}={\frac {\rho _{\mathrm {s} }-\rho _{\mathrm {a} }}{\rho _{\mathrm {w} }-\rho _{\mathrm {a} }}}.} This

660-418: A ) , {\displaystyle F_{\mathrm {s,n} }=gV(\rho _{\mathrm {s} }-\rho _{\mathrm {a} }),} where ρ s is the density of the sample. The ratio of the sample and water forces is: S G A = g V ( ρ s − ρ a ) g V ( ρ w − ρ

792-439: A ) , {\displaystyle F_{\mathrm {w,n} }=gV(\rho _{\mathrm {w} }-\rho _{\mathrm {a} }),} where the subscript n indicated that this force is net of the force of the empty bottle. The bottle is now emptied, thoroughly dried and refilled with the sample. The force, net of the empty bottle, is now: F s , n = g V ( ρ s − ρ

924-405: A m p l e {\displaystyle \rho _{\mathrm {sample} }} is the density of the sample and ρ H 2 O {\displaystyle \rho _{\mathrm {H_{2}O} }} is the density of water. The apparent specific gravity is simply the ratio of the weights of equal volumes of sample and water in air: S G a p p

1056-670: A m p l e V m H 2 O V = m s a m p l e m H 2 O g g = W V , sample W V , H 2 O , {\displaystyle SG_{\mathrm {true} }={\frac {\rho _{\mathrm {sample} }}{\rho _{\mathrm {H_{2}O} }}}={\frac {\frac {m_{\mathrm {sample} }}{V}}{\frac {m_{\mathrm {H_{2}O} }}{V}}}={\frac {m_{\mathrm {sample} }}{m_{\mathrm {H_{2}O} }}}{\frac {g}{g}}={\frac {W_{\mathrm {V} ,{\text{sample}}}}{W_{\mathrm {V} ,\mathrm {H_{2}O} }}},} where g

1188-405: A r e n t = W A , sample W A , H 2 O , {\displaystyle SG_{\mathrm {apparent} }={\frac {W_{\mathrm {A} ,{\text{sample}}}}{W_{\mathrm {A} ,\mathrm {H_{2}O} }}},} where W A , sample {\displaystyle W_{A,{\text{sample}}}} represents the weight of

1320-609: A brief period as benthic environments rapidly changed in the Cambrian. Chengjiang is the richest source of the Lobopodia , a group including many early panarthropods , with six genera represented: Luolishania , Paucipodia , Cardiodictyon , Hallucigenia (also known from the Burgess Shale), Microdictyon , and Onychodictyon . Perhaps the most important fossils from Chengjiang are eight possible members of phylum Chordata ,

1452-467: A constant rate. These " molecular clocks ", however, are fallible, and provide only approximate timing: for example, they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques may vary by a factor of two. Organisms are only rarely preserved as fossils in the best of circumstances, and only

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1584-426: A fraction of such fossils have been discovered. This is illustrated by the fact that the number of species known through the fossil record is less than 5% of the number of known living species, suggesting that the number of species known through fossils must be far less than 1% of all the species that have ever lived. Because of the specialized and rare circumstances required for a biological structure to fossilize, only

1716-495: A hierarchical classification system still in use today. Darwin and his contemporaries first linked the hierarchical structure of the tree of life with the then very sparse fossil record. Darwin eloquently described a process of descent with modification, or evolution, whereby organisms either adapt to natural and changing environmental pressures, or they perish. When Darwin wrote On the Origin of Species by Means of Natural Selection, or

1848-411: A liquid can be measured using a hydrometer. This consists of a bulb attached to a stalk of constant cross-sectional area, as shown in the adjacent diagram. First the hydrometer is floated in the reference liquid (shown in light blue), and the displacement (the level of the liquid on the stalk) is marked (blue line). The reference could be any liquid, but in practice it is usually water. The hydrometer

1980-418: A mid-Ordovician age. Such index fossils must be distinctive, be globally distributed and occupy a short time range to be useful. Misleading results are produced if the index fossils are incorrectly dated. Stratigraphy and biostratigraphy can in general provide only relative dating ( A was before B ), which is often sufficient for studying evolution. However, this is difficult for some time periods, because of

2112-412: A molar volume of 22.259 L under those same conditions. Those with SG greater than 1 are denser than water and will, disregarding surface tension effects, sink in it. Those with an SG less than 1 are less dense than water and will float on it. In scientific work, the relationship of mass to volume is usually expressed directly in terms of the density (mass per unit volume) of the substance under study. It

2244-526: A mutation first appeared. Phylogenetics and paleontology work together in the clarification of science's still dim view of the appearance of life and its evolution. Niles Eldredge 's study of the Phacops trilobite genus supported the hypothesis that modifications to the arrangement of the trilobite's eye lenses proceeded by fits and starts over millions of years during the Devonian . Eldredge's interpretation of

2376-570: A portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of vertebrates , or the chitinous or calcareous exoskeletons of invertebrates . Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces ( coprolites ). These types of fossil are called trace fossils or ichnofossils , as opposed to body fossils . Some fossils are biochemical and are called chemofossils or biosignatures . Gathering fossils dates at least to

2508-559: A reference, and/or with a substantial contribution to the development of geological sciences through history.' The fossils occur in a section of mudstone 50 metres (160 ft) thick in the Yuanshan Member of the Qiongzhusi Formation . The Yuanshan Member is extensive, covering multiple 10,000 square kilometres (3,900 sq mi) of eastern Yunnan Province, where there are many scattered outcrops yielding fossils. Studies of

2640-400: A richly diverse assembly of early multicellular eukaryotes . The fossil record and faunal succession form the basis of the science of biostratigraphy or determining the age of rocks based on embedded fossils. For the first 150 years of geology , biostratigraphy and superposition were the only means for determining the relative age of rocks. The geologic time scale was developed based on

2772-448: A small percentage of life-forms can be expected to be represented in discoveries, and each discovery represents only a snapshot of the process of evolution. The transition itself can only be illustrated and corroborated by transitional fossils, which will never demonstrate an exact half-way point. The fossil record is strongly biased toward organisms with hard-parts, leaving most groups of soft-bodied organisms with little to no role. It

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2904-462: A specific, but not necessarily accurately known volume, V and is placed upon a balance, it will exert a force F b = g ( m b − ρ a m b ρ b ) , {\displaystyle F_{\mathrm {b} }=g\left(m_{\mathrm {b} }-\rho _{\mathrm {a} }{\frac {m_{\mathrm {b} }}{\rho _{\mathrm {b} }}}\right),} where m b

3036-409: A substance with a relative density (or specific gravity) less than 1 will float in water. For example, an ice cube, with a relative density of about 0.91, will float. A substance with a relative density greater than 1 will sink. Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm (101.325 kPa ). Where it is not, it is more usual to specify

3168-430: Is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nearly always measured with respect to water at its densest (at 4 °C or 39.2 °F); for gases, the reference is air at room temperature (20 °C or 68 °F). The term "relative density" (abbreviated r.d. or RD )

3300-429: Is a dimensionless quantity , as it is the ratio of either densities or weights R D = ρ s u b s t a n c e ρ r e f e r e n c e , {\displaystyle {\mathit {RD}}={\frac {\rho _{\mathrm {substance} }}{\rho _{\mathrm {reference} }}},} where R D {\displaystyle RD}

3432-409: Is a device used to determine the density of a liquid. A pycnometer is usually made of glass , with a close-fitting ground glass stopper with a capillary tube through it, so that air bubbles may escape from the apparatus. This device enables a liquid's density to be measured accurately by reference to an appropriate working fluid, such as water or mercury , using an analytical balance . If

3564-449: Is a notable example of how knowledge encoded by the fossil record continues to contribute otherwise unattainable information on the emergence and development of life on Earth. For example, the research suggests Markuelia has closest affinity to priapulid worms, and is adjacent to the evolutionary branching of Priapulida , Nematoda and Arthropoda . Despite significant advances in uncovering and identifying paleontological specimens, it

3696-463: Is a process of fossilization that occurs when an organism is buried. The empty spaces within an organism (spaces filled with liquid or gas during life) become filled with mineral-rich groundwater . Minerals precipitate from the groundwater, occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell . Small scale permineralization can produce very detailed fossils. For permineralization to occur,

3828-412: Is any preserved remains, impression, or trace of any once-living thing from a past geological age . Examples include bones , shells , exoskeletons , stone imprints of animals or microbes , objects preserved in amber , hair , petrified wood and DNA remnants. The totality of fossils is known as the fossil record . Though the fossil record is incomplete, numerous studies have demonstrated that there

3960-420: Is being measured. For true ( in vacuo ) relative density calculations air pressure must be considered (see below). Temperatures are specified by the notation ( T s / T r ) with T s representing the temperature at which the sample's density was determined and T r the temperature at which the reference (water) density is specified. For example, SG (20 °C/4 °C) would be understood to mean that

4092-422: Is being measured. For true ( in vacuo ) specific gravity calculations, air pressure must be considered (see below). Temperatures are specified by the notation ( T s / T r ), with T s representing the temperature at which the sample's density was determined and T r the temperature at which the reference (water) density is specified. For example, SG (20 °C/4 °C) would be understood to mean that

Maotianshan Shales - Misplaced Pages Continue

4224-629: Is believed to have inhabited a delta front environment rich in oxygen, with high sedimentation rates and major fluctuations in salinity being the main environmental stressors. Located at the Yunnan Province of South China, the Guanshan biota are also Burgess shale-type fossils but slightly younger than the Chengjian biota, and is dated to 515–510 Myr falling within the Cambrian Stage 4 . Brachiopods are

4356-408: Is called the apparent relative density , denoted by subscript A, because it is what we would obtain if we took the ratio of net weighings in air from an analytical balance or used a hydrometer (the stem displaces air). Note that the result does not depend on the calibration of the balance. The only requirement on it is that it read linearly with force. Nor does RD A depend on the actual volume of

4488-488: Is described to be the earliest craniate -like chordate. This fish-like animal has many similarities to Y. lividum , but differs in several aspects: It has a discernible heart, dorsal and ventral aorta, gill filaments, and a notochord (neural chord). At present, there is no agreement as to the systematic placement of the Vetulicola , represented by seven species from Chengjiang. Originally described as crustacean arthropods,

4620-422: Is easy to measure, the volume of an irregularly shaped sample can be more difficult to ascertain. One method is to put the sample in a water-filled graduated cylinder and read off how much water it displaces. Alternatively the container can be filled to the brim, the sample immersed, and the volume of overflow measured. The surface tension of the water may keep a significant amount of water from overflowing, which

4752-538: Is enough information available to give a good understanding of the pattern of diversification of life on Earth. In addition, the record can predict and fill gaps such as the discovery of Tiktaalik in the arctic of Canada . Paleontology includes the study of fossils: their age, method of formation, and evolutionary significance. Specimens are usually considered to be fossils if they are over 10,000 years old. The oldest fossils are around 3.48 billion years to 4.1 billion years old. The observation in

4884-786: Is especially problematic for small samples. For this reason it is desirable to use a water container with as small a mouth as possible. For each substance, the density, ρ , is given by ρ = Mass Volume = Deflection × Spring Constant Gravity Displacement W a t e r L i n e × Area C y l i n d e r . {\displaystyle \rho ={\frac {\text{Mass}}{\text{Volume}}}={\frac {{\text{Deflection}}\times {\frac {\text{Spring Constant}}{\text{Gravity}}}}{{\text{Displacement}}_{\mathrm {WaterLine} }\times {\text{Area}}_{\mathrm {Cylinder} }}}.} When these densities are divided, references to

5016-469: Is extremely important that the analyst enter the table with the correct form of relative density. For example, in the brewing industry, the Plato table , which lists sucrose concentration by mass against true RD, were originally (20 °C/4 °C) that is based on measurements of the density of sucrose solutions made at laboratory temperature (20 °C) but referenced to the density of water at 4 °C which

5148-417: Is extremely important that the analyst enter the table with the correct form of specific gravity. For example, in the brewing industry, the Plato table lists sucrose concentration by weight against true SG, and was originally (20 °C/4 °C) i.e. based on measurements of the density of sucrose solutions made at laboratory temperature (20 °C) but referenced to the density of water at 4 °C which

5280-501: Is filled with air but as that air displaces an equal amount of air the weight of that air is canceled by the weight of the air displaced. Now we fill the bottle with the reference fluid e.g. pure water. The force exerted on the pan of the balance becomes: F w = g ( m b − ρ a m b ρ b + V ρ w − V ρ

5412-479: Is generally accepted that the fossil record is vastly incomplete. Approaches for measuring the completeness of the fossil record have been developed for numerous subsets of species, including those grouped taxonomically, temporally, environmentally/geographically, or in sum. This encompasses the subfield of taphonomy and the study of biases in the paleontological record. Paleontology seeks to map out how life evolved across geologic time. A substantial hurdle

Maotianshan Shales - Misplaced Pages Continue

5544-487: Is in industry where specific gravity finds wide application, often for historical reasons. True specific gravity of a liquid can be expressed mathematically as: S G t r u e = ρ s a m p l e ρ H 2 O , {\displaystyle SG_{\mathrm {true} }={\frac {\rho _{\mathrm {sample} }}{\rho _{\mathrm {H_{2}O} }}},} where ρ s

5676-607: Is more easily and perhaps more accurately measured without measuring volume. Using a spring scale, the sample is weighed first in air and then in water. Relative density (with respect to water) can then be calculated using the following formula: R D = W a i r W a i r − W w a t e r , {\displaystyle RD={\frac {W_{\mathrm {air} }}{W_{\mathrm {air} }-W_{\mathrm {water} }}},} where This technique cannot easily be used to measure relative densities less than one, because

5808-515: Is necessary to specify the temperatures and pressures at which the densities or masses were determined. It is nearly always the case that measurements are made at nominally 1 atmosphere (101.325 kPa ignoring the variations caused by changing weather patterns) but as relative density usually refers to highly incompressible aqueous solutions or other incompressible substances (such as petroleum products) variations in density caused by pressure are usually neglected at least where apparent relative density

5940-437: Is normally assumed to be water at 4 ° C (or, more precisely, 3.98 °C, which is the temperature at which water reaches its maximum density). In SI units, the density of water is (approximately) 1000  kg / m or 1  g / cm , which makes relative density calculations particularly convenient: the density of the object only needs to be divided by 1000 or 1, depending on the units. The relative density of gases

6072-623: Is often measured with respect to dry air at a temperature of 20 °C and a pressure of 101.325 kPa absolute, which has a density of 1.205 kg/m . Relative density with respect to air can be obtained by R D = ρ g a s ρ a i r ≈ M g a s M a i r , {\displaystyle {\mathit {RD}}={\frac {\rho _{\mathrm {gas} }}{\rho _{\mathrm {air} }}}\approx {\frac {M_{\mathrm {gas} }}{M_{\mathrm {air} }}},} where M {\displaystyle M}

6204-455: Is preferred in SI , whereas the term "specific gravity" is gradually being abandoned. If a substance's relative density is less than 1 then it is less dense than the reference; if greater than 1 then it is denser than the reference. If the relative density is exactly 1 then the densities are equal; that is, equal volumes of the two substances have the same mass. If the reference material is water, then

6336-455: Is relative density, ρ s u b s t a n c e {\displaystyle \rho _{\mathrm {substance} }} is the density of the substance being measured, and ρ r e f e r e n c e {\displaystyle \rho _{\mathrm {reference} }} is the density of the reference. (By convention ρ {\displaystyle \rho } ,

6468-438: Is replete with the mollusks , the vertebrates , the echinoderms , the brachiopods and some groups of arthropods . Fossil sites with exceptional preservation—sometimes including preserved soft tissues—are known as Lagerstätten —German for "storage places". These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus slowing decomposition. Lagerstätten span geological time from

6600-499: Is taken from this work which uses SG (17.5 °C/17.5 °C). As a final example, the British RD units are based on reference and sample temperatures of 60 °F and are thus (15.56 °C/15.56 °C). Relative density can be calculated directly by measuring the density of a sample and dividing it by the (known) density of the reference substance. The density of the sample is simply its mass divided by its volume. Although mass

6732-403: Is the molar mass and the approximately equal sign is used because equality pertains only if 1 mol of the gas and 1 mol of air occupy the same volume at a given temperature and pressure, i.e., they are both ideal gases . Ideal behaviour is usually only seen at very low pressure. For example, one mol of an ideal gas occupies 22.414 L at 0 °C and 1 atmosphere whereas carbon dioxide has

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6864-470: Is the difficulty of working out fossil ages. Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating . This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be accurate to within 0.5% or better. Although radiometric dating requires careful laboratory work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so

6996-511: Is the local acceleration due to gravity, V is the volume of the sample and of water (the same for both), ρ sample is the density of the sample, ρ H 2 O is the density of water, W V represents a weight obtained in vacuum, m s a m p l e {\displaystyle {\mathit {m}}_{\mathrm {sample} }} is the mass of the sample and m H 2 O {\displaystyle {\mathit {m}}_{\mathrm {H_{2}O} }}

7128-606: Is the mass of an equal volume of water. The density of water and of the sample varies with temperature and pressure, so it is necessary to specify the temperatures and pressures at which the densities or weights were determined. Measurements are nearly always made at 1 nominal atmosphere (101.325 kPa ± variations from changing weather patterns), but as specific gravity usually refers to highly incompressible aqueous solutions or other incompressible substances (such as petroleum products), variations in density caused by pressure are usually neglected at least where apparent specific gravity

7260-422: Is the mass of the bottle and g the gravitational acceleration at the location at which the measurements are being made. ρ a is the density of the air at the ambient pressure and ρ b is the density of the material of which the bottle is made (usually glass) so that the second term is the mass of air displaced by the glass of the bottle whose weight, by Archimedes Principle must be subtracted. The bottle

7392-413: Is the science of deciphering the "layer-cake" that is the sedimentary record. Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil's age is claimed to lie between the two known ages. Because rock sequences are not continuous, but may be broken up by faults or periods of erosion , it

7524-412: Is then filled with a liquid of known density, in which the powder is completely insoluble. The weight of the displaced liquid can then be determined, and hence the relative density of the powder. A gas pycnometer , the gas-based manifestation of a pycnometer, compares the change in pressure caused by a measured change in a closed volume containing a reference (usually a steel sphere of known volume) with

7656-432: Is then floated in a liquid of unknown density (shown in green). The change in displacement, Δ x , is noted. In the example depicted, the hydrometer has dropped slightly in the green liquid; hence its density is lower than that of the reference liquid. It is necessary that the hydrometer floats in both liquids. The application of simple physical principles allows the relative density of the unknown liquid to be calculated from

7788-595: Is very close to the temperature at which water has its maximum density of ρ ( H 2 O ) equal to 0.999972 g/cm (or 62.43 lb·ft ). The ASBC table in use today in North America, while it is derived from the original Plato table is for apparent relative density measurements at (20 °C/20 °C) on the IPTS-68 scale where the density of water is 0.9982071 g/cm . In the sugar, soft drink, honey, fruit juice and related industries sucrose concentration by mass

7920-567: Is very close to the temperature at which water has its maximum density, ρ H 2 O equal to 999.972 kg/m in SI units ( 0.999 972  g/cm in cgs units or 62.43 lb/cu ft in United States customary units ). The ASBC table in use today in North America for apparent specific gravity measurements at (20 °C/20 °C) is derived from the original Plato table using Plato et al.‘s value for SG(20 °C/4 °C) = 0.998 2343 . In

8052-459: Is very difficult to match up rock beds that are not directly adjacent. However, fossils of species that survived for a relatively short time can be used to match isolated rocks: this technique is called biostratigraphy . For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period. If rocks of unknown age have traces of E. pseudoplanus , they have

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8184-491: The Archimedes buoyancy principle, the buoyancy force acting on the hydrometer is equal to the weight of liquid displaced. This weight is equal to the mass of liquid displaced multiplied by g , which in the case of the reference liquid is ρ ref Vg . Setting these equal, we have m g = ρ r e f V g {\displaystyle mg=\rho _{\mathrm {ref} }Vg} or just Exactly

8316-757: The Cambrian period to the present . Worldwide, some of the best examples of near-perfect fossilization are the Cambrian Maotianshan Shales and Burgess Shale , the Devonian Hunsrück Slates , the Jurassic Solnhofen Limestone , and the Carboniferous Mazon Creek localities. A fossil is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, such as from aragonite to calcite . Replacement occurs when

8448-568: The Cambrian Explosion . One of the most intriguing locations of the Chengjiang biota is the Haiyan Lagerstätte where hundreds of juvenile specimens have been found. This unique location offers insights into the development of most animal groups and as such is a unique deposit in the Cambrian. In respect of 'the Chengjiang fossils represent[ing] an uparalleled record of the fundamentally important rapid diversification of metazoan life in

8580-581: The Phacops fossil record was that the aftermaths of the lens changes, but not the rapidly occurring evolutionary process, were fossilized. This and other data led Stephen Jay Gould and Niles Eldredge to publish their seminal paper on punctuated equilibrium in 1971. Synchrotron X-ray tomographic analysis of early Cambrian bilaterian embryonic microfossils yielded new insights of metazoan evolution at its earliest stages. The tomography technique provides previously unattainable three-dimensional resolution at

8712-559: The Renaissance . Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life. For example, Leonardo noticed discrepancies with the biblical flood narrative as an explanation for fossil origins: If the Deluge had carried the shells for distances of three and four hundred miles from the sea it would have carried them mixed with various other natural objects all heaped up together; but even at such distances from

8844-591: The Song dynasty during the 11th century, who kept a specific seashell fossil with his own poem engraved on it. In his Dream Pool Essays published in 1088, Song dynasty Chinese scholar-official Shen Kuo hypothesized that marine fossils found in a geological stratum of mountains located hundreds of miles from the Pacific Ocean was evidence that a prehistoric seashore had once existed there and shifted over centuries of time . His observation of petrified bamboos in

8976-499: The Vetulicola were later erected as a new phylum of primitive deuterostomes by D.G. Shu et al. (Shu 2001). Another researcher places them with the urochordates, based on putative affinity with the phylum Chordata. They are thought to have been swimmers that either were filter feeders or detritivores. Some two dozen animals from the Chengjiang biota are problematic regarding phylogenetic assignment. Among these, ' Anomalocaris ' saron ,

9108-402: The law of superposition ) preserved different assemblages of fossils, and that these assemblages succeeded one another in a regular and determinable order. He observed that rocks from distant locations could be correlated based on the fossils they contained. He termed this the principle of faunal succession . This principle became one of Darwin's chief pieces of evidence that biological evolution

9240-461: The mineral content of a rock or other sample. Gemologists use it as an aid in the identification of gemstones . Water is preferred as the reference because measurements are then easy to carry out in the field (see below for examples of measurement methods). As the principal use of relative density measurements in industry is determination of the concentrations of substances in aqueous solutions and these are found in tables of RD vs concentration it

9372-503: The phylogeny of groups and even the erection of the new phylum Vetulicolia of primitive deuterostomes . The Chengjiang biota has all the animal groups found in the Burgess Shale; however, since it is ten million years older, it more strongly supports the deduction that metazoans diversified earlier or faster in the early Cambrian than does the Burgess Shale fauna alone. The preservation of an extremely diverse faunal assemblage renders

9504-403: The thunderbird . There is no such direct mythological connection known from prehistoric Africa, but there is considerable evidence of tribes there excavating and moving fossils to ceremonial sites, apparently treating them with some reverence. In Japan, fossil shark teeth were associated with the mythical tengu , thought to be the razor-sharp claws of the creature, documented some time after

9636-449: The 16th century. Roman naturalist Pliny the Elder wrote of " tongue stones ", which he called glossopetra . These were fossil shark teeth, thought by some classical cultures to look like the tongues of people or snakes. He also wrote about the horns of Ammon , which are fossil ammonites , whence the group of shelled octopus-cousins ultimately draws its modern name. Pliny also makes one of

9768-741: The 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils. The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host. There are many processes that lead to fossilization , including permineralization , casts and molds, authigenic mineralization , replacement and recrystallization, adpression, carbonization , and bioimmuration. Fossils vary in size from one- micrometre (1 μm) bacteria to dinosaurs and trees, many meters long and weighing many tons. A fossil normally preserves only

9900-413: The 8th century AD. In medieval China, the fossil bones of ancient mammals including Homo erectus were often mistaken for " dragon bones" and used as medicine and aphrodisiacs . In addition, some of these fossil bones are collected as "art" by scholars, who left scripts on various artifacts, indicating the time they were added to a collection. One good example is the famous scholar Huang Tingjian of

10032-403: The Chengjian biota, and share many species. The unique species include arthropods like Guangweicaris and Astutuscaris , vetulicolians like Vetulicola gantoucunensis and V. longbaoshanensis .; chordates like Cathaymyrus haikouensis and Zhongxiniscus intermedius . Fossil A fossil (from Classical Latin fossilis , lit.   ' obtained by digging ' )

10164-474: The Greek letter rho , denotes density.) The reference material can be indicated using subscripts: R D s u b s t a n c e / r e f e r e n c e {\displaystyle RD_{\mathrm {substance/reference} }} which means "the relative density of substance with respect to reference ". If the reference is not explicitly stated then it

10296-542: The Maotianshan shale the world's most important for understanding the evolution of early multi-cellular life, particularly the members of phylum Chordata , which includes all vertebrates . The Chengjiang fossils comprise the oldest diverse metazoan assemblage above the Proterozoic - Phanerozoic transition and, thus, the fossil record 's best data source for understanding the apparently rapid diversification of life known as

10428-597: The Preservation of Favoured Races in the Struggle for Life , the oldest animal fossils were those from the Cambrian Period, now known to be about 540 million years old. He worried about the absence of older fossils because of the implications on the validity of his theories, but he expressed hope that such fossils would be found, noting that: "only a small portion of the world is known with accuracy." Darwin also pondered

10560-555: The Proterozoic and deeper still in the Archean is only "recounted by microscopic fossils and subtle chemical signals." Molecular biologists, using phylogenetics , can compare protein amino acid or nucleotide sequence homology (i.e., similarity) to evaluate taxonomy and evolutionary distances among organisms, with limited statistical confidence. The study of fossils, on the other hand, can more specifically pinpoint when and in what organism

10692-508: The above formula: ρ s u b s t a n c e = S G × ρ H 2 O . {\displaystyle \rho _{\mathrm {substance} }=SG\times \rho _{\mathrm {H_{2}O} }.} Occasionally a reference substance other than water is specified (for example, air), in which case specific gravity means density relative to that reference. The density of substances varies with temperature and pressure so that it

10824-497: The alleged predatory terror of the early Cambrian, was the most famous, although that species is later reclassified to Houcaris saron and Innovatiocaris maotianshanensis . Shu (2006) recently described Stromatoveris psygmoglena as a possible bilateran missing link between Ediacaran fronds and Cambrian ctenophores . Cambrocornulitus had a tubicolous shell which probably was biomineralized. It shares some affinities with cornulitids and lophophorates. The Chengjiang biota

10956-413: The beach, indicating the fossils were once living animals. He had previously explained them in terms of vaporous exhalations , which Persian polymath Avicenna modified into the theory of petrifying fluids ( succus lapidificatus ). Recognition of fossil seashells as originating in the sea was built upon in the 14th century by Albert of Saxony , and accepted in some form by most naturalists by

11088-710: The beginning of recorded history. The fossils themselves are referred to as the fossil record. The fossil record was one of the early sources of data underlying the study of evolution and continues to be relevant to the history of life on Earth . Paleontologists examine the fossil record to understand the process of evolution and the way particular species have evolved. Fossils have been visible and common throughout most of natural history, and so documented human interaction with them goes back as far as recorded history, or earlier. There are many examples of paleolithic stone knives in Europe, with fossil echinoderms set precisely at

11220-405: The change in displacement. (In practice the stalk of the hydrometer is pre-marked with graduations to facilitate this measurement.) In the explanation that follows, Since the floating hydrometer is in static equilibrium , the downward gravitational force acting upon it must exactly balance the upward buoyancy force. The gravitational force acting on the hydrometer is simply its weight, mg . From

11352-411: The change in pressure caused by the sample under the same conditions. The difference in change of pressure represents the volume of the sample as compared to the reference sphere, and is usually used for solid particulates that may dissolve in the liquid medium of the pycnometer design described above, or for porous materials into which the liquid would not fully penetrate. When a pycnometer is filled to

11484-660: The deity Sopdu , the Morning Star, equivalent of Venus in Roman mythology. Fossils appear to have directly contributed to the mythology of many civilizations, including the ancient Greeks. Classical Greek historian Herodotos wrote of an area near Hyperborea where gryphons protected golden treasure. There was indeed gold mining in that approximate region , where beaked Protoceratops skulls were common as fossils. A later Greek scholar, Aristotle , eventually realized that fossil seashells from rocks were similar to those found on

11616-487: The densities used here and in the rest of this article are based on that scale. On the previous IPTS-68 scale, the densities at 20 °C and 4 °C are 0.998 2041 and 0.999 9720 respectively, resulting in an SG (20 °C/4 °C) value for water of 0.998 232 . As the principal use of specific gravity measurements in industry is determination of the concentrations of substances in aqueous solutions and as these are found in tables of SG versus concentration, it

11748-406: The densities used here and in the rest of this article are based on that scale. On the previous IPTS-68 scale the densities at 20 °C and 4 °C are, respectively, 0.9982041 and 0.9999720 resulting in an RD (20 °C/4 °C) value for water of 0.99823205. The temperatures of the two materials may be explicitly stated in the density symbols; for example: where the superscript indicates

11880-585: The density directly. Temperatures for both sample and reference vary from industry to industry. In British brewing practice, the specific gravity, as specified above, is multiplied by 1000. Specific gravity is commonly used in industry as a simple means of obtaining information about the concentration of solutions of various materials such as brines , must weight ( syrups , juices, honeys, brewers wort , must , etc.) and acids. Relative density ( R D {\displaystyle RD} ) or specific gravity ( S G {\displaystyle SG} )

12012-410: The density of dry air at 101.325 kPa at 20 °C is 0.001205 g/cm and that of water is 0.998203 g/cm we see that the difference between true and apparent relative densities for a substance with relative density (20 °C/20 °C) of about 1.100 would be 0.000120. Where the relative density of the sample is close to that of water (for example dilute ethanol solutions) the correction

12144-407: The density of the sample was determined at 20 °C and of the water at 4 °C. Taking into account different sample and reference temperatures, while SG H 2 O = 1.000 000 (20 °C/20 °C), it is also the case that SG H 2 O = 0.998 2008 ⁄ 0.999 9720 = 0.998 2288 (20 °C/4 °C). Here, temperature is being specified using the current ITS-90 scale and

12276-399: The density of the sample was determined at 20 °C and of the water at 4 °C. Taking into account different sample and reference temperatures, while SG H 2 O = 1.000000 (20 °C/20 °C) it is also the case that RD H 2 O = ⁠ 0.9982008 / 0.9999720 ⁠ = 0.9982288 (20 °C/4 °C). Here temperature is being specified using the current ITS-90 scale and

12408-466: The dry northern climate zone of what is now Yan'an , Shaanxi province, China, led him to advance early ideas of gradual climate change due to bamboo naturally growing in wetter climate areas. In medieval Christendom , fossilized sea creatures on mountainsides were seen as proof of the biblical deluge of Noah's Ark . After observing the existence of seashells in mountains, the ancient Greek philosopher Xenophanes (c. 570 – 478 BC) speculated that

12540-415: The earlier known references to toadstones , thought until the 18th century to be a magical cure for poison originating in the heads of toads, but which are fossil teeth from Lepidotes , a Cretaceous ray-finned fish. The Plains tribes of North America are thought to have similarly associated fossils, such as the many intact pterosaur fossils naturally exposed in the region, with their own mythology of

12672-517: The earliest known stromatolites are over 3.4 billion years old. The fossil record is life's evolutionary epic that unfolded over four billion years as environmental conditions and genetic potential interacted in accordance with natural selection. Paleontology has joined with evolutionary biology to share the interdisciplinary task of outlining the tree of life, which inevitably leads backwards in time to Precambrian microscopic life when cell structure and functions evolved. Earth's deep time in

12804-525: The early Cambrian', the International Union of Geological Sciences (IUGS) included the 'Cambrian Chengjiang fossil site and lagerstätte' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an 'IUGS Geological Heritage Site' as 'a key place with geological elements and/or processes of international scientific relevance, used as

12936-555: The early part of the 10th century, Chengjiang was first recognized for its exquisite states of preservation with the 1984 discovery of the naraoiid Misszhouia , a soft-bodied relative of trilobites . Since then, the locality has been intensively studied by scientists throughout the world, yielding a constant flow of new discoveries and triggering an extensive scientific debate surrounding the interpretation of discoveries. Over this time, taxa have been revised or reassigned to different groups. Interpretations have led to many refinements of

13068-460: The earth during earthquake and subsidences, and petrifies whatever comes into contact with it. As a matter of fact, the petrifaction of the bodies of plants and animals is not more extraordinary than the transformation of waters. From the 13th century to the present day, scholars pointed out that the fossil skulls of Deinotherium giganteum , found in Crete and Greece, might have been interpreted as being

13200-478: The existence of a world previous to ours, destroyed by some kind of catastrophe. Interest in fossils, and geology more generally, expanded during the early nineteenth century. In Britain, Mary Anning 's discoveries of fossils, including the first complete ichthyosaur and a complete plesiosaurus skeleton, sparked both public and scholarly interest. Early naturalists well understood the similarities and differences of living species leading Linnaeus to develop

13332-412: The flask is weighed empty, full of water, and full of a liquid whose relative density is desired, the relative density of the liquid can easily be calculated. The particle density of a powder, to which the usual method of weighing cannot be applied, can also be determined with a pycnometer. The powder is added to the pycnometer, which is then weighed, giving the weight of the powder sample. The pycnometer

13464-484: The fossil record. Phylum Porifera (sponges; 15 species) and Priapulida (16 species) are also well represented. Other phyla represented are Brachiopoda , Chaetognatha , Cnidaria , Ctenophora , Echinodermata , Hyolitha , Nematomorpha , Phoronida , and Chordata . Possible molluscs include Wiwaxia and Nectocaris . About one in eight animals are problematic forms of uncertain affinity, some of which may have been evolutionary experiments that survived for only

13596-519: The hand grip, dating back to Homo heidelbergensis and Neanderthals . These ancient peoples also drilled holes through the center of those round fossil shells, apparently using them as beads for necklaces. The ancient Egyptians gathered fossils of species that resembled the bones of modern species they worshipped. The god Set was associated with the hippopotamus , therefore fossilized bones of hippo-like species were kept in that deity's temples. Five-rayed fossil sea urchin shells were associated with

13728-535: The limits of fossilization. Fossils of two enigmatic bilaterians, the worm-like Markuelia and a putative, primitive protostome , Pseudooides , provide a peek at germ layer embryonic development. These 543-million-year-old embryos support the emergence of some aspects of arthropod development earlier than previously thought in the late Proterozoic. The preserved embryos from China and Siberia underwent rapid diagenetic phosphatization resulting in exquisite preservation, including cell structures. This research

13860-428: The literature as of June 2006. Of these, nearly half are arthropods, few of which had the hard, mineral-reinforced exoskeletons characteristic of all later arthropoda; only about 3% of the organisms known from Chengjiang have hard shells. Most of those are the trilobites (of which there are five species), all of which have been found with traces of legs, antennae, and other soft body parts, an exceedingly rare occurrence in

13992-407: The most abundant species, followed by trilobites. Other species belong to sponges, chancelloriids, cnidarians, ctenophores, priapulids, lobopodians, arthropods, anomalocaridids, hyoliths, molluscs, brachiopods, echinoderms, algae and vetulicolians. There are also the earliest-known eocrinoids, unidentified soft-bodied animals and abundant trace fossils. The Guanshan biota are regarded as successors of

14124-474: The organism must become covered by sediment soon after death, otherwise the remains are destroyed by scavengers or decomposition. The degree to which the remains are decayed when covered determines the later details of the fossil. Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin , feathers or even soft tissues. This is a form of diagenesis . Specific gravity Relative density , also called specific gravity ,

14256-482: The phylum to which all vertebrates belong. The most famous is Myllokunmingia , possibly a very primitive agnathid (i.e., jawless fish ). Similar to Myllokunmingia is Haikouichthys ercaicunensis , another primitive fish-like animal. The enigmatic Yunnanozoon lividum is considered to be the earliest hemichordate , possessing many of the characteristic chordate features and providing an anatomical link between invertebrates and chordates. Haikouella lanceolata

14388-545: The problems involved in matching rocks of the same age across continents . Family-tree relationships also help to narrow down the date when lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved earlier. It is also possible to estimate how long ago two living clades diverged, in other words approximately how long ago their last common ancestor must have lived, by assuming that DNA mutations accumulate at

14520-438: The pycnometer. Further manipulation and finally substitution of RD V , the true relative density (the subscript V is used because this is often referred to as the relative density in vacuo ), for ρ s / ρ w gives the relationship between apparent and true relative density: R D A = ρ s ρ w − ρ

14652-443: The ratio of the radioactive element to its decay products shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash layers, which may provide termini for the intervening sediments. Consequently, palaeontologists rely on stratigraphy to date fossils. Stratigraphy

14784-766: The reference liquid, and the known properties of the hydrometer. If Δ x is small then, as a first-order approximation of the geometric series equation ( 4 ) can be written as: R D n e w / r e f ≈ 1 + A Δ x m ρ r e f . {\displaystyle RD_{\mathrm {new/ref} }\approx 1+{\frac {A\Delta x}{m}}\rho _{\mathrm {ref} }.} This shows that, for small Δ x , changes in displacement are approximately proportional to changes in relative density. A pycnometer (from Ancient Greek : πυκνός , romanized :  puknos , lit.   'dense'), also called pyknometer or specific gravity bottle ,

14916-462: The relative ages of rock strata as determined by the early paleontologists and stratigraphers . Since the early years of the twentieth century, absolute dating methods, such as radiometric dating (including potassium/argon , argon/argon , uranium series , and, for very recent fossils, radiocarbon dating ) have been used to verify the relative ages obtained by fossils and to provide absolute ages for many fossils. Radiometric dating has shown that

15048-417: The same equation applies when the hydrometer is floating in the liquid being measured, except that the new volume is V − A Δ x (see note above about the sign of Δ x ). Thus, Combining ( 1 ) and ( 2 ) yields But from ( 1 ) we have V = m / ρ ref . Substituting into ( 3 ) gives This equation allows the relative density to be calculated from the change in displacement, the known density of

15180-500: The sample measured in air and W A , H 2 O {\displaystyle {W_{\mathrm {A} ,\mathrm {H_{2}O} }}} the weight of an equal volume of water measured in air. It can be shown that true specific gravity can be computed from different properties: S G t r u e = ρ s a m p l e ρ H 2 O = m s

15312-405: The sample will then float. W water becomes a negative quantity, representing the force needed to keep the sample underwater. Another practical method uses three measurements. The sample is weighed dry. Then a container filled to the brim with water is weighed, and weighed again with the sample immersed, after the displaced water has overflowed and been removed. Subtracting the last reading from

15444-429: The sea and that they were still living when the strait of Gibraltar was cut through. In the mountains of Parma and Piacenza multitudes of shells and corals with holes may be seen still sticking to the rocks.... In 1666, Nicholas Steno examined a shark, and made the association of its teeth with the "tongue stones" of ancient Greco-Roman mythology, concluding that those were not in fact the tongues of venomous snakes, but

15576-423: The sea we see the oysters all together and also the shellfish and the cuttlefish and all the other shells which congregate together, found all together dead; and the solitary shells are found apart from one another as we see them every day on the sea-shores. And we find oysters together in very large families, among which some may be seen with their shells still joined together, indicating that they were left there by

15708-456: The shell, bone, or other tissue is replaced with another mineral. In some cases mineral replacement of the original shell occurs so gradually and at such fine scales that microstructural features are preserved despite the total loss of original material. Scientists can use such fossils when researching the anatomical structure of ancient species. Several species of saurids have been identified from mineralized dinosaur fossils. Permineralization

15840-581: The skulls of the Cyclopes of Greek mythology , and are possibly the origin of that Greek myth. Their skulls appear to have a single eye-hole in the front, just like their modern elephant cousins, though in fact it's actually the opening for their trunk. In Norse mythology , echinoderm shells (the round five-part button left over from a sea urchin) were associated with the god Thor , not only being incorporated in thunderstones , representations of Thor's hammer and subsequent hammer-shaped crosses as Christianity

15972-608: The slightly younger Guanshan biota from Malong District in Yunnan, Kaili biota and Balang fauna in Guizhou, Shipai fauna in Hubei, and sponge faunas of Guizhou and Anhui. Along with the Burgess Shale, the Maotianshan Shales are remarked as "our best window into the Cambrian 'explosion' ", especially on the origin of chordates . Although fossils from the region have been known from

16104-1256: The spring constant, gravity and cross-sectional area simply cancel, leaving R D = ρ o b j e c t ρ r e f = Deflection O b j . Displacement O b j . Deflection R e f . Displacement R e f . = 3   i n 20   m m 5   i n 34   m m = 3   i n × 34   m m 5   i n × 20   m m = 1.02. {\displaystyle RD={\frac {\rho _{\mathrm {object} }}{\rho _{\mathrm {ref} }}}={\frac {\frac {{\text{Deflection}}_{\mathrm {Obj.} }}{{\text{Displacement}}_{\mathrm {Obj.} }}}{\frac {{\text{Deflection}}_{\mathrm {Ref.} }}{{\text{Displacement}}_{\mathrm {Ref.} }}}}={\frac {\frac {3\ \mathrm {in} }{20\ \mathrm {mm} }}{\frac {5\ \mathrm {in} }{34\ \mathrm {mm} }}}={\frac {3\ \mathrm {in} \times 34\ \mathrm {mm} }{5\ \mathrm {in} \times 20\ \mathrm {mm} }}=1.02.} Relative density

16236-909: The strata are consistent with a tropical environment with sea level changes and tectonic activity. The region is believed to have been a shallow sea with a muddy bottom. The preserved fauna is primarily benthic and was likely buried by periodic turbidity currents , since most fossils do not show evidence of post-mortem transport. Like the younger Burgess Shale fossils, the paleo-environment enabled preservation of non-mineralized, soft body parts. Fossils are found in thin layers less than an inch thick. The soft parts are preserved as aluminosilicate films, often with high oxidized iron content and often exhibiting exquisite details. The Chengjiang beds are very deeply weathered, as evidenced by their low specific gravity (i.e., they are very lightweight). Trace fossils are abundant. The Chengjiang biota comprises an extremely diverse faunal assembly, with some 185 species described in

16368-467: The sudden appearance of many groups (i.e. phyla ) in the oldest known Cambrian fossiliferous strata. Since Darwin's time, the fossil record has been extended to between 2.3 and 3.5 billion years. Most of these Precambrian fossils are microscopic bacteria or microfossils . However, macroscopic fossils are now known from the late Proterozoic. The Ediacara biota (also called Vendian biota) dating from 575 million years ago collectively constitutes

16500-481: The sugar, soft drink, honey, fruit juice and related industries, sucrose concentration by weight is taken from a table prepared by A. Brix , which uses SG (17.5 °C/17.5 °C). As a final example, the British SG units are based on reference and sample temperatures of 60 °F and are thus (15.56 °C/15.56 °C). Given the specific gravity of a substance, its actual density can be calculated by rearranging

16632-418: The sum of the first two readings gives the weight of the displaced water. The relative density result is the dry sample weight divided by that of the displaced water. This method allows the use of scales which cannot handle a suspended sample. A sample less dense than water can also be handled, but it has to be held down, and the error introduced by the fixing material must be considered. The relative density of

16764-451: The teeth of some long-extinct species of shark. Robert Hooke (1635–1703) included micrographs of fossils in his Micrographia and was among the first to observe fossil forams . His observations on fossils, which he stated to be the petrified remains of creatures some of which no longer existed, were published posthumously in 1705. William Smith (1769–1839) , an English canal engineer, observed that rocks of different ages (based on

16896-426: The temperature at which the density of the material is measured, and the subscript indicates the temperature of the reference substance to which it is compared. Relative density can also help to quantify the buoyancy of a substance in a fluid or gas, or determine the density of an unknown substance from the known density of another. Relative density is often used by geologists and mineralogists to help determine

17028-453: The usual case we will have measured weights and want the true relative density. This is found from R D V = R D A − ρ a ρ w ( R D A − 1 ) . {\displaystyle RD_{\mathrm {V} }=RD_{\mathrm {A} }-{\rho _{\mathrm {a} } \over \rho _{\mathrm {w} }}(RD_{\mathrm {A} }-1).} Since

17160-511: The world was once inundated in a great flood that buried living creatures in drying mud. In 1027, the Persian Avicenna explained fossils' stoniness in The Book of Healing : If what is said concerning the petrifaction of animals and plants is true, the cause of this (phenomenon) is a powerful mineralizing and petrifying virtue which arises in certain stony spots, or emanates suddenly from

17292-458: Was adopted, but also kept in houses to garner Thor's protection. These grew into the shepherd's crowns of English folklore, used for decoration and as good luck charms, placed by the doorway of homes and churches. In Suffolk , a different species was used as a good-luck charm by bakers, who referred to them as fairy loaves , associating them with the similarly shaped loaves of bread they baked. More scientific views of fossils emerged during

17424-408: Was real. Georges Cuvier came to believe that most if not all the animal fossils he examined were remains of extinct species. This led Cuvier to become an active proponent of the geological school of thought called catastrophism . Near the end of his 1796 paper on living and fossil elephants he said: All of these facts, consistent among themselves, and not opposed by any report, seem to me to prove

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