Scavengers are animals that consume dead organisms that have died from causes other than predation. While scavenging refers to carnivores feeding on carrion, it is a herbivorous feeding behavior. Scavengers play an important role in the ecosystem by consuming dead plant material. Decomposers and detritivores complete this process, by consuming the remains left by scavengers. Scavengers aid in overcoming fluctuations of food resources in the environment; the process and rate of scavenging is affected by both biotic and abiotic factors, such as carcass size, habitat and seasons. Scavenger is an alteration of scavager, from Middle English skawager meaning "customs collector", from skawage meaning "customs", from Old North French escauwage meaning "inspection", from schauwer meaning "to inspect", of Germanic origin. Obligate scavenging is rare among vertebrates, due to the difficulty of finding enough carrion without expending too much energy. In vertebrates, only vultures and some pterosaurs are obligate scavengers, as terrestrial soaring flyers are the only animals able to find enough carrion.
Well-known invertebrate scavengers of animal material include burying beetles and blowflies, which are obligate scavengers, yellowjackets. Most scavenging animals are facultative scavengers that gain most of their food through other methods predation. Many large carnivores that hunt such as hyenas and jackals, but animals thought of as scavengers, such as African lions and wolves will scavenge if given the chance, they may use their size and ferocity to intimidate the original hunters. All scavengers above insect size are predators and will hunt if not enough carrion is available, as few ecosystems provide enough dead animals year-round to keep its scavengers fed on that alone. Scavenging wild dogs and crows exploit roadkill. Scavengers of dead plant material include termites that build nests in grasslands and collect dead plant material for consumption within the nest; the interaction between scavenging animals and humans is seen today most in suburban settings with animals such as opossums and raccoons.
In some African towns and villages, scavenging from hyenas is common. In the prehistoric eras, the species Tyrannosaurus rex may have been an apex predator, preying upon hadrosaurs and juvenile sauropods, although some experts have suggested the dinosaur was a scavenger; the debate about whether Tyrannosaurus was an apex predator or scavenger was among the longest ongoing feuds in paleontology. Recent research shows that while an adult Tyrannosaurus rex would energetically gain little though scavenging, smaller theropods of 500 kg may have gained levels similar to that of hyenas, though not enough for them to rely on scavenging. There are an info that Otodus megalodon, Ceratosaurus and some more prehistoric animals were scavengers. Animals which consume feces, such as dung beetles, are referred to as coprovores. Animals that collect small particles of dead organic material of both animal and plant origin are referred to as detritivores. Scavengers play a fundamental role in the environment through the removal of decaying organisms, serving as a natural sanitation service.
While microscopic and invertebrate decomposers break down dead organisms into simple organic matter which are used by nearby autotrophs, scavengers help conserve energy and nutrients obtained from carrion within the upper trophic levels, are able to disperse the energy and nutrients farther away from the site of the carrion than decomposers. Scavenging unites animals which would not come into contact, results in the formation of structured and complex communities which engage in nonrandom interactions. Scavenging communities function in the redistribution of energy obtained from carcasses and reducing diseases associated with decomposition. Oftentimes, scavenger communities differ in consistency due to carcass size and carcass types, as well as by seasonal effects as consequence of differing invertebrate and microbial activity. Competition for carrion results in the inclusion or exclusion of certain scavengers from access to carrion, shaping the scavenger community; when carrion decomposes at a slower rate during cooler seasons, competitions between scavengers decrease, while the number of scavenger species present increases.
Alterations in scavenging communities may result in drastic changes to the scavenging community in general, reduce ecosystem services and have detrimental effects on animal and humans. The reintroduction of gray wolves into Yellowstone National Park in the United States caused drastic changes to the prevalent scavenging community, resulting in the provision of carrion to many mammalian and avian species; the reduction of vulture species in India lead to the increase of opportunistic species such as feral dogs and rats. The presence of both species at carcasses resulted in the increase of diseases such as rabies and bubonic plague in wildlife and livestock, as feral dogs and rats are transmitters of such diseases. Furthermore, the decline of vulture populations in India has been linked to the increased rates of anthrax in humans due to the handling and ingestion of infected livestock carcasses. An increase of disease transmission has been observed in mammalian scavengers in Kenya due to the decrease in vulture populations in the ar
Trilobites are a group of extinct marine arachnomorph arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods; the first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period, they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders except the Proetids died out. Trilobites disappeared in the mass extinction at the end of the Permian about 252 million years ago; the trilobites were among the most successful of all early animals, existing in oceans for over 300 million years. By the time trilobites first appeared in the fossil record, they were highly diversified and geographically dispersed; because trilobites had wide diversity and an fossilized exoskeleton, they left an extensive fossil record, with some 50,000 known species spanning Paleozoic time. The study of these fossils has facilitated important contributions to biostratigraphy, evolutionary biology, plate tectonics.
Trilobites are placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha, although several alternative taxonomies are found in the literature. Trilobites had many lifestyles. Most lifestyles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism; some trilobites are thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food. The earliest trilobites known from the fossil record are redlichiids and ptychopariid bigotinids dated to some 540 to 520 million years ago. Contenders for the earliest trilobites include Fritzaspis spp.. Hupetina antiqua and Serrania gordaensis. All trilobites are thought to have originated in present-day Siberia, with subsequent distribution and radiation from this location. All Olenellina lack facial sutures, this is thought to represent the original state; the earliest sutured trilobite found so far, occurs at the same time as the earliest Olenellina, suggesting the trilobites origin lies before the start of the Atdabanian, but without leaving fossils.
Other groups show secondary lost facial sutures, such as all some Phacopina. Another common feature of the Olenellina suggests this suborder to be the ancestral trilobite stock: early protaspid stages have not been found because these were not calcified, this is supposed to represent the original state. Earlier trilobites could shed more light on the origin of trilobites. Three specimens of a trilobite from Morocco, Megistaspis hammondi, dated 478 million years old contain fossilized soft parts. Early trilobites show all the features of the trilobite group as a whole. Morphological similarities between trilobites and early arthropod-like creatures such as Spriggina and other "trilobitomorphs" of the Ediacaran period of the Precambrian are ambiguous enough to make a detailed analysis of their ancestry complex. Morphological similarities between early trilobites and other Cambrian arthropods make analysis of ancestral relationships difficult as well; that trilobites share a common ancestor with other arthropods before the Ediacaran-Cambrian boundary is still reasonable to assume.
Evidence suggests that significant diversification had occurred before trilobites were preserved in the fossil record, allowing for the "sudden" appearance of diverse trilobite groups with complex derived characteristics. For such a long-lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where some groups perished and surviving groups diversified to fill ecological niches with comparable or unique adaptations. Trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn-out decline in the Devonian, culminating in the final extinction of the last few survivors at the end of the Permian period. Principal evolutionary trends from primitive morphologies, such as exemplified by Eoredlichia, include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium, development of extreme spinosity in certain groups. Changes included narrowing of the thorax and increasing or decreasing numbers of thoracic segments.
Specific changes to the cephalon are noted. Several morphologies appeared independently within different major taxa. Effacement, the loss of surface detail in the cephalon, pygidium, or the thoracic furrows, is a common evolutionary trend. Notable examples of this were the orders Agnostida and Asaphida, the suborder Illaenina of the Corynexochida. Effacement is believed to be an indication of either a pelagic one. Effacement poses a problem for taxonomists since the loss of details can make the determination of phylogenetic relationships difficult. Phylogenetic biogeographic analysis of Early Cambrian Olenellidae and Redlichiidae
The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At 508 million years old, it is one of the earliest fossil beds containing soft-part imprints; the rock unit is a black shale and crops out at a number of localities near the town of Field in Yoho National Park and the Kicking Horse Pass. Another outcrop is in Kootenay National Park 42 km to the south; the Burgess Shale was discovered by palaeontologist Charles Walcott on 30 August 1909, towards the end of the season's fieldwork. He returned in 1910 with his sons and wife, establishing a quarry on the flanks of Fossil Ridge; the significance of soft-bodied preservation, the range of organisms he recognised as new to science, led him to return to the quarry every year until 1924. At that point, aged 74, he had amassed over 65,000 specimens. Describing the fossils was a vast task, pursued by Walcott until his death in 1927.
Walcott, led by scientific opinion at the time, attempted to categorise all fossils into living taxa, as a result, the fossils were regarded as little more than curiosities at the time. It was not until 1962 that a first-hand reinvestigation of the fossils was attempted, by Alberto Simonetta; this led scientists to recognise that Walcott had scratched the surface of information available in the Burgess Shale, made it clear that the organisms did not fit comfortably into modern groups. Excavations were resumed at the Walcott Quarry by the Geological Survey of Canada under the persuasion of trilobite expert Harry Blackmore Whittington, a new quarry, the Raymond, was established about 20 metres higher up Fossil Ridge. Whittington, with the help of research students Derek Briggs and Simon Conway Morris of the University of Cambridge, began a thorough reassessment of the Burgess Shale, revealed that the fauna represented were much more diverse and unusual than Walcott had recognized. Indeed, many of the animals present had bizarre anatomical features and only the slightest resemblance to other known animals.
Examples include Opabinia, with five eyes and a snout like a vacuum cleaner hose and Hallucigenia, reconstructed upside down, walking on bilaterally symmetrical spines. With Parks Canada and UNESCO recognising the significance of the Burgess Shale, collecting fossils became politically more difficult from the mid-1970s. Collections continued to be made by the Royal Ontario Museum; the curator of invertebrate palaeontology, Desmond Collins, identified a number of additional outcrops, stratigraphically both higher and lower than the original Walcott quarry. These localities continue to yield new organisms faster. Stephen Jay Gould's book Wonderful Life, published in 1989, brought the Burgess Shale fossils to the public's attention. Gould suggests that the extraordinary diversity of the fossils indicates that life forms at the time were much more disparate in body form than those that survive today, that many of the unique lineages were evolutionary experiments that became extinct. Gould's interpretation of the diversity of Cambrian fauna relied on Simon Conway Morris's reinterpretation of Charles Walcott's original publications.
However, Conway Morris disagreed with Gould's conclusions, arguing that all the Cambrian fauna could be classified into modern day phyla. The Burgess Shale has attracted the interest of paleoclimatologists who want to study and predict long-term future changes in Earth's climate. According to Peter Ward and Donald Brownlee in the 2003 book The Life and Death of Planet Earth, climatologists study the fossil records in the Burgess Shale to understand the climate of the Cambrian explosion, use it to predict what Earth's climate would look like 500 million years in the future when a warming and expanding Sun combined with declining CO2 and oxygen levels heat the Earth toward temperatures not seen since the Archean Eon 3 billion years ago, before the first plants and animals appeared, therefore understand how and when the last living things will die out. See Future of the Earth. After the Burgess Shale site was registered as a World Heritage Site in 1980, it was included in the Canadian Rocky Mountain Parks WHS designation in 1984.
In February 2014, the discovery was announced of another Burgess Shale outcrop in Kootenay National Park to the south. In just 15 days of field collecting in 2013, 50 animal species were unearthed at the new site; the fossil-bearing deposits of the Burgess Shale correlate to the Stephen Formation, a collection of calcareous dark mudstones, about 508 million years old. The beds were deposited at the base of a cliff about 160 m tall, below the depth agitated by waves during storms; this vertical cliff was composed of the calcareous reefs of the Cathedral Formation, which formed shortly before the deposition of the Burgess Shale. The precise formation mechanism is not known for certain, but the most accepted hypothesis suggests that the edge of the Cathedral Formation reef became detached from the rest of the reef and being transported some distance — kilometers — away from the reef edge. Reactivation of faults at the base of the formation led to its disintegration from about 509 million years ago.
This would have left a steep cliff, the bottom of which would be protected from tectonic decompression because the limestone of the Cathedral Formation is difficult to compress. This protection explains why fossils preserved further from the Cathedral Formation are impossible to work with — tectonic squeezing of the beds has produced a vertical cleavage that fractures the rocks, so they split perpendicular to the fossils; the Walcott quarry
The Cambrian Period was the first geological period of the Paleozoic Era, of the Phanerozoic Eon. The Cambrian lasted 55.6 million years from the end of the preceding Ediacaran Period 541 million years ago to the beginning of the Ordovician Period 485.4 mya. Its subdivisions, its base, are somewhat in flux; the period was established by Adam Sedgwick, who named it after Cambria, the Latin name of Wales, where Britain's Cambrian rocks are best exposed. The Cambrian is unique in its unusually high proportion of lagerstätte sedimentary deposits, sites of exceptional preservation where "soft" parts of organisms are preserved as well as their more resistant shells; as a result, our understanding of the Cambrian biology surpasses that of some periods. The Cambrian marked a profound change in life on Earth. Complex, multicellular organisms became more common in the millions of years preceding the Cambrian, but it was not until this period that mineralized—hence fossilized—organisms became common; the rapid diversification of life forms in the Cambrian, known as the Cambrian explosion, produced the first representatives of all modern animal phyla.
Phylogenetic analysis has supported the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates. Although diverse life forms prospered in the oceans, the land is thought to have been comparatively barren—with nothing more complex than a microbial soil crust and a few molluscs that emerged to browse on the microbial biofilm. Most of the continents were dry and rocky due to a lack of vegetation. Shallow seas flanked the margins of several continents created during the breakup of the supercontinent Pannotia; the seas were warm, polar ice was absent for much of the period. Despite the long recognition of its distinction from younger Ordovician rocks and older Precambrian rocks, it was not until 1994 that the Cambrian system/period was internationally ratified; the base of the Cambrian lies atop a complex assemblage of trace fossils known as the Treptichnus pedum assemblage. The use of Treptichnus pedum, a reference ichnofossil to mark the lower boundary of the Cambrian, is difficult since the occurrence of similar trace fossils belonging to the Treptichnids group are found well below the T. pedum in Namibia and Newfoundland, in the western USA.
The stratigraphic range of T. pedum overlaps the range of the Ediacaran fossils in Namibia, in Spain. The Cambrian Period was followed by the Ordovician Period; the Cambrian is divided into ten ages. Only three series and six stages are named and have a GSSP; because the international stratigraphic subdivision is not yet complete, many local subdivisions are still used. In some of these subdivisions the Cambrian is divided into three series with locally differing names – the Early Cambrian, Middle Cambrian and Furongian. Rocks of these epochs are referred to as belonging to Upper Cambrian. Trilobite zones allow biostratigraphic correlation in the Cambrian; each of the local series is divided into several stages. The Cambrian is divided into several regional faunal stages of which the Russian-Kazakhian system is most used in international parlance: *Most Russian paleontologists define the lower boundary of the Cambrian at the base of the Tommotian Stage, characterized by diversification and global distribution of organisms with mineral skeletons and the appearance of the first Archaeocyath bioherms.
The International Commission on Stratigraphy list the Cambrian period as beginning at 541 million years ago and ending at 485.4 million years ago. The lower boundary of the Cambrian was held to represent the first appearance of complex life, represented by trilobites; the recognition of small shelly fossils before the first trilobites, Ediacara biota earlier, led to calls for a more defined base to the Cambrian period. After decades of careful consideration, a continuous sedimentary sequence at Fortune Head, Newfoundland was settled upon as a formal base of the Cambrian period, to be correlated worldwide by the earliest appearance of Treptichnus pedum. Discovery of this fossil a few metres below the GSSP led to the refinement of this statement, it is the T. pedum ichnofossil assemblage, now formally used to correlate the base of the Cambrian. This formal designation allowed radiometric dates to be obtained from samples across the globe that corresponded to the base of the Cambrian. Early dates of 570 million years ago gained favour, though the methods used to obtain this number are now considered to be unsuitable and inaccurate.
A more precise date using modern radiometric dating yield a date of 541 ± 0.3 million years ago. The ash horizon in Oman from which this date was recovered corresponds to a marked fall in the abundance of carbon-13 that correlates to equivalent excursions elsewhere in the world, to the disappearance of distinctive Ediacaran fossils. There are arguments that the dated horizon in Oman does not correspond to the Ediacaran-Cambrian boundary, but represents a facies change from marine to evaporite-dominated strata — which w
The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago, to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era; as with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when 60% of marine species were wiped out. A significant evolutionary milestone during the Silurian was the diversification of jawed fish and bony fish. Multi-cellular life began to appear on land in the form of small, bryophyte-like and vascular plants that grew beside lakes and coastlines, terrestrial arthropods are first found on land during the Silurian. However, terrestrial life would not diversify and affect the landscape until the Devonian; the Silurian system was first identified by British geologist Roderick Murchison, examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s.
He named the sequences for a Celtic tribe of Wales, the Silures, inspired by his friend Adam Sedgwick, who had named the period of his study the Cambrian, from the Latin name for Wales. This naming does not indicate any correlation between the occurrence of the Silurian rocks and the land inhabited by the Silures. In 1835 the two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales, the germ of the modern geological time scale; as it was first identified, the "Silurian" series when traced farther afield came to overlap Sedgwick's "Cambrian" sequence, provoking furious disagreements that ended the friendship. Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds. An early alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland; the French geologist Joachim Barrande, building on Murchison's work, used the term Silurian in a more comprehensive sense than was justified by subsequent knowledge.
He divided the Silurian rocks of Bohemia into eight stages. His interpretation was questioned in 1854 by Edward Forbes, the stages of Barrande, F, G and H, have since been shown to be Devonian. Despite these modifications in the original groupings of the strata, it is recognized that Barrande established Bohemia as a classic ground for the study of the earliest fossils; the Llandovery Epoch lasted from 443.8 ± 1.5 to 433.4 ± 2.8 mya, is subdivided into three stages: the Rhuddanian, lasting until 440.8 million years ago, the Aeronian, lasting to 438.5 million years ago, the Telychian. The epoch is named for the town of Llandovery in Wales; the Wenlock, which lasted from 433.4 ± 1.5 to 427.4 ± 2.8 mya, is subdivided into the Sheinwoodian and Homerian ages. It is named after Wenlock Edge in England. During the Wenlock, the oldest-known tracheophytes of the genus Cooksonia, appear; the complexity of later Gondwana plants like Baragwanathia, which resembled a modern clubmoss, indicates a much longer history for vascular plants, extending into the early Silurian or Ordovician.
The first terrestrial animals appear in the Wenlock, represented by air-breathing millipedes from Scotland. The Ludlow, lasting from 427.4 ± 1.5 to 423 ± 2.8 mya, comprises the Gorstian stage, lasting until 425.6 million years ago, the Ludfordian stage. It is named for the town of Ludlow in England; the Přídolí, lasting from 423 ± 1.5 to 419.2 ± 2.8 mya, is the final and shortest epoch of the Silurian. It is named after one locality at the Homolka a Přídolí nature reserve near the Prague suburb Slivenec in the Czech Republic. Přídolí is the old name of a cadastral field area. In North America a different suite of regional stages is sometimes used: Cayugan Lockportian Tonawandan Ontarian Alexandrian In Estonia the following suite of regional stages is used: Ohessaare stage Kaugatuma stage Kuressaare stage Paadla stage Rootsiküla stage Jaagarahu stage Jaani stage Adavere stage Raikküla stage Juuru stage With the supercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe.
The high sea levels of the Silurian and the flat land resulted in a number of island chains, thus a rich diversity of environmental settings. During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late-Ordovician glaciation; the southern continents remained united during this period. The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity; the continents of Avalonia and Laurentia drifted together near the equator, starting the formation of a second supercontinent known as Euramerica. When the proto-Europe coll
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as "shells". Examples of animals with exoskeletons include insects such as grasshoppers and cockroaches, crustaceans such as crabs and lobsters; the shells of certain sponges and the various groups of shelled molluscs, including those of snails, tusk shells and nautilus, are exoskeletons. Some animals, such as the tortoise, have both an exoskeleton. Exoskeletons contain rigid and resistant components that fulfill a set of functional roles in many animals including protection, sensing, support and acting as a barrier against desiccation in terrestrial organisms. Exoskeletons have a role in defense from pests and predators, in providing an attachment framework for musculature. Exoskeletons contain chitin. Ingrowths of the arthropod exoskeleton known as apodemes serve as attachment sites for muscles.
These structures are composed of chitin, are six times as strong and twice as stiff as vertebrate tendons. Similar to tendons, apodemes can stretch to store elastic energy for jumping, notably in locusts. Many different species produce exoskeletons. Bone, cartilage, or dentine turtles. Chitin forms the exoskeleton in arthropods including insects, arachnids such as spiders, crustaceans such as crabs and lobsters, in some fungi and bacteria. Calcium carbonates constitute the shells of molluscs and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria. One species of mollusc, the scaly-foot gastropod makes use of the iron sulfides greigite and pyrite; some organisms, such as some foraminifera, agglutinate exoskeletons by sticking grains of sand and shell to their exterior. Contrary to a common misconception, echinoderms do not possess an exoskeleton, as their test is always contained within a layer of living tissue. Exoskeletons have evolved independently many times.
Further, other lineages have produced tough outer coatings analogous to an exoskeleton, such as some mammals. This coating is constructed from bone in the armadillo, hair in the pangolin; the armor of reptiles like turtles and dinosaurs like Ankylosaurs is constructed of bone. Since exoskeletons are rigid, they present some limits to growth. Organisms with open shells can grow by adding new material to the aperture of their shell, as is the case in snails and other molluscans. A true exoskeleton, like that found in arthropods, must be shed. A new exoskeleton is produced beneath the old one; as the old one is shed, the new skeleton is pliable. The animal will pump itself up to expand the new shell to maximal size let it harden; when the shell has set, the empty space inside the new skeleton can be filled up. Failure to shed the exoskeleton once outgrown can result in the animal being suffocated within its own shell, will stop subadults from reaching maturity, thus preventing them from reproducing.
This is the mechanism such as Azadirachtin. Exoskeletons, as hard parts of organisms, are useful in assisting preservation of organisms, whose soft parts rot before they can be fossilized. Mineralized exoskeletons can be preserved "as is", as shell fragments, for example; the possession of an exoskeleton permits a couple of other routes to fossilization. For instance, the tough layer can resist compaction, allowing a mold of the organism to be formed underneath the skeleton, which may decay. Alternatively, exceptional preservation may result in chitin being mineralized, as in the Burgess Shale, or transformed to the resistant polymer keratin, which can resist decay and be recovered. However, our dependence on fossilized skeletons significantly limits our understanding of evolution. Only the parts of organisms that were mineralized are preserved, such as the shells of molluscs, it helps that exoskeletons contain "muscle scars", marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of much of an organism's internal parts from its exoskeleton alone.
The most significant limitation is that, although there are 30-plus phyla of living animals, two-thirds of these phyla have never been found as fossils, because most animal species are soft-bodied and decay before they can become fossilized. Mineralized skeletons first appear in the fossil record shortly before the base of the Cambrian period, 550 million years ago; the evolution of a mineralized exoskeleton is seen by some as a possible driving force of the Cambrian explosion of animal life, resulting in a diversification of predatory and defensive tactics. However, some Precambrian organisms produced tough outer shells while others, such as Cloudina, had a calcified exoskeleton; some Cloudina shells show evidence of predation, in the form of borings. On the whole, the fossil record only contains mineralised exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started out with a non-mineralised exoskeleton which they mineralised, this makes it difficult to comment on the early evolution of each lineage's exoskeleton.
It is known, that in a short course of time, just before the Cambrian period, exoskeletons
International Standard Serial Number
An International Standard Serial Number is an eight-digit serial number used to uniquely identify a serial publication, such as a magazine. The ISSN is helpful in distinguishing between serials with the same title. ISSN are used in ordering, interlibrary loans, other practices in connection with serial literature; the ISSN system was first drafted as an International Organization for Standardization international standard in 1971 and published as ISO 3297 in 1975. ISO subcommittee TC 46/SC 9 is responsible for maintaining the standard; when a serial with the same content is published in more than one media type, a different ISSN is assigned to each media type. For example, many serials are published both in electronic media; the ISSN system refers to these types as electronic ISSN, respectively. Conversely, as defined in ISO 3297:2007, every serial in the ISSN system is assigned a linking ISSN the same as the ISSN assigned to the serial in its first published medium, which links together all ISSNs assigned to the serial in every medium.
The format of the ISSN is an eight digit code, divided by a hyphen into two four-digit numbers. As an integer number, it can be represented by the first seven digits; the last code digit, which may be 0-9 or an X, is a check digit. Formally, the general form of the ISSN code can be expressed as follows: NNNN-NNNC where N is in the set, a digit character, C is in; the ISSN of the journal Hearing Research, for example, is 0378-5955, where the final 5 is the check digit, C=5. To calculate the check digit, the following algorithm may be used: Calculate the sum of the first seven digits of the ISSN multiplied by its position in the number, counting from the right—that is, 8, 7, 6, 5, 4, 3, 2, respectively: 0 ⋅ 8 + 3 ⋅ 7 + 7 ⋅ 6 + 8 ⋅ 5 + 5 ⋅ 4 + 9 ⋅ 3 + 5 ⋅ 2 = 0 + 21 + 42 + 40 + 20 + 27 + 10 = 160 The modulus 11 of this sum is calculated. For calculations, an upper case X in the check digit position indicates a check digit of 10. To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by its position in the number, counting from the right.
The modulus 11 of the sum must be 0. There is an online ISSN checker. ISSN codes are assigned by a network of ISSN National Centres located at national libraries and coordinated by the ISSN International Centre based in Paris; the International Centre is an intergovernmental organization created in 1974 through an agreement between UNESCO and the French government. The International Centre maintains a database of all ISSNs assigned worldwide, the ISDS Register otherwise known as the ISSN Register. At the end of 2016, the ISSN Register contained records for 1,943,572 items. ISSN and ISBN codes are similar in concept. An ISBN might be assigned for particular issues of a serial, in addition to the ISSN code for the serial as a whole. An ISSN, unlike the ISBN code, is an anonymous identifier associated with a serial title, containing no information as to the publisher or its location. For this reason a new ISSN is assigned to a serial each time it undergoes a major title change. Since the ISSN applies to an entire serial a new identifier, the Serial Item and Contribution Identifier, was built on top of it to allow references to specific volumes, articles, or other identifiable components.
Separate ISSNs are needed for serials in different media. Thus, the print and electronic media versions of a serial need separate ISSNs. A CD-ROM version and a web version of a serial require different ISSNs since two different media are involved. However, the same ISSN can be used for different file formats of the same online serial; this "media-oriented identification" of serials made sense in the 1970s. In the 1990s and onward, with personal computers, better screens, the Web, it makes sense to consider only content, independent of media; this "content-oriented identification" of serials was a repressed demand during a decade, but no ISSN update or initiative occurred. A natural extension for ISSN, the unique-identification of the articles in the serials, was the main demand application. An alternative serials' contents model arrived with the indecs Content Model and its application, the digital object identifier, as ISSN-independent initiative, consolidated in the 2000s. Only in 2007, ISSN-L was defined in the