Saltasauridae — a family of armored herbivorous sauropods from the Upper Cretaceous. They are known from fossils found in South America, North America, Europe, they are characterized by their vertebrae and feet, which are similar to those of Saltasaurus, the first of the group to be discovered and the source of the name. The last and largest of the group and only one found in North America, was thirty-four metres in length and the last sauropod to go extinct. Most of the saltasaurids were smaller, around fifteen metres in length, one, was only eight metres long. Like all sauropods, the saltasaurids were quadrupeds, their necks and tails were held parallel to the ground, their small heads had only tiny, peg-like teeth, they were herbivorous, digesting them in their enormous guts. Although large animals, they were smaller than other sauropods of their time, many possessed distinctive additional defenses in the form of scutes along their backs; as sauropods, the Saltasauridae are herbivorous saurischians with the characteristic body plan of a small head, long neck, four erect legs, a counterbalancing tail.
Most sauropods are from the clade Neosauropoda, further split into the narrow-toothed Diplodocoidea and the broad-toothed Macronaria. The Macronarians emerged in the Jurassic and a subclade, the Titanosauria, survived into the Cretaceous and spread across the continents; because of their diversity, wide distribution, the fragmentary or incomplete nature of most specimens, little is known about the titanosaurs beyond their size and tendency to have scutes. The saltasaurids, one of the several titanosaur families, are recognized by the convexities in certain caudal vertebrae and the markings on their coracoid bones. All saltasaurids have thirty-five or fewer caudal vertebrae, each of, convex on both sides of its centrum, the one closest to the tail is shorter than the others, their coracoid bones have rectangular margins on the anteroventral side, as well as a lip where they meet the infraglenoid. The Opisthocoelicaudiinae, a subfamily of the saltasaurids, are unique in that they lack phalanges in their forelimbs.
Although Saltasaurus is known to possess dorsal osteoderms, scutes have not been discovered in all saltasaurids, it is unclear when and where the evolution of osteoderms occurred in saltasaurids and titanosaurs in general. The first saltasaurid to be discovered was Alamosaurus, found by paleontologist Charles Gilmore in Utah in 1922; the next species would not be described until Opisthocoelicaudia was named by Magdalena Borsuk-Bialynicka from a postcranial material in Mongolia in 1977. In 1980, Jose Bonaparte and Jaime Powell discovered Saltasaurus in Argentina; this was the first sauropod to be discovered with armor and proved that sauropods had thrived in Cretaceous South America. Paul Sereno recognized a cladistic relationship between Opisthocoelicauda and Saltasaurus to create the family Saltasauridae; the group is defined by the characteristics shared by all with the two best-known members and Opisthocoelicaudia. Paleontologists J Wilson and P Upchurch defined the Saltasauridae in 2003 as the least inclusive clade containing Opisthocoelicaudia skarzynskii, Saltasaurus loricatus, their most recent common ancestor, all that species’ descendants.
This taxonomy is based on those of Curry Rogers & Wilson. Family Saltasauridae Unclear Subfamilies Dongyangosaurus sinensis? Jiangshanosaurus Petrobrasaurus puestohernandezi Trigonosaurus pricei Subfamily Opisthocoelicaudinae Alamosaurus sanjuanensis Borealosaurus wimani Opisthocoelicaudia skarzynskii Subfamily Saltasaurinae Bonatitan reigi Microcoelus patagonicus Neuquensaurus australis Neuquensaurus rubustus Rocasaurus muniozi Saltasaurus loricatus The family is further divided into two subfamilies. Wilson and Upchurch defined Saltasaurinae in 2003 as the least-inclusive clade containing Saltasaurus but not Opisthocoelicaudia; the same paleontologists defined Opisthocoelicaudiinae as the inverse: the least-inclusive clade containing Opisthocoelicaudia but not Saltasaurus. Some species, due to the incompleteness of their skeletons, cannot yet be placed in either subfamily. Many fragmentary saltasaurids have been discovered since 1980, placing members of the family in territories as dispersed as today’s Australia and France, in addition to their earlier-known residencies in North and South America.
Like the other titanosaurs, the saltasaurids where a widespread, successful group that colonized all continents in the Cretaceous. Like all titanosaurs, the saltasaurids possessed small, peg-like teeth that were not usable for chewing. Coproliths from an unidentified titanosaur found in India suggest a diet of conifers and early species of grasses. Unable to chew and lacking gastroliths, sauropods survived by retaining plant matter in their stomachs for long periods of time, fermenting it to extract as many resources as possible, their long necks allowed them to graze over a large area while standing. The osteoderms of Saltasaurus consisted of numerous, large bony plates embedded in the dorsal skin, each surrounded by a pattern of smaller plates; the large osteoderms contained some hollow spaces for blood vessels and spongy trabecular bone, while the small ones were solid. Patches of skin from unidentified Cretaceous titanosaurs have revealed similar scale patterns in embryos but no bone or mineralized structure, suggesting that, like crocodiles, those saltasaurids that possessed armor only developed it some time after hatching.
Analysis of th
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago to the start of the Silurian Period 443.8 Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, placed them in a system of their own; the Ordovician received international approval in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events.
Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today; the Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya. It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary from those found in other sources; this second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is significant, it correlates well with the occurrence of widespread graptolite and trilobite species.
The base of the Tremadocian allows scientists to relate these species not only to each other, but to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions. There exist Baltoscandic, Siberian, North American, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes; the Ordovician Period in Britain was traditionally broken into Early and Late epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column; the faunal stages from youngest to oldest are: Late Ordovician Hirnantian/Gamach Rawtheyan/Richmond Cautleyan/Richmond Pusgillian/Maysville/Richmond Middle Ordovician Trenton Onnian/Maysville/Eden Actonian/Eden Marshbrookian/Sherman Longvillian/Sherman Soudleyan/Kirkfield Harnagian/Rockland Costonian/Black River Chazy Llandeilo Whiterock Llanvirn Early Ordovician Cassinian Arenig/Jefferson/Castleman Tremadoc/Deming/Gaconadian The Tremadoc corresponds to the Tremadocian.
The Floian corresponds to the lower Arenig. The Llanvirn occupies the rest of the Darriwilian, terminates with it at the base of the Late Ordovician; the Sandbian represents the first half of the Caradoc. During the Ordovician, the southern continents were collected into Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents of Laurentia and Baltica were still independent continents, but Baltica began to move towards Laurentia in the period, causing the Iapetus Ocean between them to shrink; the small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia. The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. In the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.
Sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped. At first, low-lying sea beds increased diversity, but glaciation led to mass extinctions as the seas drained and continental shelves became dry land. During the Ordovician, in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved; these volcanic island arcs collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician the volcanic emissions had stopped. Gondwana had by that time neared the South Pole and was glaciated
Kotasaurus is a genus of sauropod dinosaur from the Early Jurassic period. The only known species is Kotasaurus yamanpalliensis, it was discovered in the Kota Formation of Telangana and shared its habitat with the related Barapasaurus. So far the remains of at least 12 individuals are known; the greater part of the skeleton is known, but the skull is missing, with the exception of two teeth. Like all sauropods, it was a quadrupedal herbivore with long neck and tail. Kotasaurus is one of the most basal sauropods known; the general body plan was that of a typical sauropod, but in several basal features it resembles prosauropods. Like all sauropods, Kotasaurus was an obligate quadruped, while prosauropods were primitively bipedal; the body length is estimated at nine meters, with a weight of 2.5 tonnes, therefore comparable with that of sauropods. The femur was straight and oval in cross section, which means that the limbs were columnar; the teeth were spoon-shaped, like those of sauropods. Basal features, on the other hand, include the short and twisted humerus, as well as the retention of a lesser trochanter on the femur.
The neural spines of the vertebrae were built and their centra are massive, in contrast to those of the related Barapasaurus, which show more hollowing, be it without pneumatisation, of the sides as a weight-saving measure. Autapomorphies include the slender limb bones as well as the low and elongated preacetabular process, it was not clear if Kotasaurus represents a true sauropod or a basal sauropodomorph that has to be classified outside Sauropoda. Some paleontologists placed it inside a basal sauropod family called Vulcanodontidae though, together with Barapasaurus and the fragmentary Ohmdenosaurus and Zizhongosaurus; this grouping is now recognized to be paraphyletic. Today Kotasaurus is recognized as one of the most basal sauropods known; the exact relationships are not clear, however. A recent study by Bandyopadhyay and colleagues renders Kotasaurus to be more basal than Barapasaurus and Vulcanodon but more derived than Jingshanosaurus and Chinshakiangosaurus. All known fossils come from an area of 2,400 m² near the village of Yamanpalli in Telangana forty kilometres north of the Barapasaurus type locality.
These finds, altogether 840 skeletal parts, were found in the late 1970s. In 1988 they were named and described by P. M. Yadagiri as a new genus and species of sauropod, Kotasaurus yamanpalliensis; the generic name refers to the Kota Formation. The specific name reflects the provenance from Yamanpalli; the holotype is an ilium. The Geological Survey of India combined several elements into a skeletal mount and displayed it at the Birla Science Museum, Hyderabad. In 2001, Yadagiri described the osteology in more detail
Sauropodomorpha is an extinct clade of long-necked, saurischian dinosaurs that includes the sauropods and their ancestral relatives. Sauropods grew to large sizes, had long necks and tails, were quadrupedal, became the largest animals to walk the Earth; the "prosauropods", which preceded the sauropods, were smaller and were able to walk on two legs. The sauropodomorphs were the dominant terrestrial herbivores throughout much of the Mesozoic Era, from their origins in the mid-Triassic until their decline and extinction at the end of the Cretaceous. Sauropodomorphs were adapted to browsing higher than any other contemporary herbivore, giving them access to high tree foliage; this feeding strategy is supported by many of their defining characteristics, such as: a light, tiny skull on the end of a long neck and a counterbalancing long tail. Their teeth were weak, shaped like leaves or spoons. Instead of grinding teeth, they had stomach stones, similar to the gizzard stones of modern birds and crocodiles, to help digest tough plant fibers.
The front of the upper mouth bends down in. One of the earliest known sauropodomorphs, was small and slender; the largest sauropods, like Supersaurus, Diplodocus hallorum and Argentinosaurus, reached 30–40 metres in length, 60,000–100,000 kilograms or more in mass. Bipedal, as their size increased they evolved a four-legged graviportal gait adapted only to walking on land, like elephants; the early sauropodomorphs were most omnivores as their shared common ancestor with the other saurischian lineage was a carnivore. Therefore, their evolution to herbivory went hand in hand with their increasing size and neck length, they had large nostrils, retained a thumb with a big claw, which may have been used for defense — though their primary defensive adaptation was their extreme size. Sauropodomorphs can be distinguished as a group on the basis of some of the following synapomorphies: The presence of large nares; the distal part of the tibia is covered by an ascending process of the astragalus. Their hind limbs are short.
The presence of three or more sacral vertebrae. The teeth are thin and are spatula-like, with bladed and serrated crowns; the presence of a minimum of 10 cervical vertebrae that are elongated The presence of 25 presacral vertebrae The manus had a large digit I Among the first dinosaurs to evolve in the Late Triassic Period, about 230 million years ago, they became the dominant herbivores by halfway through the late Triassic. Their perceived decline in the early Cretaceous is most a bias in fossil sampling, as most fossils are known from Europe and North America. Sauropods were still the dominant herbivores in the Gondwanan landmasses, however; the spread of flowering plants and "advanced" ornithischians, another major group of herbivorous dinosaurs, are most not a major factor in sauropod decline in the northern continents. Like all non-avian dinosaurs, the sauropodomorphs became extinct 66 Mya, during the Cretaceous–Paleogene extinction event; the earliest and most basal sauropodomorphs known are Chromogisaurus novasi and Panphagia protos, both from the Ischigualasto Formation, dated to 231.4 million years ago.
Some studies have found Eoraptor lunensis, traditionally considered a theropod, to be an early member of the sauropodomorph lineage, which would make it the most basal sauropodomorph known. Sauropodomorpha is one of the two major clades within the order Saurischia; the sauropodomorphs' sister group, the Theropoda, includes bipedal carnivores like Velociraptor and Tyrannosaurus. However, sauropodomorphs share a number of characteristics with the Ornithischia, so a small minority of palaeontologists, like Bakker, have placed both sets of herbivores within a group called "Phytodinosauria" or "Ornithischiformes". In Linnaean taxonomy, Sauropodomorpha is left unranked, it was established by Friedrich von Huene in 1932, who broke it into two groups: the basal forms within Prosauropoda, their descendants, the giant Sauropoda. Phylogenetic analyses by Adam Yates and others placed Sauropoda within a paraphyletic "Prosauropoda". Recent cladistic analyses suggest that the clade Prosauropoda, named by Huene in 1920 and was defined by Sereno, in 1998, as all animals more related to Plateosaurus engelhardti than to Saltasaurus loricatus, is a junior synonym of Plateosauridae as both contain the same taxa.
Most modern classification schemes break the prosauropods into a half-dozen groups that evolved separately from one common lineage. While they have a number of shared characteristics, the evolutionary requirements for giraffe-like browsing high in the trees may have caused convergent evolution, where similar traits evolve separately because they faced the same evolutionary pressure, instead of trait
Brazil the Federative Republic of Brazil, is the largest country in both South America and Latin America. At 8.5 million square kilometers and with over 208 million people, Brazil is the world's fifth-largest country by area and the fifth most populous. Its capital is Brasília, its most populated city is São Paulo; the federation is composed of the union of the 26 states, the Federal District, the 5,570 municipalities. It is the largest country to have Portuguese as an official language and the only one in the Americas. Bounded by the Atlantic Ocean on the east, Brazil has a coastline of 7,491 kilometers, it borders all other South American countries except Ecuador and Chile and covers 47.3% of the continent's land area. Its Amazon River basin includes a vast tropical forest, home to diverse wildlife, a variety of ecological systems, extensive natural resources spanning numerous protected habitats; this unique environmental heritage makes Brazil one of 17 megadiverse countries, is the subject of significant global interest and debate regarding deforestation and environmental protection.
Brazil was inhabited by numerous tribal nations prior to the landing in 1500 of explorer Pedro Álvares Cabral, who claimed the area for the Portuguese Empire. Brazil remained a Portuguese colony until 1808, when the capital of the empire was transferred from Lisbon to Rio de Janeiro. In 1815, the colony was elevated to the rank of kingdom upon the formation of the United Kingdom of Portugal and the Algarves. Independence was achieved in 1822 with the creation of the Empire of Brazil, a unitary state governed under a constitutional monarchy and a parliamentary system; the ratification of the first constitution in 1824 led to the formation of a bicameral legislature, now called the National Congress. The country became a presidential republic in 1889 following a military coup d'état. An authoritarian military junta came to power in 1964 and ruled until 1985, after which civilian governance resumed. Brazil's current constitution, formulated in 1988, defines it as a democratic federal republic. Due to its rich culture and history, the country ranks thirteenth in the world by number of UNESCO World Heritage Sites.
Brazil is considered an advanced emerging economy. It has the ninth largest GDP in the world by nominal, eight and PPP measures, it is one of the world's major breadbaskets, being the largest producer of coffee for the last 150 years. It is classified as an upper-middle income economy by the World Bank and a newly industrialized country, with the largest share of global wealth in Latin America. Brazil is a regional power and sometimes considered a great or a middle power in international affairs. On account of its international recognition and influence, the country is subsequently classified as an emerging power and a potential superpower by several analysts. Brazil is a founding member of the United Nations, the G20, BRICS, Union of South American Nations, Organization of American States, Organization of Ibero-American States and the Community of Portuguese Language Countries, it is that the word "Brazil" comes from the Portuguese word for brazilwood, a tree that once grew plentifully along the Brazilian coast.
In Portuguese, brazilwood is called pau-brasil, with the word brasil given the etymology "red like an ember", formed from brasa and the suffix -il. As brazilwood produces a deep red dye, it was valued by the European textile industry and was the earliest commercially exploited product from Brazil. Throughout the 16th century, massive amounts of brazilwood were harvested by indigenous peoples along the Brazilian coast, who sold the timber to European traders in return for assorted European consumer goods; the official Portuguese name of the land, in original Portuguese records, was the "Land of the Holy Cross", but European sailors and merchants called it the "Land of Brazil" because of the brazilwood trade. The popular appellation eclipsed and supplanted the official Portuguese name; some early sailors called it the "Land of Parrots". In the Guarani language, an official language of Paraguay, Brazil is called "Pindorama"; this was the name the indigenous population gave to the region, meaning "land of the palm trees".
Some of the earliest human remains found in the Americas, Luzia Woman, were found in the area of Pedro Leopoldo, Minas Gerais and provide evidence of human habitation going back at least 11,000 years. The earliest pottery found in the Western Hemisphere was excavated in the Amazon basin of Brazil and radiocarbon dated to 8,000 years ago; the pottery was found near Santarém and provides evidence that the tropical forest region supported a complex prehistoric culture. The Marajoara culture flourished on Marajó in the Amazon delta from 800 CE to 1400 CE, developing sophisticated pottery, social stratification, large populations, mound building, complex social formations such as chiefdoms. Around the time of the Portuguese arrival, the territory of current day Brazil had an estimated indigenous population of 7 million people semi-nomadic who subsisted on hunting, fishing and migrant agriculture; the indigenous population of Brazil comprised several large indigenous ethnic groups. The Tupí people were subdivided into the Tupiniquins and Tupinambás, there were many subdivisions of the other gro
Titanosaurs were a diverse group of sauropod dinosaurs which included Saltasaurus and Isisaurus. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still thriving at the time of the extinction event at the end of the Cretaceous; the group includes the largest land animals known to have existed, such as Patagotitan—estimated at 37 m long with a weight of 69 tonnes —and the comparably sized Argentinosaurus and Puertasaurus from the same region. The group's name alludes to the mythological Titans of Ancient Greece, via the type genus Titanosaurus. Together with the brachiosaurids and relatives, titanosaurs make up the larger clade Titanosauriformes. Titanosaurs had small heads when compared with other sauropods; the head was wide, similar to the heads of Camarasaurus and Brachiosaurus but more elongated. Their nostrils were large and they all had crests formed by these nasal bones, their teeth were either somewhat spatulate or like pegs or pencils, but were always small.
Their necks were of average length, for sauropods, their tails were whip-like, but not as long as in the diplodocids. While the pelvis was slimmer than some sauropods, the pectoral was much wider, giving them a uniquely'wide-gauged' stance; as a result, the fossilised trackways of titanosaurs are distinctly broader than other sauropods. Their forelimbs were stocky, longer than their hind limbs, their vertebrae were solid. Their spinal column was more flexible, so they were more agile than their cousins and better at rearing up. Unlike other sauropods, some titanosaurs had no digits or digit bones, walked only on horseshoe-shaped "stumps" made up of the columnar metacarpal bones. From skin impressions found with the fossils, it has been determined that the skin of many titanosaur species was armored with a small mosaic of small, bead-like scales around a larger scale. One species, has been discovered with bony plates, like the ankylosaurs. Studies published in 2011 indicate that titanosaurs such as Rapetosaurus, may have used the osteoderms common in the various genera for storing minerals during harsh changes in climate, such as drought.
While they were all huge, many were average in size compared with the other giant dinosaurs. There were some island-dwelling dwarf species such as Magyarosaurus the result of allopatric speciation and insular dwarfism; the family Titanosauridae was once used for derived titanosaurs, but Wilson and Upchurch found the type genus Titanosaurus dubious based on the figures and original description. Weishampel et al. in the second edition of The Dinosauria did not use the family Titanosauridae, instead used several smaller titanosaur families such as Saltasauridae and Nemegtosauridae, coining Lithostrotia for derived titanosaurs. A handful of Argentine sauropod workers, continue to use Titanosauridae for titanosaurs now placed in Lithostrotia. In the second edition of The Dinosauria, the clade Titanosauria was defined as all sauropods closer to Saltasaurus than to Brachiosaurus. Subsequent cladistic analyses have defined Titanosauria as including Saltasaurus but not Euhelopus or Brachiosaurus. Relationships within the Titanosauria have been variable from study to study, complicated by the fact that clade and rank names have been applied inconsistently by various scientists.
One possible cladogram is presented here, follows a 2007 analysis by Calvo and colleagues. The authors notably used the family Titanosauridae in a broader fashion than other recent studies, coined the new clade name Lognkosauria. In the description of Mansourasaurus, Sallam et al. published a phylogenetic analysis of Titanosauria including the most taxa of any analysis of the clade. The relationships within Titanosauria can be seen below. Fossilized dung associated with late Cretaceous titanosaurids has revealed phytoliths, silicified plant fragments, that offer clues to a broad, unselective plant diet. Besides the plant remains that might have been expected, such as cycads and conifers, discoveries published in 2005 revealed an unexpectedly wide range of monocotyledons, including palms and grasses, including ancestors of rice and bamboo, which has given rise to speculation that herbivorous dinosaurs and grasses co-evolved. A large titanosaurid nesting ground was discovered in Auca Mahuevo, in Patagonia and another colony has been discovered in Spain.
Several hundred female saltasaurs dug holes with their back feet, laid eggs in clutches averaging around 25 eggs each, buried the nests under dirt and vegetation. The small eggs, about 11–12 centimetres in diameter, contained fossilised embryos, complete with skin impressions; the impressions showed. The huge number of individuals gives evidence of herd behavior, along with their armor, could have helped provide protection against large predators such as Abelisaurus; the titanosaurs were the last great group of sauropods, which existed from about 136 to 66 million years ago, before the Cretaceous–Paleogene extinction event, were the dominant herbivores of their time. The fossil evidence suggests they replaced the other sauropods, like the diplodocids and the brachiosaurids, which died out between the late Jurassic and the mid-Cretaceous Periods. Titanosaurs were widespread. In December 2011, Argentine scientists announced titanosaur fossils had been found on Antarctica—meaning that titanosaur fossils
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