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
A chordate is an animal constituting the phylum Chordata. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, a post-anal tail: these five anatomical features define this phylum. Chordates are bilaterally symmetric; the Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata. There are extinct taxa such as the Vetulicolia. Hemichordata has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes. Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically, vertebrates - chordates with the notochord replaced by a vertebral column during development - are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull.
The Craniata and Tunicata compose the clade Olfactores. Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: A notochord, a stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, in wholly aquatic species this helps the animal to swim by flexing its tail. A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system. Pharyngeal slits; the pharynx is the part of the throat behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live. Post-anal tail. A muscular tail that extends backwards behind the anus. An endostyle; this is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus.
It stores iodine, may be a precursor of the vertebrate thyroid gland. There are soft constraints that separate chordates from certain other biological lineages, but are not part of the formal definition: All chordates are deuterostomes; this means. All chordates are based on a bilateral body plan. All chordates are coelomates, have a fluid filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm; the following schema is from the third edition of Vertebrate Palaeontology. The invertebrate chordate classes are from Fishes of the World. While it is structured so as to reflect evolutionary relationships, it retains the traditional ranks used in Linnaean taxonomy. Phylum Chordata †Vetulicolia? Subphylum Cephalochordata – Class Leptocardii Clade Olfactores Subphylum Tunicata – Class Ascidiacea Class Thaliacea Class Appendicularia Class Sorberacea Subphylum Vertebrata Infraphylum incertae sedis Cyclostomata Superclass'Agnatha' paraphyletic Class Myxini Class Petromyzontida or Hyperoartia Class †Conodonta Class †Myllokunmingiida Class †Pteraspidomorphi Class †Thelodonti Class †Anaspida Class †Cephalaspidomorphi Infraphylum Gnathostomata Class †Placodermi Class Chondrichthyes Class †Acanthodii Superclass Osteichthyes Class Actinopterygii Class Sarcopterygii Superclass Tetrapoda Class Amphibia Class Sauropsida Class Synapsida Craniates, one of the three subdivisions of chordates, all have distinct skulls.
They include the hagfish. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or all deuterostomes, are by their tails". Most craniates are vertebrates; these consist of a series of bony or cartilaginous cylindrical vertebrae with neural arches that protect the spinal cord, with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, are therefore not regarded as vertebrates, but as members of the craniates, the group from which vertebrates are thought to have evolved; however the cladistic exclusion of hagfish from the vertebrates is controversial, as they ma
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
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago, although the exact origin and timing of the evolution of dinosaurs is the subject of active research. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201 million years ago. Reverse genetic engineering and the fossil record both demonstrate that birds are modern feathered dinosaurs, having evolved from earlier theropods during the late Jurassic Period; as such, birds were the only dinosaur lineage to survive the Cretaceous–Paleogene extinction event 66 million years ago. Dinosaurs can therefore be divided into birds; this article deals with non-avian dinosaurs. Dinosaurs are a varied group of animals from taxonomic and ecological standpoints. Birds, at over 10,000 living species, are the most diverse group of vertebrates besides perciform fish. Using fossil evidence, paleontologists have identified over 500 distinct genera and more than 1,000 different species of non-avian dinosaurs.
Dinosaurs are represented on every continent by fossil remains. Through the first half of the 20th century, before birds were recognized to be dinosaurs, most of the scientific community believed dinosaurs to have been sluggish and cold-blooded. Most research conducted since the 1970s, has indicated that all dinosaurs were active animals with elevated metabolisms and numerous adaptations for social interaction; some were herbivorous, others carnivorous. Evidence suggests that egg-laying and nest-building are additional traits shared by all dinosaurs and non-avian alike. While dinosaurs were ancestrally bipedal, many extinct groups included quadrupedal species, some were able to shift between these stances. Elaborate display structures such as horns or crests are common to all dinosaur groups, some extinct groups developed skeletal modifications such as bony armor and spines. While the dinosaurs' modern-day surviving avian lineage are small due to the constraints of flight, many prehistoric dinosaurs were large-bodied—the largest sauropod dinosaurs are estimated to have reached lengths of 39.7 meters and heights of 18 meters and were the largest land animals of all time.
Still, the idea that non-avian dinosaurs were uniformly gigantic is a misconception based in part on preservation bias, as large, sturdy bones are more to last until they are fossilized. Many dinosaurs were quite small: Xixianykus, for example, was only about 50 cm long. Since the first dinosaur fossils were recognized in the early 19th century, mounted fossil dinosaur skeletons have been major attractions at museums around the world, dinosaurs have become an enduring part of world culture; the large sizes of some dinosaur groups, as well as their monstrous and fantastic nature, have ensured dinosaurs' regular appearance in best-selling books and films, such as Jurassic Park. Persistent public enthusiasm for the animals has resulted in significant funding for dinosaur science, new discoveries are covered by the media; the taxon'Dinosauria' was formally named in 1841 by paleontologist Sir Richard Owen, who used it to refer to the "distinct tribe or sub-order of Saurian Reptiles" that were being recognized in England and around the world.
The term is derived from Ancient Greek δεινός, meaning'terrible, potent or fearfully great', σαῦρος, meaning'lizard or reptile'. Though the taxonomic name has been interpreted as a reference to dinosaurs' teeth and other fearsome characteristics, Owen intended it to evoke their size and majesty. Other prehistoric animals, including pterosaurs, ichthyosaurs and Dimetrodon, while popularly conceived of as dinosaurs, are not taxonomically classified as dinosaurs. Pterosaurs are distantly related to dinosaurs; the other groups mentioned are, like dinosaurs and pterosaurs, members of Sauropsida, except Dimetrodon. Under phylogenetic nomenclature, dinosaurs are defined as the group consisting of the most recent common ancestor of Triceratops and Neornithes, all its descendants, it has been suggested that Dinosauria be defined with respect to the MRCA of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria. Both definitions result in the same set of animals being defined as dinosaurs: "Dinosauria = Ornithischia + Saurischia", encompassing ankylosaurians, ceratopsians, ornithopods and sauropodomorphs.
Birds are now recognized as being the sole surviving lineage of theropod dinosaurs. In traditional taxonomy, birds were considered a separate class that had evolved from dinosaurs, a distinct superorder. However, a majority of contemporary paleontologists concerned with dinosaurs reject the traditional style of classification in favor of phylogenetic taxonomy. Birds are thus considered to be dinosaurs and dinosaurs are, not extinct. Birds are classified as belonging to the subgroup M
The Devonian is a geologic period and system of the Paleozoic, spanning 60 million years from the end of the Silurian, 419.2 million years ago, to the beginning of the Carboniferous, 358.9 Mya. It is named after Devon, where rocks from this period were first studied; the first significant adaptive radiation of life on dry land occurred during the Devonian. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods became well-established. Fish reached substantial diversity during this time, leading the Devonian to be dubbed the "Age of Fishes." The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating every known aquatic environment. The ancestors of all four-limbed vertebrates began adapting to walking on land, as their strong pectoral and pelvic fins evolved into legs.
In the oceans, primitive sharks became more numerous than in the Late Ordovician. The first ammonites, species of molluscs, appeared. Trilobites, the mollusc-like brachiopods and the great coral reefs, were still common; the Late Devonian extinction which started about 375 million years ago affected marine life, killing off all placodermi, all trilobites, save for a few species of the order Proetida. The palaeogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, the early formation of the small continent of Euramerica in between; the period is named after Devon, a county in southwestern England, where a controversial argument in the 1830s over the age and structure of the rocks found distributed throughout the county was resolved by the definition of the Devonian period in the geological timescale. The Great Devonian Controversy was a long period of vigorous argument and counter-argument between the main protagonists of Roderick Murchison with Adam Sedgwick against Henry De la Beche supported by George Bellas Greenough.
Murchison and Sedgwick named the period they proposed as the Devonian System. While the rock beds that define the start and end of the Devonian period are well identified, the exact dates are uncertain. According to the International Commission on Stratigraphy, the Devonian extends from the end of the Silurian 419.2 Mya, to the beginning of the Carboniferous 358.9 Mya. In nineteenth-century texts the Devonian has been called the "Old Red Age", after the red and brown terrestrial deposits known in the United Kingdom as the Old Red Sandstone in which early fossil discoveries were found. Another common term is "Age of the Fishes", referring to the evolution of several major groups of fish that took place during the period. Older literature on the Anglo-Welsh basin divides it into the Downtonian, Dittonian and Farlovian stages, the latter three of which are placed in the Devonian; the Devonian has erroneously been characterised as a "greenhouse age", due to sampling bias: most of the early Devonian-age discoveries came from the strata of western Europe and eastern North America, which at the time straddled the Equator as part of the supercontinent of Euramerica where fossil signatures of widespread reefs indicate tropical climates that were warm and moderately humid but in fact the climate in the Devonian differed during its epochs and between geographic regions.
For example, during the Early Devonian, arid conditions were prevalent through much of the world including Siberia, North America, China, but Africa and South America had a warm temperate climate. In the Late Devonian, by contrast, arid conditions were less prevalent across the world and temperate climates were more common; the Devonian Period is formally broken into Early and Late subdivisions. The rocks corresponding to those epochs are referred to as belonging to the Lower and Upper parts of the Devonian System. Early DevonianThe Early Devonian lasted from 419.2 ± 2.8 to 393.3 ± 2.5 and began with the Lochkovian stage, which lasted until the Pragian. It spanned from 410.8 ± 2.8 to 407.6 ± 2.5, was followed by the Emsian, which lasted until the Middle Devonian began, 393.3± 2.7 million years ago. During this time, the first ammonoids appeared. Ammonoids during this time period differed little from their nautiloid counterparts; these ammonoids belong to the order Agoniatitida, which in epochs evolved to new ammonoid orders, for example Goniatitida and Clymeniida.
This class of cephalopod molluscs would dominate the marine fauna until the beginning of the Mesozoic era. Middle DevonianThe Middle Devonian comprised two subdivisions: first the Eifelian, which gave way to the Givetian 387.7± 2.7 million years ago. During this time the jawless agnathan fishes began to decline in diversity in freshwater and marine environments due to drastic environmental changes and due to the increasing competition and diversity of jawed fishes; the shallow, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop such essential characteristics as well developed lungs, the ability to crawl out of the water and onto the land for short periods of time. Late DevonianFinally, the Late Devonian started with the Frasnian, 382.7 ± 2.8 to 372.2 ± 2.5, during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuing Famennian subdivisi
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the city of Perm. The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles and archosaurs; the world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors; the Permian ended with the Permian–Triassic extinction event, the largest mass extinction in Earth's history, in which nearly 96% of marine species and 70% of terrestrial species died out.
It would take well into the Triassic for life to recover from this catastrophe. Recovery from the Permian–Triassic extinction event was protracted; the term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Édouard de Verneuil; the region now lies in the Perm Krai of Russia. Official ICS 2017 subdivisions of the Permian System from most recent to most ancient rock layers are: Lopingian epoch Changhsingian Wuchiapingian Others: Waiitian Makabewan Ochoan Guadalupian epoch Capitanian stage Wordian stage Roadian stage Others: Kazanian or Maokovian Braxtonian stage Cisuralian epoch Kungurian stage Artinskian stage Sakmarian stage Asselian stage Others: Telfordian Mangapirian Sea levels in the Permian remained low, near-shore environments were reduced as all major landmasses collected into a single continent—Pangaea; this could have in part caused the widespread extinctions of marine species at the end of the period by reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean, the Paleo-Tethys Ocean, a large ocean that existed between Asia and Gondwana; the Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys Ocean to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic era. Large continental landmass interiors experience climates with extreme variations of heat and cold and monsoon conditions with seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea; such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores in a wetter environment. The first modern trees appeared in the Permian. Three general areas are noted for their extensive Permian deposits—the Ural Mountains and the southwest of North America, including the Texas red beds.
The Permian Basin in the U. S. states of Texas and New Mexico is so named because it has one of the thickest deposits of Permian rocks in the world. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an ice age. Glaciers receded around the mid-Permian period as the climate warmed, drying the continent's interiors. In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Permian marine deposits are rich in fossil mollusks and brachiopods. Fossilized shells of two kinds of invertebrates are used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist, one of the foraminiferans, ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct. Terrestrial life in the Permian included diverse plants, fungi and various types of tetrapods; the period saw a massive desert covering the interior of Pangaea.
The warm zone spread in the northern hemisphere. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals became marginal elements; the Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began; the swamp-loving
The Precambrian is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, named after Cambria, the Latinised name for Wales, where rocks from this age were first studied; the Precambrian accounts for 88% of the Earth's geologic time. The Precambrian is an informal unit of geologic time, subdivided into three eons of the geologic time scale, it spans from the formation of Earth about 4.6 billion years ago to the beginning of the Cambrian Period, about 541 million years ago, when hard-shelled creatures first appeared in abundance. Little is known about the Precambrian, despite it making up seven-eighths of the Earth's history, what is known has been discovered from the 1960s onwards; the Precambrian fossil record is poorer than that of the succeeding Phanerozoic, fossils from the Precambrian are of limited biostratigraphic use. This is because many Precambrian rocks have been metamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain buried beneath Phanerozoic strata.
It is thought that the Earth coalesced from material in orbit around the Sun at 4,543 Ma, may have been struck by a large planetesimal shortly after it formed, splitting off material that formed the Moon. A stable crust was in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma; the term "Precambrian" is recognized by the International Commission on Stratigraphy as the only "supereon" in geologic time. "Precambrian" is still used by geologists and paleontologists for general discussions not requiring the more specific eon names. As of 2010, the United States Geological Survey considers the term informal, lacking a stratigraphic rank. A specific date for the origin of life has not been determined. Carbon found in 3.8 billion-year-old rocks from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area.
However, there is evidence. There is a solid record of bacterial life throughout the remainder of the Precambrian. Excluding a few contested reports of much older forms from North America and India, the first complex multicellular life forms seem to have appeared at 1500 Ma, in the Mesoproterozoic era of the Proterozoic eon. Fossil evidence from the Ediacaran period of such complex life comes from the Lantian formation, at least 580 million years ago. A diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma; these are referred to as Vendian biota. Hard-shelled creatures appeared toward the end of that time span, marking the beginning of the Phanerozoic eon. By the middle of the following Cambrian period, a diverse fauna is recorded in the Burgess Shale, including some which may represent stem groups of modern taxa; the increase in diversity of lifeforms during the early Cambrian is called the Cambrian explosion of life. While land seems to have been devoid of plants and animals and other microbes formed prokaryotic mats that covered terrestrial areas.
Tracks from an animal with leg like appendages have been found in what was mud 551 million years ago. Evidence of the details of plate motions and other tectonic activity in the Precambrian has been poorly preserved, it is believed that small proto-continents existed prior to 4280 Ma, that most of the Earth's landmasses collected into a single supercontinent around 1130 Ma. The supercontinent, known as Rodinia, broke up around 750 Ma. A number of glacial periods have been identified going as far back as the Huronian epoch 2400–2100 Ma. One of the best studied is the Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth"; the atmosphere of the early Earth is not well understood. Most geologists believe it was composed of nitrogen, carbon dioxide, other inert gases, was lacking in free oxygen. There is, evidence that an oxygen-rich atmosphere existed since the early Archean. At present, it is still believed that molecular oxygen was not a significant fraction of Earth's atmosphere until after photosynthetic life forms evolved and began to produce it in large quantities as a byproduct of their metabolism.
This radical shift from a chemically inert to an oxidizing atmosphere caused an ecological crisis, sometimes called the oxygen catastrophe. At first, oxygen would have combined with other elements in Earth's crust iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive banded iron formations that were laid down as iron oxides. A terminology has evolved covering the early years of the Earth's existence, as radiometric dating has allowed real dates to be assigned to specific formations and features; the Precambrian is divided into