Aglaophyton major was the sporophyte generation of a diplohaplontic, pre-vascular, free-sporing land plant of the Lower Devonian. It had anatomical features intermediate between those of the bryophytes and vascular plants or tracheophytes. A. major was first described by Lang in 1920 as the new species Rhynia major. The species is known only from the Rhynie chert in Aberdeenshire, where it grew in the vicinity of a silica-rich hot spring, together with a number of associated vascular plants such as a smaller species Rhynia gwynne-vaughanii which may be interpreted as a representative of the ancestors of modern vascular plants and Asteroxylon mackei, an ancestor of modern clubmosses; the stems of Aglaophyton were round in cross-section, unornamented, up to about 6mm in diameter. Kidston and Lang interpreted the plant as growing upright, to about 50 cm in height, but Edwards has re-interpreted it as having prostrate habit, with shorter aerial axes of about 15 cm height; the axes branched dichotomously, the aerial axes branching at a comparatively wide angle of up to 90°, were terminated with elliptical, thick-walled sporangia, which when mature, opened by spiral slits, so that the sporangia appear to be spiral in form.
Sporangia contained many identical spores bearing trilete marks. The spores may therefore be interpreted as meiospores, the product of meiotic divisions, thus the plants described by Edwards and by Kidston and Lang were diploid, sporophytes; the plant was interpreted as a tracheophyte, because the stem has a simple central vascular cylinder or protostele, but more recent interpretations in the light of additional data indicated that Rhynia major had water-conducting tissue lacking the secondary thickening bars seen in the xylem of Rhynia gwynne-vaughanii, more like the water-conducting system of moss sporophytes. Edwards renamed it Aglaophyton major. Aglaophyton is among the first plants known to have had a mycorrhizal relationship with fungi, which formed arbuscules in a well-defined zone in the cortex of its stems. Aglaophyton lacked roots, like other rootless land plants of the Silurian and early Devonian may have relied on mycorrhizal fungi for acquisition of water and nutrients from the soil.
The male gametophyte of the species has been formally described, assigned to a new form taxon Lyonophyton rhyniensis, but is now properly referred to as an Aglaophyton gametophyte. The Rhynie chert bears many examples of male and female gametophytes, which are loosely similar in their construction to the sporophyte phase, down to bearing rhizoids. Aglaophyton major was first described as Rhynia major by Kidston and Lang in 1920. In 1986 D. S. Edwards re-examined fossil specimens and reported that they did not contain true vascular tissue, but rather conducting tissue more similar to that of bryophytes; as the diagnosis of Rhynia was that it was a vascular plant, he created a new genus, for this species. As Rhynia major the species had been placed in the rhyniophytes, but no alternative higher level classification was proposed for the new genus. In 2004, Crane et al. published a cladogram for the polysporangiophytes which places Aglaophyton as a sister of the vascular plants, with the Horneophytopsida being sister to both.
The basis of the cladogram is that Aglaophyton has more developed conducting tissue than the Horneophytopsida, but does not have true vascular tissue. Cladogram from Crane, Herendeen & Friis 2004
The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago, to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō and ferō, was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern'system' names to be employed, reflects the fact that many coal beds were formed globally during that time; the Carboniferous is treated in North America as two geological periods, the earlier Mississippian and the Pennsylvanian. Terrestrial animal life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would evolve into amniotes, the first terrestrial vertebrates. Arthropods were very common, many were much larger than those of today. Vast swaths of forest covered the land, which would be laid down and become the coal beds characteristic of the Carboniferous stratigraphy evident today.
The atmospheric content of oxygen reached its highest levels in geological history during the period, 35% compared with 21% today, allowing terrestrial invertebrates to evolve to great size. The half of the period experienced glaciations, low sea level, mountain building as the continents collided to form Pangaea. A minor marine and terrestrial extinction event, the Carboniferous rainforest collapse, occurred at the end of the period, caused by climate change. In the United States the Carboniferous is broken into Mississippian and Pennsylvanian subperiods; the Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal-bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration. In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian and Stephanian Series, dated at 332.9-298.9 Ma.
The Silesian is contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, the Westphalian as the Coal Measures and Pennant Sandstone; the International Commission on Stratigraphy faunal stages from youngest to oldest, together with some of their regional subdivisions, are: A global drop in sea level at the end of the Devonian reversed early in the Carboniferous. There was a drop in south polar temperatures; these conditions had little effect in the deep tropics, where lush swamps to become coal, flourished to within 30 degrees of the northernmost glaciers. Mid-Carboniferous, a drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites hard; this sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod. This happened about 323 million years ago, at the onset of the Permo-Carboniferous Glaciation.
The Carboniferous was a time of active mountain-building as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe along the present line of eastern North America; this continental collision resulted in the Hercynian orogeny in Europe, the Alleghenian orogeny in North America. In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural Mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China, South China continents were still separated from Laurasia; the Late Carboniferous Pangaea was shaped like an "O." There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and closed - Rheic Ocean, the small, shallow Ural Ocean and Proto-Tethys Ocean. Average global temperatures in the Early Carboniferous Period were high: 20 °C.
However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C. Lack of growth rings of fossilized trees suggest a lack of seasons of a tropical climate. Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are referred to as Permo-Carboniferous in age; the cooling and drying of the climate led to the Carboniferous Rainforest Collapse during the late Carboniferous. Tropical rainforests fragmented and were devastated by climate change. Carboniferous rocks in Europe and eastern North America consist of a repeated sequence of limestone, sandstone and coal beds. In North America, the early Carboniferous is marine
The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, the longest period of the Phanerozoic Eon; the Cretaceous Period is abbreviated K, for its German translation Kreide. The Cretaceous was a period with a warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas; these oceans and seas were populated with now-extinct marine reptiles and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared; the Cretaceous ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.
The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin and named for the extensive beds of chalk, found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from Latin creta; the Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian and Senonian. A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use; as with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact age of the system's base is uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is defined, being placed at an iridium-rich layer found worldwide, believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico.
This layer has been dated at 66.043 Ma. A 140 Ma age for the Jurassic-Cretaceous boundary instead of the accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina. Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age sees the study as a "first step" toward formally changing the age in the International Union of Geological Sciences. From youngest to oldest, the subdivisions of the Cretaceous period are: Late Cretaceous Maastrichtian – Campanian – Santonian – Coniacian – Turonian – Cenomanian – Early Cretaceous Albian – Aptian – Barremian – Hauterivian – Valanginian – Berriasian – The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms; the Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of marine limestone, a rock type, formed under warm, shallow marine circumstances.
Due to the high sea level, there was extensive space for such sedimentation. Because of the young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide. Chalk is a rock type characteristic for the Cretaceous, it consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas. In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast; the group is found in England, northern France, the low countries, northern Germany, Denmark and in the subsurface of the southern part of the North Sea. Chalk is not consolidated and the Chalk Group still consists of loose sediments in many places; the group has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites and sea reptiles such as Mosasaurus. In southern Europe, the Cretaceous is a marine system consisting of competent limestone beds or incompetent marls.
Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf, at the margin of the Tethys Ocean. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half the worlds petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval; these shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. During th
The Jurassic period was a geologic period and system that spanned 56 million years from the end of the Triassic Period 201.3 million years ago to the beginning of the Cretaceous Period 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era known as the Age of Reptiles; the start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, the Tithonian event at the end; the Jurassic period is divided into three epochs: Early and Late. In stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, Upper Jurassic series of rock formations; the Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period were first identified. By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses: Laurasia to the north, Gondwana to the south; this created more coastlines and shifted the continental climate from dry to humid, many of the arid deserts of the Triassic were replaced by lush rainforests.
On land, the fauna transitioned from the Triassic fauna, dominated by both dinosauromorph and crocodylomorph archosaurs, to one dominated by dinosaurs alone. The first birds appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, the evolution of therian mammals, including primitive placentals. Crocodilians made the transition from a terrestrial to an aquatic mode of life; the oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates. The chronostratigraphic term "Jurassic" is directly linked to the Jura Mountains, a mountain range following the course of the France–Switzerland border. During a tour of the region in 1795, Alexander von Humboldt recognized the limestone dominated mountain range of the Jura Mountains as a separate formation that had not been included in the established stratigraphic system defined by Abraham Gottlob Werner, he named it "Jura-Kalkstein" in 1799.
The name "Jura" is derived from the Celtic root *jor via Gaulish *iuris "wooded mountain", borrowed into Latin as a place name, evolved into Juria and Jura. The Jurassic period is divided into three epochs: Early and Late. In stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, Upper Jurassic series of rock formations known as Lias and Malm in Europe; the separation of the term Jurassic into three sections originated with Leopold von Buch. The faunal stages from youngest to oldest are: During the early Jurassic period, the supercontinent Pangaea broke up into the northern supercontinent Laurasia and the southern supercontinent Gondwana; the Jurassic North Atlantic Ocean was narrow, while the South Atlantic did not open until the following Cretaceous period, when Gondwana itself rifted apart. The Tethys Sea closed, the Neotethys basin appeared. Climates were warm, with no evidence of a glacier having appeared; as in the Triassic, there was no land over either pole, no extensive ice caps existed.
The Jurassic geological record is good in western Europe, where extensive marine sequences indicate a time when much of that future landmass was submerged under shallow tropical seas. In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface. Though the epicontinental Sundance Sea left marine deposits in parts of the northern plains of the United States and Canada during the late Jurassic, most exposed sediments from this period are continental, such as the alluvial deposits of the Morrison Formation; the Jurassic was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus common, along with calcitic ooids, calcitic cements, invertebrate faunas with dominantly calcitic skeletons; the first of several massive batholiths were emplaced in the northern American cordillera beginning in the mid-Jurassic, marking the Nevadan orogeny. Important Jurassic exposures are found in Russia, South America, Japan and the United Kingdom.
In Africa, Early Jurassic strata are distributed in a similar fashion to Late Triassic beds, with more common outcrops in the south and less common fossil beds which are predominated by tracks to the north. As the Jurassic proceeded and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa. Middle Jurassic strata are neither well studied in Africa. Late Jurassic strata are poorly represented apart from the spectacular Tendaguru fauna in Tanzania; the Late Jurassic life of Tendaguru is similar to that found in western North America's Morrison Formation. During the Jurassic period, the primary vertebrates living in the sea were marine reptiles; the latter include ichthyosaurs, which were at the peak of their diversity, plesiosaurs and marine crocodiles of the families Teleosauridae and Metriorhynchidae. Numerous turtles could be found in rivers. In the invertebrate world, several new groups appeared, including rudists (a reef-formi
Vascular plants known as tracheophytes, form a large group of plants that are defined as those land plants that have lignified tissues for conducting water and minerals throughout the plant. They have a specialized non-lignified tissue to conduct products of photosynthesis. Vascular plants include the clubmosses, ferns and angiosperms. Scientific names for the group include Tracheophyta and Equisetopsida sensu lato; the term higher plants should be avoided as a synonym for vascular plants as it is a remnant of the abandoned concept of the great chain of being. Vascular plants are defined by three primary characteristics: Vascular plants have vascular tissues which distribute resources through the plant; this feature allows vascular plants to evolve to a larger size than non-vascular plants, which lack these specialized conducting tissues and are thereby restricted to small sizes. In vascular plants, the principal generation phase is the sporophyte, which produce spores and is diploid. By contrast, the principal generation phase in non-vascular plants is the gametophyte, which produces gametes and is haploid.
They have true roots and stems if one or more of these traits are secondarily lost in some groups. The formal definition of the division Tracheophyta encompasses both these characteristics in the Latin phrase "facies diploida xylem et phloem instructa". One possible mechanism for the presumed switch from emphasis on the haploid generation to emphasis on the diploid generation is the greater efficiency in spore dispersal with more complex diploid structures. In other words, elaboration of the spore stalk enabled the production of more spores, enabled the development of the ability to release them higher and to broadcast them farther; such developments may include more photosynthetic area for the spore-bearing structure, the ability to grow independent roots, woody structure for support, more branching. A proposed phylogeny of the vascular plants after Kenrick and Crane is as follows, with modification to the gymnosperms from Christenhusz et al. Pteridophyta from Smith et al. and lycophytes and ferns by Christenhusz et al.
This phylogeny is supported by several molecular studies. Other researchers state that taking fossils into account leads to different conclusions, for example that the ferns are not monophyletic. Water and nutrients in the form of inorganic solutes are drawn up from the soil by the roots and transported throughout the plant by the xylem. Organic compounds such as sucrose produced by photosynthesis in leaves are distributed by the phloem sieve tube elements; the xylem consists of vessels in flowering plants and tracheids in other vascular plants, which are dead hard-walled hollow cells arranged to form files of tubes that function in water transport. A tracheid cell wall contains the polymer lignin; the phloem however consists of living cells called sieve-tube members. Between the sieve-tube members are sieve plates, which have pores to allow molecules to pass through. Sieve-tube members lack such organs as nuclei or ribosomes, but cells next to them, the companion cells, function to keep the sieve-tube members alive.
The most abundant compound in all plants, as in all cellular organisms, is water which serves an important structural role and a vital role in plant metabolism. Transpiration is the main process of water movement within plant tissues. Water is transpired from the plant through its stomata to the atmosphere and replaced by soil water taken up by the roots; the movement of water out of the leaf stomata creates a transpiration pull or tension in the water column in the xylem vessels or tracheids. The pull is the result of water surface tension within the cell walls of the mesophyll cells, from the surfaces of which evaporation takes place when the stomata are open. Hydrogen bonds exist between water molecules; the draw of water upwards may be passive and can be assisted by the movement of water into the roots via osmosis. Transpiration requires little energy to be used by the plant. Transpiration assists the plant in absorbing nutrients from the soil as soluble salts. Living root cells passively absorb water in the absence of transpiration pull via osmosis creating root pressure.
It is possible for there to be no evapotranspiration and therefore no pull of water towards the shoots and leaves. This is due to high temperatures, high humidity, darkness or drought. Xylem and phloem tissues are involved in the conduction processes within plants. Sugars are conducted throughout the plant in the phloem and other nutrients through the xylem. Conduction occurs from a source to a sink for each separate nutrient. Sugars are produced in the leaves by photosynthesis and transported to the growing shoots and roots for use in growth, cellular respiration or storage. Minerals are transported to the shoots to allow cell division and growth. Fern allies Non-vascular plant “Higher plants” or “vascular plants”
A gametophyte is one of the two alternating phases in the life cycle of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes; the gametophyte is the sexual phase in the life cycle of plants and algae. It develops sex organs that produce gametes, haploid sex cells that participate in fertilization to form a diploid zygote which has a double set of chromosomes. Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte; the sporophyte can produce haploid spores by meiosis. In some multicellular green algae, red algae and brown algae and gametophytes may be externally indistinguishable. In Ulva the gametes are isogamous, all of one size and general morphology. In land plants, anisogamy is universal; as in animals and male gametes are called eggs and sperm. In extant land plants, either the sporophyte or the gametophyte may be reduced. In bryophytes, the gametophyte is the most visible stage of the life cycle.
The bryophyte gametophyte is longer lived, nutritionally independent, the sporophytes are attached to the gametophytes and dependent on them. When a moss spore germinates it grows to produce a filament of cells; the mature gametophyte of mosses develops into leafy shoots that produce sex organs that produce gametes. Eggs develop in sperm in antheridia. In some bryophyte groups such as many liverworts of the order Marchantiales, the gametes are produced on specialized structures called gametophores. All vascular plants are sporophyte dominant, a trend toward smaller and more sporophyte-dependent female gametophytes is evident as land plants evolved towards reproduction by seeds. Vascular plants such as ferns that produce only one type of spore are said to be homosporous, they have exosporic gametophytes—that is, the gametophyte is free-living and develops outside of the spore wall. Exosporic gametophytes can either be bisexual, capable of producing both sperm and eggs in the same thallus, or specialized into separate male and female organisms.
In heterosporous vascular plants, the gametophyte develops endosporically, within the spore wall. These gametophytes are dioicous, producing eggs but not both. In most ferns, for example, in the leptosporangiate fern Dryopteris, the gametophyte is a photosynthetic free living autotrophic organism called a prothallus that produces gametes and maintains the sporophyte during its early multicellular development. However, in some groups, notably the clade that includes Ophioglossaceae and Psilotaceae, the gametophytes are subterranean and subsist by forming mycotrophic relationships with fungi. Extant lycophytes produce two different types of gametophytes. In the families Lycopodiaceae and Huperziaceae, spores germinate into free-living and mycotrophic gametophytes that derive nutrients from symbiosis with fungi. In Isoetes and Selaginella, which are heterosporous, the megaspore remains attached to the parent sporophyte and a reduced megagametophyte develops inside. At maturity, the megaspore cracks open at the trilete suture to allow the male gametes to access the egg cells in the archegonia inside.
The gametophytes of Isoetes appear to be similar in this respect to those of the extinct Carboniferous giant arborescent clubmosses and Lepidostrobus. The seed plants are heterosporous; the gametophytes develop into multicellular organisms while still enclosed within the spore wall, the megaspores are retained within the sporangium. In plants with heteromorphic gametophytes, there are two distinct kinds of gametophytes; because the two gametophytes differ in form and function, they are termed heteromorphic, from hetero- "different" and morph "form". The egg producing gametophyte is known as a megagametophyte, because it is larger, the sperm producing gametophyte is known as a microgametophyte. Gametophytes which produce egg and sperm on separate plants are termed dioicous. In heterosporous plants, there are two distinct sporangia, each of which produces a single kind of spore and single kind of gametophyte. However, not all heteromorphic gametophytes come from heterosporous plants; that is, some plants have distinct egg-producing and sperm-producing gametophytes, but these gametophytes develop from the same kind of spore inside the same sporangium.
In the seed plants, the microgametophyte is called pollen. Seed plant microgametophytes consists of two or three cells when the pollen grains exit the sporangium; the megagametophyte develops within the megaspore of extant seedless vascular plants and within the megasporangium in a cone or flower in seed plants. In seed plants, the microgametophyte travels to the vicinity of the egg cell, produces two sperm by mitosis. In gymnosperms the megagametophyte consists of several thousand cells and produces one to several archegonia, each with a single egg cell; the gametophyte becomes a food storage tissue in the seed. In angiosperms, the megagametophyte is reduced to only a few nuclei and cells, is sometimes called the embryo sac. A typical embryo sac contains seven cells and eight nuclei, one of, the egg cell. Two nuclei fuse with a sperm nucleus to form the endosperm, which becomes the food storage tissue in the seed. Sporophyte Alternation of generations Arc
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