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Precambrian time

Currently, seven phyla are proposed: As in the related chrytrids, neocallimastigomycetes form zoospores that are posteriorly uniflagellate or polyflagellate.

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Evolution of the atmosphere and ocean

These organisms are classified as a kingdom , Fungi , which is separate from the other eukaryotic life kingdoms of plants and animals. A characteristic that places fungi in a different kingdom from plants, bacteria, and some protists is chitin in their cell walls. Similar to animals, fungi are heterotrophs ; they acquire their food by absorbing dissolved molecules, typically by secreting digestive enzymes into their environment.

Fungi do not photosynthesise. Growth is their means of mobility, except for spores a few of which are flagellated , which may travel through the air or water. Fungi are the principal decomposers in ecological systems.

These and other differences place fungi in a single group of related organisms, named the Eumycota true fungi or Eumycetes , which share a common ancestor form a monophyletic group , an interpretation that is also strongly supported by molecular phylogenetics. This fungal group is distinct from the structurally similar myxomycetes slime molds and oomycetes water molds.

In the past, mycology was regarded as a branch of botany , although it is now known fungi are genetically more closely related to animals than to plants. Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, and their cryptic lifestyles in soil or on dead matter. Fungi include symbionts of plants, animals, or other fungi and also parasites. They may become noticeable when fruiting , either as mushrooms or as molds.

Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange in the environment.

They have long been used as a direct source of human food, in the form of mushrooms and truffles ; as a leavening agent for bread; and in the fermentation of various food products, such as wine , beer , and soy sauce. Since the s, fungi have been used for the production of antibiotics , and, more recently, various enzymes produced by fungi are used industrially and in detergents.

Fungi are also used as biological pesticides to control weeds, plant diseases and insect pests. Many species produce bioactive compounds called mycotoxins , such as alkaloids and polyketides , that are toxic to animals including humans. The fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies.

Fungi can break down manufactured materials and buildings, and become significant pathogens of humans and other animals.

Losses of crops due to fungal diseases e. The fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, and morphologies ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of Kingdom Fungi, which has been estimated at 2.

Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, which has sometimes challenged the historical groupings based on morphology and other traits. Phylogenetic studies published in the last decade have helped reshape the classification within Kingdom Fungi, which is divided into one subkingdom , seven phyla , and ten subphyla. The English word fungus is directly adopted from the Latin fungus mushroom , used in the writings of Horace and Pliny.

Before the introduction of molecular methods for phylogenetic analysis, taxonomists considered fungi to be members of the plant kingdom because of similarities in lifestyle: Like plants, fungi often grow in soil and, in the case of mushrooms , form conspicuous fruit bodies , which sometimes resemble plants such as mosses. The fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged around one billion years ago around the start of the Neoproterozoic Era.

Most fungi lack an efficient system for the long-distance transport of water and nutrients, such as the xylem and phloem in many plants.

To overcome this limitation, some fungi, such as Armillaria , form rhizomorphs , [29] which resemble and perform functions similar to the roots of plants. As eukaryotes, fungi possess a biosynthetic pathway for producing terpenes that uses mevalonic acid and pyrophosphate as chemical building blocks. Fungi have a worldwide distribution, and grow in a wide range of habitats, including extreme environments such as deserts or areas with high salt concentrations [33] or ionizing radiation , [34] as well as in deep sea sediments.

This organism spends part of its life cycle as a motile zoospore , enabling it to propel itself through water and enter its amphibian host. Around , species of fungi have been described by taxonomists , [39] but the global biodiversity of the fungus kingdom is not fully understood.

Classification based on morphological characteristics, such as the size and shape of spores or fruiting structures, has traditionally dominated fungal taxonomy. The biological species concept discriminates species based on their ability to mate.

The application of molecular tools, such as DNA sequencing and phylogenetic analysis, to study diversity has greatly enhanced the resolution and added robustness to estimates of genetic diversity within various taxonomic groups. Mycology is the branch of biology concerned with the systematic study of fungi, including their genetic and biochemical properties, their taxonomy, and their use to humans as a source of medicine, food, and psychotropic substances consumed for religious purposes, as well as their dangers, such as poisoning or infection.

The field of phytopathology , the study of plant diseases, is closely related because many plant pathogens are fungi. The use of fungi by humans dates back to prehistory; Ötzi the Iceman , a well-preserved mummy of a 5,year-old Neolithic man found frozen in the Austrian Alps, carried two species of polypore mushrooms that may have been used as tinder Fomes fomentarius , or for medicinal purposes Piptoporus betulinus.

Some of the oldest written records contain references to the destruction of crops that were probably caused by pathogenic fungi. Mycology is a relatively new science that became systematic after the development of the microscope in the 17th century. Although fungal spores were first observed by Giambattista della Porta in , the seminal work in the development of mycology is considered to be the publication of Pier Antonio Micheli 's work Nova plantarum genera.

Later, Elias Magnus Fries — further elaborated the classification of fungi, using spore color and microscopic characteristics, methods still used by taxonomists today.

Buller , Curtis G. Lloyd , and Pier Andrea Saccardo. The 20th century has seen a modernization of mycology that has come from advances in biochemistry , genetics , molecular biology , and biotechnology. The use of DNA sequencing technologies and phylogenetic analysis has provided new insights into fungal relationships and biodiversity , and has challenged traditional morphology-based groupings in fungal taxonomy.

Hyphae grow at their tips apices ; new hyphae are typically formed by emergence of new tips along existing hyphae by a process called branching , or occasionally growing hyphal tips fork, giving rise to two parallel-growing hyphae.

These growth processes lead to the development of a mycelium , an interconnected network of hyphae. Septate hyphae are divided into compartments separated by cross walls internal cell walls, called septa, that are formed at right angles to the cell wall giving the hypha its shape , with each compartment containing one or more nuclei; coenocytic hyphae are not compartmentalized.

Many species have developed specialized hyphal structures for nutrient uptake from living hosts; examples include haustoria in plant-parasitic species of most fungal phyla, and arbuscules of several mycorrhizal fungi, which penetrate into the host cells to consume nutrients. Although fungi are opisthokonts —a grouping of evolutionarily related organisms broadly characterized by a single posterior flagellum —all phyla except for the chytrids have lost their posterior flagella.

Fungal mycelia can become visible to the naked eye, for example, on various surfaces and substrates , such as damp walls and spoiled food, where they are commonly called molds. Mycelia grown on solid agar media in laboratory petri dishes are usually referred to as colonies.

These colonies can exhibit growth shapes and colors due to spores or pigmentation that can be used as diagnostic features in the identification of species or groups. The apothecium —a specialized structure important in sexual reproduction in the ascomycetes—is a cup-shaped fruit body that is often macroscopic and holds the hymenium , a layer of tissue containing the spore-bearing cells. The growth of fungi as hyphae on or in solid substrates or as single cells in aquatic environments is adapted for the efficient extraction of nutrients, because these growth forms have high surface area to volume ratios.

The mechanical pressure exerted by the appressorium is generated from physiological processes that increase intracellular turgor by producing osmolytes such as glycerol. The fungi are traditionally considered heterotrophs , organisms that rely solely on carbon fixed by other organisms for metabolism. Fungi have evolved a high degree of metabolic versatility that allows them to use a diverse range of organic substrates for growth, including simple compounds such as nitrate , ammonia , acetate , or ethanol.

This form of "radiotrophic" growth has been described for only a few species, the effects on growth rates are small, and the underlying biophysical and biochemical processes are not well known. Fungal reproduction is complex, reflecting the differences in lifestyles and genetic makeup within this diverse kingdom of organisms.

These structures aid reproduction by efficiently dispersing spores or spore-containing propagules. Asexual reproduction occurs via vegetative spores conidia or through mycelial fragmentation. Mycelial fragmentation occurs when a fungal mycelium separates into pieces, and each component grows into a separate mycelium.

Mycelial fragmentation and vegetative spores maintain clonal populations adapted to a specific niche , and allow more rapid dispersal than sexual reproduction. Sexual reproduction with meiosis has been directly observed in all fungal phyla except Glomeromycota [78] genetic analysis suggests meiosis in Glomeromycota as well. It differs in many aspects from sexual reproduction in animals or plants. Differences also exist between fungal groups and can be used to discriminate species by morphological differences in sexual structures and reproductive strategies.

Some species may allow mating only between individuals of opposite mating type , whereas others can mate and sexually reproduce with any other individual or itself. Species of the former mating system are called heterothallic , and of the latter homothallic. Most fungi have both a haploid and a diploid stage in their life cycles.

In sexually reproducing fungi, compatible individuals may combine by fusing their hyphae together into an interconnected network; this process, anastomosis , is required for the initiation of the sexual cycle. Many ascomycetes and basidiomycetes go through a dikaryotic stage, in which the nuclei inherited from the two parents do not combine immediately after cell fusion, but remain separate in the hyphal cells see heterokaryosis. In ascomycetes, dikaryotic hyphae of the hymenium the spore-bearing tissue layer form a characteristic hook at the hyphal septum.

During cell division , formation of the hook ensures proper distribution of the newly divided nuclei into the apical and basal hyphal compartments. An ascus plural asci is then formed, in which karyogamy nuclear fusion occurs. Asci are embedded in an ascocarp , or fruiting body. Karyogamy in the asci is followed immediately by meiosis and the production of ascospores. After dispersal, the ascospores may germinate and form a new haploid mycelium. Sexual reproduction in basidiomycetes is similar to that of the ascomycetes.

Compatible haploid hyphae fuse to produce a dikaryotic mycelium. However, the dikaryotic phase is more extensive in the basidiomycetes, often also present in the vegetatively growing mycelium.

A specialized anatomical structure, called a clamp connection , is formed at each hyphal septum. As with the structurally similar hook in the ascomycetes, the clamp connection in the basidiomycetes is required for controlled transfer of nuclei during cell division, to maintain the dikaryotic stage with two genetically different nuclei in each hyphal compartment. In glomeromycetes formerly zygomycetes , haploid hyphae of two individuals fuse, forming a gametangium , a specialized cell structure that becomes a fertile gamete -producing cell.

The gametangium develops into a zygospore , a thick-walled spore formed by the union of gametes. When the zygospore germinates, it undergoes meiosis , generating new haploid hyphae, which may then form asexual sporangiospores. These sporangiospores allow the fungus to rapidly disperse and germinate into new genetically identical haploid fungal mycelia. Both asexual and sexual spores or sporangiospores are often actively dispersed by forcible ejection from their reproductive structures.

This ejection ensures exit of the spores from the reproductive structures as well as traveling through the air over long distances.

Specialized mechanical and physiological mechanisms, as well as spore surface structures such as hydrophobins , enable efficient spore ejection. The bird's nest fungi use the force of falling water drops to liberate the spores from cup-shaped fruiting bodies. Besides regular sexual reproduction with meiosis, certain fungi, such as those in the genera Penicillium and Aspergillus , may exchange genetic material via parasexual processes, initiated by anastomosis between hyphae and plasmogamy of fungal cells.

It is known to play a role in intraspecific hybridization [94] and is likely required for hybridization between species, which has been associated with major events in fungal evolution. In contrast to plants and animals , the early fossil record of the fungi is meager. Factors that likely contribute to the under-representation of fungal species among fossils include the nature of fungal fruiting bodies , which are soft, fleshy, and easily degradable tissues and the microscopic dimensions of most fungal structures, which therefore are not readily evident.

Fungal fossils are difficult to distinguish from those of other microbes, and are most easily identified when they resemble extant fungi. It is presumed that the fungi colonized the land during the Cambrian — Fungal fossils do not become common and uncontroversial until the early Devonian — Some time after the Permian—Triassic extinction event Although commonly included in botany curricula and textbooks, fungi are more closely related to animals than to plants and are placed with the animals in the monophyletic group of opisthokonts.

These current phylogenetic analyses often overturn classifications based on older and sometimes less discriminative methods based on morphological features and biological species concepts obtained from experimental matings. There is no unique generally accepted system at the higher taxonomic levels and there are frequent name changes at every level, from species upwards. Efforts among researchers are now underway to establish and encourage usage of a unified and more consistent nomenclature.

Web sites such as Index Fungorum and ITIS list current names of fungal species with cross-references to older synonyms. The classification of Kingdom Fungi is the result of a large-scale collaborative research effort involving dozens of mycologists and other scientists working on fungal taxonomy. The accompanying cladogram depicts the major fungal taxa and their relationship to opisthokont and unikont organisms, based on the work of Philippe Silar, [] "The Mycota: The major phyla sometimes called divisions of fungi have been classified mainly on the basis of characteristics of their sexual reproductive structures.

Currently, seven phyla are proposed: Microsporidia, Chytridiomycota, Blastocladiomycota, Neocallimastigomycota, Glomeromycota, Ascomycota, and Basidiomycota. Phylogenetic analysis has demonstrated that the Microsporidia , unicellular parasites of animals and protists, are fairly recent and highly derived endobiotic fungi living within the tissue of another species. The Chytridiomycota are commonly known as chytrids.

These fungi are distributed worldwide. Chytrids and their close relatives Neocallimastigomycota and Blastocladiomycota below are the only fungi with active motility, producing zoospores that are capable of active movement through aqueous phases with a single flagellum , leading early taxonomists to classify them as protists. Molecular phylogenies , inferred from rRNA sequences in ribosomes , suggest that the Chytrids are a basal group divergent from the other fungal phyla, consisting of four major clades with suggestive evidence for paraphyly or possibly polyphyly.

The Blastocladiomycota were previously considered a taxonomic clade within the Chytridiomycota. The chert dates to 3. Some of these species were early photosynthesizers , whereas others had metabolic processes that relied on methane cycling. Such diversity suggests that the first forms of life were much older than the chert in which they were discovered, possibly as old as 4 billion years.

A much younger, but no less fascinating, collection of microfossils occurred in the 2. These beds are notable because they contain carbonaceous columnar microfossils up to 7 mm slightly less than 0. They probably extracted gold from their environment in much the same way that modern fungi and lichens do. Stromatolites are stratiform, domal, or columnar structures made from sheetlike mats precipitated by communities of microorganisms, particularly filamentous blue-green algae.

The early Archean examples form domes as tall as about 10 cm 4 inches. Stromatolites continued to form all the way through the geologic record and today grow in warm intertidal waters, as exemplified by those of Shark Bay in Western Australia. They provide indisputable evidence that life had begun on Earth using algal photosynthesis in complex, integrated biological communities by 3.

These Archean organisms were prokaryotes that were incapable of cell division. The prokaryotes were predominant until about 1. The latter made use of oxygen in metabolism and for growth and thus developed profusely in the increasingly oxygen-rich atmosphere of the early Proterozoic the Proterozoic Eon extended from 2. The eukaryotes were capable of cell division, which allowed DNA deoxyribonucleic acid , the genetic coding material, to be passed on to succeeding generations.

By early Proterozoic time both microfossils and stromatolites had proliferated. The best-known occurrence of microorganisms is in the 2-billion-year-old stromatolite-bearing Gunflint iron formation in the Huronian Basin of southern Ontario. These microbial fossils include some 30 different types with spheroidal, filamentous, and sporelike forms up to about 20 micrometres 0.

Sixteen species in 14 genera have been classified so far. Microfossils of this kind are abundant, contain beautifully preserved organic matter, and are extremely similar to such present-day microorganisms as blue-green algae and microbacteria.

These microbiota lived at the time of the transition in the chemical composition of the atmosphere when oxygen began accumulating for the first time. During the late Proterozoic, stromatolites reached their peak of development, became distributed worldwide, and diversified into complex, branching forms. From about million years ago, however, they began to decline significantly in number.

Possibly the newly arrived metazoans multicelled organisms whose cells are differentiated into tissues and organs ate the stromatolitic algae, and their profuse growth destroyed the habitats of the latter. There is the intriguing question as to when sexual division arose in life-forms. In the late s, American paleobiologist J. William Schopf pointed out that the abundant microflora of the million-year-old Bitter Springs Formation of central Australia includes some eukaryotic algae that have cells in various stages of division arranged into tetrahedral sporelike forms.

These resemble the tetrad of spore cells of living plants known to develop by sexual division. In effect, by the end of the Precambrian the conditions were set for the explosion of life at the start of the Phanerozoic Eon which extends from about million years ago to the present.

Metazoans developed rapidly from the beginning of the Cambrian, when organisms acquired the ability to produce the protein collagen and, thus, skeletons and shells. However, more-primitive metazoans without skeletons—the Ediacara fauna —appeared earlier more than million years ago , after the end of the Varanger-Marinoan ice age at million years ago and before the onset of the Cambrian Period at million years ago.

They are found as impressions of soft-bodied, multicellular animals in the rocks and have the form of tiny blobs, circular discs, or plantlike fronds ranging from less than 1 cm less than 0. The type locality is the Ediacara Hills in South Australia, where over 1, well-preserved specimens have been collected, resulting in the naming of more than 60 species and 30 genera.

They occur in a quartzite that is stratigraphically situated some metres 1, feet below the base of the Cambrian System. These organisms resemble modern jellyfish , worms , sponges , seaweed , sea anemones , and sea pens. Comparable impressions in the youngest Precambrian sediments have been found in over 30 localities from every continent except Antarctica. Ediacaran fossils have been deposited in environments ranging from tidal marine habitats to the deep seafloor.

Some forms show evidence of sophisticated adaptations , such as the use of multiple modes of reproduction. The Ediacaran organisms were probably the ancestors of shelled organisms that mark the beginning of the Phanerozoic. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind.

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Page 1 of 2. Next page Precambrian geology. Learn More in these related Britannica articles: There are also several smaller platforms that were deformed to a…. The Precambrian portion of the record, prior to ,, years ago, is complicated by the widely metamorphosed character of the surviving rocks, although even here the typical cross-bedding of shallow-water sands can be recognized in many places.

The Cambrian and post-Cambrian succession of the last ,,…. This greatly reduced the net fixation of CO 2 into sugars…. The pre-Phanerozoic is a poorly understood phase of Earth system history. Much of the sedimentary record of the atmosphere, oceans, biota, and crust of the early Earth has been obliterated…. More About Precambrian time 12 references found in Britannica articles Assorted References geological time scale In geochronology: Development of radioactive dating methods and their application geological development of Appalachian Mountains In Appalachian Mountains: Geology Asia In Asia: They can destroy foliage faster than plants can grow, thus killing even fairly large plants.

They also feed on fruits and vegetables prior to harvest, making holes in the crop, which can make individual items unsuitable to sell for aesthetic reasons, and can make the crop more vulnerable to rot and disease.

As control measures, baits are the norm in both agriculture and the garden. In recent years, iron phosphate baits have emerged and are preferred over the more toxic metaldehyde , especially because domestic or wild animals may be exposed to the bait. The environmentally safer iron phosphate has been shown to be at least as effective as baits. Parasitic nematodes Phasmarhabditis hermaphrodita are a commercially available biological control method that are effective against a wide range of common slug species.

The nematodes are applied in water and actively seek out slugs in the soil and infect them, leading to the death of the slug. This control method is suitable for use in organic growing systems.

Other slug control methods are generally ineffective on a large scale, but can be somewhat useful in small gardens. These include beer traps, [41] [42] diatomaceous earth , crushed eggshells, coffee grounds, and copper.

It is of scientific interest that salt kills slugs by causing water to leave the body owing to osmosis [43] but this is not used for agricultural control as soil salinity is detrimental to crops.

In a few rare cases, humans have developed Angiostrongylus cantonensis -induced meningitis from eating raw slugs. Belocaulus angustipes mating; note the white penis extending to the reproduction pore on the underside of the smaller slug.

A banana slug eating a fly agaric, Amanita muscaria. From Wikipedia, the free encyclopedia. For marine slugs, see Sea slug. For snails, see Snail. For other uses, see Slug disambiguation. Various species of British land slugs, including from the top the larger drawings: Arion ater , Kerry slug , Limax maximus and Limax flavus. The ghost slug Selenochlamys ysbryda. Ambigolimax from Calicut Kerala India.

Gastropoda and their impact on phylogenetic relationships". Journal of Experimental Biology. The slug families Limacidae, Arionidae, Veronicellidae, and Philomycidae". Proceedings of the Oklahoma Academy of Science. Pulmonata with a checklist of nominal species". Redescription and Comparison with Valiguna siamensis Martens, Gastropoda: Retrieved 14 December Archived from the original on 13 April Retrieved 15 August Trends in Ecology and Evolution. University of Michigan Museum of Zoology. Biology, ecology and control.

Boundary Row, London, UK: Archived from the original on 21 May Archived from the original PDF on 25 April Archived from the original on 2 April Investigations using monoclonal antibodies". Journal of Natural History. Archived from the original PDF on 5 July The Medical Journal of Australia 8: Part 8, pages 1— Monograph of the land and freshwater Mollusca of the British Isles. Annals of the Entomological Society of America.

Archived from the original on 3 December Dipteras as predators and parasitoids of terrestrial gastropods, with emphasis on Phoridae, Calliphoridae, Sarcophagidae, Muscidae and Fannidae". Natural Enemies of Terrestrial Molluscs.

Archived from the original PDF on 19 July Die Landschnecken Nord- und Mitteleuropas in German.

The Precambrian environment

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