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
In alpha taxonomy, a grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a phylogenetic unit. An evolutionary grade is a group of species united by morphological or physiological traits, that has given rise to another group that has major differences from the ancestral condition, is thus not considered part of the ancestral group, while still having enough similarities that we can group them under the same clade; the ancestral group will not be phylogenetically complete. In order to understand evolutionary grades, one must first get a better understanding of Phylogenetics, defined as "In biology, is the study of the evolutionary history and relationships among individuals or groups of organisms; these relationships are discovered through phylogenetic inference methods that evaluate observed heritable traits, such as DNA sequences or morphology under a model of evolution of these traits" The most cited example is that of reptiles.
In the early 19th century, the French naturalist Latreille was the first to divide tetrapods into the four familiar classes of amphibians, reptiles and mammals. In this system, reptiles are characterized by traits such as laying membranous or shelled eggs, having skin covered in scales or scutes, having a'cold-blooded' metabolism. However, the ancestors of mammals and birds had these traits and so birds and mammals can be said to "have evolved from reptiles", making the reptiles, when defined by these traits, a grade rather than a clade. In microbiology, taxa that are thus seen as excluded from their evolutionary grade parent group are called taxa in disguise. Paraphyletic taxa will but not always, represent evolutionary grades. In some cases paraphyletic taxa are united by not being part of any other groups, give rise to so-called wastebasket taxa which may be polyphyletic; the traditional Linnaean way of defining taxa is through the use of anatomical traits. When the actual phylogenetic relationship is unknown, well defined groups sometimes turn out to be defined by traits that are primitive rather than derived.
In Linnaean systematics, evolutionary grades are accepted in higher taxonomic ranks, though avoided at family level and below. In phylogenetic nomenclature evolutionary grades are not accepted. Where information about phylogenetic relationships is available, organisms are preferentially grouped into clades. Where data is lacking, or groups of uncertain relationship are to be compared, the cladistic method is limited and grade provides a useful tool for comparing organisms; this is common in palaeontology, where fossils are fragmentary and difficult to interpret. Thus, traditional palaeontological works are using evolutionary grades as formal or informal taxa, including examples such as labyrinthodonts, synapsids, ammonites, eurypterids and many of the more well known taxa of human evolution. Organizing organisms into grades rather than strict clades can be useful to understand the evolutionary sequence behind major diversification of both animals and plants. Evolutionary grades, being united by gross morphological traits, are eminently recognizable in the field.
While taxonomy seeks to eliminate paraphyletic taxa, such grades are sometimes kept as formal or informal groups on the basis of their usefulness for laymen and field researchers. In bacteriology, the renaming of species or groups that turn out to be evolutionary grades is kept to a minimum to avoid misunderstanding, which in the case of pathogens could have fatal consequences; when referring to a group of organisms, the term "grade" is enclosed in quotation marks to denote its status as a paraphyletic term. With the rise of phylogenetic nomenclature, the use of evolutionary grades as formal taxa has come under debate. Under a strict phylogenetic approach, only monophyletic taxa are recognized; this differs from the more traditional approach of evolutionary taxonomy. The difference in approach has led to a vigorous debate between proponents of the two approaches to taxonomy in well established fields like vertebrate palaeontology and botany; the difference between the statement "B is part of A" and "B has evolved from A" is, one of semantics rather than of phylogeny.
Both express the same phylogeny, but the former emphasizes the phylogenetic continuum while the latter emphasizes a distinct shift in anatomy or ecology in B relative to A. Fish represent a grade, inasmuch as they have given rise to the land vertebrates. In fact, the three traditional classes of fish all represent evolutionary grades. Amphibians in the biological sense represent a grade, in that they are the ancestors of the amniotes. Reptiles are composed of the cold-blooded amniotes, this excludes mammals. Dinosaurs were proposed to be the ancestors of birds as early as the 1860s, yet the term sees popular use as an evolutionary grade excluding birds, though most scientists use a monophyletic Dinosauria. Lizards as a unit represent an evolutionary grade, defined by their retention of limbs relative to snakes and Amphisbaenans. However, defining lizards by the presence of limbs is incorrect, as there are many species of legless lizards, which are considered true lizards. Green represent a grade, in that they are the ancestors of land plants.
Prokaryotes, which include cellular organisms lacking a nucleus, represent a grade, in that they are the ancestors of eukary
An earthworm is a tube-shaped, segmented worm found in the phylum Annelida. They have a world-wide distribution and are found living in soil, feeding on live and dead organic matter. An earthworm's digestive system runs through the length of its body, it conducts respiration through its skin. It has a double transport system composed of coelomic fluid that moves within the fluid-filled coelom and a simple, closed blood circulatory system, it has a peripheral nervous system. The central nervous system consists of two ganglia above the mouth, one on either side, connected to a nerve cord running back along its length to motor neurons and sensory cells in each segment. Large numbers of chemoreceptors are concentrated near its mouth. Circumferential and longitudinal muscles on the periphery of each segment enable the worm to move. Similar sets of muscles line the gut, their actions move the digesting food toward the worm's anus. Earthworms are hermaphrodites: each individual carries both male and female sex organs.
As invertebrates, they lack either an internal skeleton or exoskeleton, but maintain their structure with fluid-filled coelom chambers that function as a hydrostatic skeleton. "Earthworm" is the common name for the largest members of Oligochaeta. In classical systems, they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, though the internal male segments are anterior to the female. Theoretical cladistic studies have placed them, instead, in the suborder Lumbricina of the order Haplotaxida, but this may again soon change. Folk names for the earthworm include "dew-worm", "rainworm", "night crawler", "angleworm". Larger terrestrial earthworms are called megadriles, as opposed to the microdriles in the semiaquatic families Tubificidae and Enchytraeidae, among others; the megadriles are characterized by having a distinct clitellum and a vascular system with true capillaries. Depending on the species, an adult earthworm can be from 10 mm long and 1 mm wide to 3 m long and over 25 mm wide, but the typical Lumbricus terrestris grows to about 360 mm long.
The longest worm on confirmed records is Amynthas mekongianus that extends up to 3 m in the mud along the banks of the 4,350 km Mekong River in Southeast Asia. From front to back, the basic shape of the earthworm is a cylindrical tube, divided into a series of segments that compartmentalize the body. Furrows are externally visible on the body demarking the segments. Except for the mouth and anal segments, each segment carries bristle-like hairs called lateral setae used to anchor parts of the body during movement. Special ventral setae are used to anchor mating earthworms by their penetration into the bodies of their mates. Within a species, the number of segments found is consistent across specimens, individuals are born with the number of segments they will have throughout their lives; the first body segment features both the earthworm's mouth and, overhanging the mouth, a fleshy lobe called the prostomium, which seals the entrance when the worm is at rest, but is used to feel and chemically sense the worm's surroundings.
Some species of earthworm can use the prehensile prostomium to grab and drag items such as grasses and leaves into their burrow. An adult earthworm develops a belt-like glandular swelling, called the clitellum, which covers several segments toward the front part of the animal; this produces egg capsules. The posterior is most cylindrical like the rest of the body, but depending on the species, may be quadrangular, trapezoidal, or flattened; the last segment is called the periproct. The exterior of an individual segment is a thin cuticle over skin pigmented red to brown, which has specialized cells that secrete mucus over the cuticle to keep the body moist and ease movement through soil. Under the skin is a layer of nerve tissue, two layers of muscles—a thin outer layer of circular muscle, a much thicker inner layer of longitudinal muscle. Interior to the muscle layer is a fluid-filled chamber called a coelom that by its pressurization provides structure to the worm's boneless body; the segments are separated from each other by septa which are perforated transverse walls, allowing the coelomic fluid to pass between segments.
A pair of structures called. This tubule leads to the main body fluid filtering organ, the nephridium or metanephridium, which removes metabolic waste from the coelomic fluid and expels it through pores called nephridiopores on the worm's sides. At the center of a worm is the digestive tract, which runs straight through from mouth to anus without coiling, is flanked above and below by blood vessels and the ventral nerve cord, is surrounded in each segment by a pair of pallial blood vessels that connect the dorsal to 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
In ecology, a niche is the match of a species to a specific environmental condition. It describes how an organism or population responds to the distribution of resources and competitors and how it in turn alters those same factors. "The type and number of variables comprising the dimensions of an environmental niche vary from one species to another the relative importance of particular environmental variables for a species may vary according to the geographic and biotic contexts". A Grinnellian niche is determined by the habitat in which a species lives and its accompanying behavioral adaptations. An Eltonian niche emphasizes that a species not only grows in and responds to an environment, it may change the environment and its behavior as it grows; the Hutchinsonian niche uses mathematics and statistics to try to explain how species coexist within a given community. The concept of ecological niche is central to ecological biogeography, which focuses on spatial patterns of ecological communities.
"Species distributions and their dynamics over time result from properties of the species, environmental variation... and interactions between the two—in particular the abilities of some species our own, to modify their environments and alter the range dynamics of many other species." Alteration of an ecological niche by its inhabitants is the topic of niche construction. The majority of species exist in a standard ecological niche, sharing behaviors and functional traits similar to the other related species within the same broad taxonomic class, but there are exceptions. A premier example of a non-standard niche filling species is the flightless, ground-dwelling kiwi bird of New Zealand, which feeds on worms and other ground creatures, lives its life in a mammal-like niche. Island biogeography can help associated unfilled niches; the ecological meaning of niche comes from the meaning of niche as a recess in a wall for a statue, which itself is derived from the Middle French word nicher, meaning to nest.
The term was coined by the naturalist Roswell Hill Johnson but Joseph Grinnell was the first to use it in a research program in 1917, in his paper "The niche relationships of the California Thrasher". The Grinnellian niche concept embodies the idea that the niche of a species is determined by the habitat in which it lives and its accompanying behavioral adaptations. In other words, the niche is the sum of the habitat requirements and behaviors that allow a species to persist and produce offspring. For example, the behavior of the California thrasher is consistent with the chaparral habitat it lives in—it breeds and feeds in the underbrush and escapes from its predators by shuffling from underbrush to underbrush. Its'niche' is defined by the felicitous complementing of the thrasher's behavior and physical traits with this habitat; this perspective of niche allows for the existence of empty niches. An ecological equivalent to an organism is an organism from a different taxonomic group exhibiting similar adaptations in a similar habitat, an example being the different succulents found in American and African deserts and euphorbia, respectively.
As another example, the anole lizards of the Greater Antilles are a rare example of convergent evolution, adaptive radiation, the existence of ecological equivalents: the anole lizards evolved in similar microhabitats independently of each other and resulted in the same ecomorphs across all four islands. In 1927 Charles Sutherland Elton, a British ecologist, defined a niche as follows: "The'niche' of an animal means its place in the biotic environment, its relations to food and enemies."Elton classified niches according to foraging activities: For instance there is the niche, filled by birds of prey which eat small animals such as shrews and mice. In an oak wood this niche is filled by tawny owls, while in the open grassland it is occupied by kestrels; the existence of this carnivore niche is dependent on the further fact that mice form a definite herbivore niche in many different associations, although the actual species of mice may be quite different. Conceptually, the Eltonian niche introduces the idea of a species' response to and effect on the environment.
Unlike other niche concepts, it emphasizes that a species not only grows in and responds to an environment based on available resources and climatic conditions, but changes the availability and behavior of those factors as it grows. In an extreme example, beavers require certain resources in order to survive and reproduce, but construct dams that alter water flow in the river where the beaver lives. Thus, the beaver affects the biotic and abiotic conditions of other species that live in and near the watershed. In a more subtle case, competitors that consume resources at different rates can lead to cycles in resource density that differ between species. Not only do species grow differently with respect to resource density, but their own population growth can affect resource density over time; the Hutchinsonian niche is an "n-dimensional hypervolume", where the dimensions are environmental conditions and resources, that define the requirements of an individual or a species to practice "its" way of life, more for its population to persist.
The "hypervolume" defines the multi-dimensional space of resources available to organisms, "all species
The Neogene is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period 23.03 million years ago to the beginning of the present Quaternary Period 2.58 Mya. The Neogene is sub-divided into two epochs, the earlier Miocene and the Pliocene; some geologists assert that the Neogene cannot be delineated from the modern geological period, the Quaternary. The term "Neogene" was coined in 1853 by the Austrian palaeontologist Moritz Hörnes. During this period and birds continued to evolve into modern forms, while other groups of life remained unchanged. Early hominids, the ancestors of humans, appeared in Africa near the end of the period; some continental movement took place, the most significant event being the connection of North and South America at the Isthmus of Panama, late in the Pliocene. This cut off the warm ocean currents from the Pacific to the Atlantic Ocean, leaving only the Gulf Stream to transfer heat to the Arctic Ocean; the global climate cooled over the course of the Neogene, culminating in a series of continental glaciations in the Quaternary Period that follows.
In ICS terminology, from upper to lower: The Pliocene Epoch is subdivided into 2 ages: Piacenzian Age, preceded by Zanclean AgeThe Miocene Epoch is subdivided into 6 ages: Messinian Age, preceded by Tortonian Age Serravallian Age Langhian Age Burdigalian Age Aquitanian AgeIn different geophysical regions of the world, other regional names are used for the same or overlapping ages and other timeline subdivisions. The terms Neogene System and upper Tertiary System describe the rocks deposited during the Neogene Period; the continents in the Neogene were close to their current positions. The Isthmus of Panama formed, connecting South America; the Indian subcontinent continued forming the Himalayas. Sea levels fell, creating land bridges between Africa and Eurasia and between Eurasia and North America; the global climate became seasonal and continued an overall drying and cooling trend which began at the start of the Paleogene. The ice caps on both poles began to grow and thicken, by the end of the period the first of a series of glaciations of the current Ice Age began.
Marine and continental flora and fauna have a modern appearance. The reptile group Choristodera became extinct in the early part of the period, while the amphibians known as Allocaudata disappeared at the end. Mammals and birds continued to be the dominant terrestrial vertebrates, took many forms as they adapted to various habitats; the first hominins, the ancestors of humans, may have appeared in southern Europe and migrated into Africa. In response to the cooler, seasonal climate, tropical plant species gave way to deciduous ones and grasslands replaced many forests. Grasses therefore diversified, herbivorous mammals evolved alongside it, creating the many grazing animals of today such as horses and bison. Eucalyptus fossil leaves occur in the Miocene of New Zealand, where the genus is not native today, but have been introduced from Australia; the Neogene traditionally ended at the end of the Pliocene Epoch, just before the older definition of the beginning of the Quaternary Period. However, there was a movement amongst geologists to include ongoing geological time in the Neogene, while others insist the Quaternary to be a separate period of distinctly different record.
The somewhat confusing terminology and disagreement amongst geologists on where to draw what hierarchical boundaries is due to the comparatively fine divisibility of time units as time approaches the present, due to geological preservation that causes the youngest sedimentary geological record to be preserved over a much larger area and to reflect many more environments than the older geological record. By dividing the Cenozoic Era into three periods instead of seven epochs, the periods are more comparable to the duration of periods in the Mesozoic and Paleozoic eras; the International Commission on Stratigraphy once proposed that the Quaternary be considered a sub-era of the Neogene, with a beginning date of 2.58 Ma, namely the start of the Gelasian Stage. In the 2004 proposal of the ICS, the Neogene would have consisted of the Miocene and Pliocene epochs; the International Union for Quaternary Research counterproposed that the Neogene and the Pliocene end at 2.58 Ma, that the Gelasian be transferred to the Pleistocene, the Quaternary be recognized as the third period in the Cenozoic, citing key changes in Earth's climate and biota that occurred 2.58 Ma and its correspondence to the Gauss-Matuyama magnetostratigraphic boundary.
In 2006 ICS and INQUA reached a compromise that made Quaternary a subera, subdividing Cenozoic into the old classical Tertiary and Quaternary, a compromise, rejected by International Union of Geological Sciences because it split both Neogene and Pliocene in two. Following formal discussions at the 2008 International Geological Congress in Oslo, the ICS decided in May 2009 to make the Quaternary the youngest period of the Cenozoic Era with its base at 2.58 Mya and including the Gelasian age, considered part of the Neogene Period and Pliocene Epoch. Thus the Neogene Period ends bounding the succeeding Quaternary Period at 2.58 Mya. "Digital Atlas of Neogene Life for the Southeastern United States". San Jose State University. Archived from the original on 2013-04-23. Retrieved 21 September 2018
The opossum is a marsupial of the order Didelphimorphia endemic to the Americas. The largest order of marsupials in the Western Hemisphere, it comprises 103 or more species in 19 genera. Opossums originated in South America and entered North America in the Great American Interchange following the connection of the two continents, their unspecialized biology, flexible diet, reproductive habits make them successful colonizers and survivors in diverse locations and conditions. Although the animal is called a possum in North America, which would refer to the Virginia opossum species, it should not be confused with the suborder Phalangeriformes, which are arboreal marsupials in the Eastern Hemisphere called "possums" because of their resemblance to Didelphimorphia; the word "opossum" is borrowed from the Powhatan language and was first recorded between 1607 and 1611 by John Smith and William Strachey. Both men encountered the language at the British settlement of Jamestown, which Smith helped to found and where Strachey served as its first secretary.
Strachey's notes describe the opossum as a "beast in bigness of a pig and in taste alike," while Smith recorded it "hath an head like a swine... tail like a rat... of the bigness of a cat." The Powhatan word derives from a Proto-Algonquian word meaning "white dog or dog-like beast."Following the arrival of Europeans in Australia, the term "possum" was borrowed to describe distantly related Australian marsupials of the suborder Phalangeriformes, which are more related to other Australian marsupials such as kangaroos. "Didelphimorphia" refers to the fact. Didelphimorphs are small to medium-sized marsupials, they tend to be semi-arboreal omnivores. Most members of this taxon have long snouts, a narrow braincase, a prominent sagittal crest; the dental formula is: 18.104.22.168.1.3.4 × 2 = 50 teeth. By mammalian standards, this is an unusually full jaw; the incisors are small, the canines large, the molars are tricuspid. Didelphimorphs have a plantigrade stance and the hind feet have an opposable digit with no claw.
Like some New World monkeys, opossums have prehensile tails. Like that of all marsupials, the fur consists of awn hair only, the females have a pouch; the tail and parts of the feet bear scutes. The stomach is simple, with a small cecum. Like most marsupials, the male opossum has a forked penis bearing twin glandes. Although all living opossums are opportunistic omnivores, different species vary in the amount of meat and vegetation they include in their diet. Members of the Caluromyinae are frugivorous; the yapok is unusual, as it is the only living semi-aquatic marsupial, using its webbed hindlimbs to dive in search of freshwater mollusks and crayfish. The extinct Thylophorops, the largest known opossum at 4–7 kg, was a macropredator. Most opossums are scansorial, well-adapted to life in the trees or on the ground, but members of the Caluromyinae and Glironiinae are arboreal, whereas species of Metachirus, to a lesser degree Didelphis show adaptations for life on the ground; the Metachirus nudicaudatus, found in the upper Amazon basin, consumes fruit seeds, small vertebrate creatures like birds and reptiles and invertebrates like crayfish and snails, but seems to be most insectivorous.
As a marsupial, the female opossum has a reproductive system that includes a bifurcated vagina, a divided uterus and a marsupium, her pouch. The average estrous cycle of the opossum is about 28 days. Opossums do possess a placenta, but it is short-lived, simple in structure, unlike that of placental mammals, not functional; the young are therefore born at a early stage, although the gestation period is similar to that of many other small marsupials, at only 12 to 14 days. Once born, the offspring must find their way into the marsupium to hold on to and nurse from a teat. Baby opossums, like their Australian cousins, are called joeys. Female opossums give birth to large numbers of young, most of which fail to attach to a teat, although as many as thirteen young can attach, therefore survive, depending on species; the young are weaned between 125 days, when they detach from the teat and leave the pouch. The opossum lifespan is unusually short for a mammal of its size only one to two years in the wild and as long as four or more years in captivity.
Senescence is rapid. The species are moderately sexually dimorphic with males being larger, much heavier, having larger canines than females; the largest difference between the opossum and non-marsupial mammals is the bifurcated penis of the male and bifurcated vagina of the female. Opossum spermatozoa exhibit sperm-pairing; this may ensure that flagella movement can be coordinated for maximal motility. Conjugate pairs dissociate into separate spermatozoa before fertilization. Opossums are solitary and nomadic, staying in one area as long as food and water are available; some families will group together in ready-made burrows or under houses. Though they will temporarily occupy abandoned burrows, they do not dig or put much effort into building their own; as nocturnal animals, they favor secure areas. These areas may be below ground or above; when threatened or harmed, they will "play possum", mimicking the app