A Medley of Potpourri

The Cretaceous (IPA: /krɪˈteɪʃəs/ krih-TAY-shəs) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of the entire Phanerozoic. The name is derived from the Latin creta, "chalk", which is abundant in the latter half of the period. It is usually abbreviated K, for its German translation Kreide.

The Cretaceous was a period with a relatively 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, ammonites, and rudists, while dinosaurs continued to dominate on land. The world was ice- free, and forests extended to the poles. During this time, new groups of mammals and birds appeared. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant group of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of previously widespread gymnosperm groups.

The Cretaceous (along with the Mesozoic) ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary (K–Pg boundary), a geologic signature associated with the mass extinction that lies between the Mesozoic and Cenozoic Eras.

Etymology and history

The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822 as the Terrain Crétacé, using strata in the Paris Basin and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates, principally coccoliths), found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from the Latin creta, meaning chalk. The twofold division of the Cretaceous was implemented by Conybeare and Phillips in 1822. Alcide d'Orbigny in 1840 divided the French Cretaceous into five étages (stages): the Neocomian, Aptian, Albian, Turonian, and Senonian, later adding the Urgonian between Neocomian and Aptian and the Cenomanian between the Albian and Turonian.

Geology

Subdivisions

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 (lower/early), Gallic (middle) and Senonian (upper/late). A subdivision into 12 stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use.

From youngest to oldest, the subdivisions of the Cretaceous period are:

Subdivisions of the Cretaceous
Epoch	Stage	Start (base)	End (top)	Definition	Etymology
Epoch	Stage	(Mya)		Definition	Etymology
Late Cretaceous	Maastrichtian	72.1 ± 0.2	66.0	top: iridium anomaly at the Cretaceous–Paleogene boundary base:first occurrence of Pachydiscus neubergicus	Maastricht Formation, Maastricht, Netherlands
	Campanian	83.6 ± 0.2	72.1 ± 0.2	base: last occurrence of Marsupites testudinarius	Champagne, France
	Santonian	86.3 ± 0.5	83.6 ± 0.2	base: first occurrence of Cladoceramus undulatoplicatus	Saintes, France
	Coniacian	89.8 ± 0.3	86.3 ± 0.5	base: first occurrence of Cremnoceramus rotundatus	Cognac, France
	Turonian	93.9 ± 0.8	89.8 ± 0.3	base: first occurrence of Watinoceras devonense	Tours, France
	Cenomanian	100.5 ± 0.9	93.9 ± 0.8	base: first occurrence of Rotalipora globotruncanoides	Cenomanum; Le Mans, France
Early Cretaceous	Albian	113.0 ± 1.0	100.5 ± 0.9	base: first occurrence of Praediscosphaera columnata	Aube, France
	Aptian	121.4	113.0 ± 1.0	base: magnetic anomaly M0r	Apt, France
	Barremian	125.77 ± 1.5	121.4 ± 1.0	base: first occurrence of Spitidiscus hugii and S. vandeckii	Barrême, France
	Hauterivian	132.6 ± 2.0	125.77 ± 1.5	base: first occurrence of Acanthodiscus	Hauterive, Switzerland
	Valanginian	139.8 ± 3.0	132.6 ± 2.0	base: first occurrence of Calpionellites darderi	Valangin, Switzerland
	Berriasian	145.0 ± 4.0	139.8 ± 3.0	base: first occurrence of Berriasella jacobi (traditionally); first occurrence of Calpionella alpina (since 2016)	Berrias, France

Boundaries

The lower boundary of the Cretaceous is currently undefined, and the Jurassic–Cretaceous boundary is currently the only system boundary to lack a defined Global Boundary Stratotype Section and Point (GSSP). Placing a GSSP for this boundary has been difficult because of the strong regionality of most biostratigraphic markers, and the lack of any chemostratigraphic events, such as isotope excursions (large sudden changes in ratios of isotopes) that could be used to define or correlate a boundary. Calpionellids, an enigmatic group of planktonic protists with urn-shaped calcitic tests briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested as the most promising candidates for fixing the Jurassic–Cretaceous boundary. In particular, the first appearance Calpionella alpina, coinciding with the base of the eponymous Alpina subzone, has been proposed as the definition of the base of the Cretaceous. The working definition for the boundary has often been placed as the first appearance of the ammonite Strambergella jacobi, formerly placed in the genus Berriasella, but its use as a stratigraphic indicator has been questioned, as its first appearance does not correlate with that of C. alpina. The boundary is officially considered by the International Commission on Stratigraphy to be approximately 145 million years ago, but other estimates have been proposed based on U-Pb geochronology, ranging as young as 140 million years ago.

The upper boundary of the Cretaceous is sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and extending into the Gulf of Mexico. This layer has been dated at 66.043 Mya.

At the end of the Cretaceous, the impact of a large body with the Earth may have been the punctuation mark at the end of a progressive decline in biodiversity during the Maastrichtian age. The result was the extinction of three-quarters of Earth's plant and animal species. The impact created the sharp break known as the K–Pg boundary (formerly known as the K–T boundary). Earth's biodiversity required substantial time to recover from this event, despite the probable existence of an abundance of vacant ecological niches.

Despite the severity of the K-Pg extinction event, there were significant variations in the rate of extinction between and within different clades. Species that depended on photosynthesis declined or became extinct as atmospheric particles blocked solar energy. As is the case today, photosynthesizing organisms, such as phytoplankton and land plants, formed the primary part of the food chain in the late Cretaceous, and all else that depended on them suffered, as well. Herbivorous animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, the top predators, such as Tyrannosaurus rex, also perished. Yet only three major groups of tetrapods disappeared completely; the nonavian dinosaurs, the plesiosaurs and the pterosaurs. The other Cretaceous groups that did not survive into the Cenozoic Era — the ichthyosaurs, last remaining temnospondyls (Koolasuchus), and nonmammalian cynodonts (Tritylodontidae) — were already extinct millions of years before the event occurred.

Coccolithophorids and molluscs, including ammonites, rudists, freshwater snails, and mussels, as well as organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, ammonites are thought to have been the principal food of mosasaurs, a group of giant marine lizards related to snakes that became extinct at the boundary.

Omnivores, insectivores, and carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous, there seem to have been no purely herbivorous or carnivorous mammals. Mammals and birds that survived the extinction fed on insects, larvae, worms, and snails, which in turn fed on dead plant and animal matter. Scientists theorise that these organisms survived the collapse of plant-based food chains because they fed on detritus.

In stream communities, few groups of animals became extinct. Stream communities rely less on food from living plants and more on detritus that washes in from land. This particular ecological niche buffered them from extinction. Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the water column than among animals living on or in the seafloor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton, while animals living on or in the ocean floor feed on detritus or can switch to detritus feeding.

The largest air-breathing survivors of the event, crocodilians and champsosaurs, were semiaquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food and go into hibernation when conditions are unfavorable, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous.

Geologic formations

Drawing of fossil jaws of *Mosasaurus hoffmanni*, from the Maastrichtian of Dutch Limburg, by Dutch geologist Pieter Harting (1866)

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 that is formed under warm, shallow marine conditions. Due to the high sea level, there was extensive space for such sedimentation. Because of the relatively young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide.

Chalk is a rock type characteristic for (but not restricted to) the Cretaceous. It consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas.

Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half of the world's 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, such as the Mancos Shale of western North America. These shales are an important source rock for oil and gas, for example in the subsurface of the North Sea.

Europe

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 easily consolidated and the Chalk Group still consists of loose sediments in many places. The group also has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites, ammonites and sea reptiles such as Mosasaurus.

In southern Europe, the Cretaceous is usually 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.

North America

During the Cretaceous, the present North American continent was isolated from the other continents. In the Jurassic, the North Atlantic already opened, leaving a proto-ocean between Europe and North America. From north to south across the continent, the Western Interior Seaway started forming. This inland sea separated the elevated areas of Laramidia in the west and Appalachia in the east. Three dinosaur clades found in Laramidia (troodontids, therizinosaurids and oviraptorosaurs) are absent from Appalachia from the Coniacian through the Maastrichtian.

Paleogeography

During the Cretaceous, the late-Paleozoic-to-early-Mesozoic supercontinent of Pangaea completed its tectonic breakup into the present-day continents, although their positions were substantially different at the time. As the Atlantic Ocean widened, the convergent-margin mountain building (orogenies) that had begun during the Jurassic continued in the North American Cordillera, as the Nevadan orogeny was followed by the Sevier and Laramide orogenies.

Gondwana had begun to break up during the Jurassic Period, but its fragmentation accelerated during the Cretaceous and was largely complete by the end of the period. South America, Antarctica, and Australia rifted away from Africa (though India and Madagascar remained attached to each other until around 80 million years ago); thus, the South Atlantic and Indian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide. To the north of Africa the Tethys Sea continued to narrow. During the most of the Late Cretaceous, North America would be divided in two by the Western Interior Seaway, a large interior sea, separating Laramidia to the west and Appalachia to the east, then receded late in the period, leaving thick marine deposits sandwiched between coal beds. Bivalve palaeobiogeography also indicates that Africa was split in half by a shallow sea during the Coniacian and Santonian, connecting the Tethys with the South Atlantic by way of the central Sahara and Central Africa, which were then underwater. Yet another shallow seaway ran between what is now Norway and Greenland, connecting the Tethys to the Arctic Ocean and enabling biotic exchange between the two oceans. At the peak of the Cretaceous transgression, one-third of Earth's present land area was submerged.

The Cretaceous is justly famous for its chalk; indeed, more chalk formed in the Cretaceous than in any other period in the Phanerozoic. Mid-ocean ridge activity—or rather, the circulation of seawater through the enlarged ridges—enriched the oceans in calcium; this made the oceans more saturated, as well as increased the bioavailability of the element for calcareous nanoplankton. These widespread carbonates and other sedimentary deposits make the Cretaceous rock record especially fine. Famous formations from North America include the rich marine fossils of Kansas's Smoky Hill Chalk Member and the terrestrial fauna of the late Cretaceous Hell Creek Formation. Other important Cretaceous exposures occur in Europe (e.g., the Weald) and China (the Yixian Formation). In the area that is now India, massive lava beds called the Deccan Traps were erupted in the very late Cretaceous and early Paleocene.

Climate

Palynological evidence indicates the Cretaceous climate had three broad phases: a Berriasian–Barremian warm-dry phase, a Aptian–Santonian warm-wet phase, and a Campanian–Maastrichtian cool-dry phase.

The cooling trend of the last epoch of the Jurassic, the Tithonian, continued into the Berriasian, the first age of the Cretaceous. There is evidence that snowfalls were common in the higher latitudes during this age, and the tropics became wetter than during the Triassic and Jurassic. Glaciation was restricted to high-latitude mountains, though seasonal snow may have existed farther from the poles. After the end of the first age, however, temperatures began to increase again, with a number of thermal excursions, such as the middle Valanginian Weissert Thermal Excursion (WTX), which was caused by the Paraná-Etendeka Large Igneous Province's activity. It was followed by the middle Hauterivian Faraoni Thermal Excursion (FTX) and the early Barremian Hauptblatterton Thermal Event (HTE). The HTE marked the ultimate end of the Tithonian-early Barremian Cool Interval (TEBCI). The TEBCI was followed by the Barremian-Aptian Warm Interval (BAWI). Early Aptian tropical sea surface temperatures (SSTs) were 27–32 °C, based on TEX₈₆ measurements from the equatorial Pacific. The BAWI itself was followed by the Aptian-Albian Cold Snap (AACS) that began about 118 million years ago. A short, relatively minor ice age may have occurred during this so-called "cold snap", as evidenced by glacial dropstones in the western parts of the Tethys Ocean and the expansion of calcareous nannofossils that dwelt in cold water into lower latitudes. The AACS is associated with an arid period in the Iberian Peninsula.

Temperatures increased drastically after the end of the AACS, which ended around 111 million years ago with the Paquier/Urbino Thermal Maximum, giving way to the Mid-Cretaceous Hothouse (MKH), which lasted from the early Albian until the early Campanian. Faster rates of seafloor spreading and entry of carbon dioxide into the atmosphere are believed to have initiated this period of extreme warmth. The MKH was punctuated by multiple thermal maxima of extreme warmth. The Leenhardt Thermal Event (LTE) occurred around 110 million years ago, followed shortly by the l’Arboudeyesse Thermal Event (ATE) a million years later. Following these two hyperthermals was the Amadeus Thermal Maximum around 106 million years ago, during the middle Albian. Then, around a million years after that, occurred the Petite Verol Thermal Event (PVTE). Afterwards, around 102.5 million years ago, the Event 6 Thermal Event (EV6) took place; this event was itself followed by the Breistroffer Thermal Maximum around 101 million years ago, during the latest Albian. Approximately 94 million years ago, the Cenomanian-Turonian Thermal Maximum occurred, with this hyperthermal being the most extreme hothouse interval of the Cretaceous. Temperatures cooled down slightly over the next few million years, but then another thermal maximum, the Coniacian Thermal Maximum, happened, with this thermal event being dated to around 87 million years ago. Atmospheric CO₂ levels may have varied by thousands of ppm throughout the MKH. Mean annual temperatures at the poles during the MKH exceeded 14 °C. Such hot temperatures during the MKH resulted in a very gentle temperature gradient from the equator to the poles; the latitudinal temperature gradient during the Cenomanian-Turonian Thermal Maximum was 0.54 °C per ° latitude for the Southern Hemisphere and 0.49 °C per ° latitude for the Northern Hemisphere, in contrast to present day values of 1.07 and 0.69 °C per ° latitude for the Southern and Northern hemispheres, respectively. This meant weaker global winds, which drive the ocean currents, and resulted in less upwelling and more stagnant oceans than today. This is evidenced by widespread black shale deposition and frequent anoxic events. Tropical SSTs during the late Albian most likely averaged around 30 °C. Despite this high SST, seawater was not hypersaline at this time, as this would have required significantly higher temperatures still. Tropical SSTs during the Cenomanian-Turonian Thermal Maximum were at least 30 °C, though one study estimated them as high as between 33 and 42 °C. An intermediate estimate of ~33-34 °C has also been given. Meanwhile, deep ocean temperatures were as much as 15 to 20 °C (27 to 36 °F) warmer than today's; one study estimated that deep ocean temperatures were between 12 and 20 °C during the MKH. The poles were so warm that ectothermic reptiles were able to inhabit them.

Beginning in the Santonian, near the end of the MKH, the global climate began to cool, with this cooling trend continuing across the Campanian. This period of cooling, driven by falling levels of atmospheric carbon dioxide, caused the end of the MKH and the transition into a cooler climatic interval, known formally as the Late Cretaceous-Early Palaeogene Cool Interval (LKEPCI). Tropical SSTs declined from around 35 °C in the early Campanian to around 28 °C in the Maastrichtian. Deep ocean temperatures declined to 9 to 12 °C, though the shallow temperature gradient between tropical and polar seas remained. Regional conditions in the Western Interior Seaway changed little between the MKH and the LKEPCI. Two upticks in global temperatures are known to have occurred during the Maastrichtian, bucking the trend of overall cooler temperatures during the LKEPCI. Between 70 and 69 Ma and 66–65 Ma, isotopic ratios indicate elevated atmospheric CO₂ pressures with levels of 1000–1400 ppmV and mean annual temperatures in west Texas between 21 and 23 °C (70 and 73 °F). Atmospheric CO₂ and temperature relations indicate a doubling of pCO₂ was accompanied by a ~0.6 °C increase in temperature. The latter warming interval, occurring at the very end of the Cretaceous, was triggered by the activity of the Deccan Traps. The LKEPCI lasted into the Late Palaeocene, when it gave way to another supergreenhouse interval.

A computer-simulated model of surface conditions in Middle Cretaceous, 100 mya, displaying the approximate shoreline and calculated isotherms

The production of large quantities of magma, variously attributed to mantle plumes or to extensional tectonics, further pushed sea levels up, so that large areas of the continental crust were covered with shallow seas. The Tethys Sea connecting the tropical oceans east to west also helped to warm the global climate. Warm-adapted plant fossils are known from localities as far north as Alaska and Greenland, while dinosaur fossils have been found within 15 degrees of the Cretaceous south pole. It was suggested that there was Antarctic marine glaciation in the Turonian Age, based on isotopic evidence. However, this has subsequently been suggested to be the result of inconsistent isotopic proxies, with evidence of polar rainforests during this time interval at 82° S. Rafting by ice of stones into marine environments occurred during much of the Cretaceous, but evidence of deposition directly from glaciers is limited to the Early Cretaceous of the Eromanga Basin in southern Australia.

Flora

Flowering plants (angiosperms) make up around 90% of living plant species today. Prior to the rise of angiosperms, during the Jurassic and the Early Cretaceous, the higher flora was dominated by gymnosperm groups, including cycads, conifers, ginkgophytes, gnetophytes and close relatives, as well as the extinct Bennettitales. Other groups of plants included pteridosperms or "seed ferns", a collective term that refers to disparate groups of extinct seed plants with fern-like foliage, including groups such as Corystospermaceae and Caytoniales. The exact origins of angiosperms are uncertain, although molecular evidence suggests that they are not closely related to any living group of gymnosperms.

The earliest widely accepted evidence of flowering plants are monosulcate (single-grooved) pollen grains from the late Valanginian (~ 134 million years ago) found in Israel and Italy, initially at low abundance. Molecular clock estimates conflict with fossil estimates, suggesting the diversification of crown-group angiosperms during the Upper Triassic or Jurassic, but such estimates are difficult to reconcile with the heavily sampled pollen record and the distinctive tricolpate to tricolporoidate (triple grooved) pollen of eudicot angiosperms. Among the oldest records of Angiosperm macrofossils are Montsechia from the Barremian aged Las Hoyas beds of Spain and Archaefructus from the Barremian-Aptian boundary Yixian Formation in China. Tricolpate pollen distinctive of eudicots first appears in the Late Barremian, while the earliest remains of monocots are known from the Aptian. Flowering plants underwent a rapid radiation beginning during the middle Cretaceous, becoming the dominant group of land plants by the end of the period, coincident with the decline of previously dominant groups such as conifers. The oldest known fossils of grasses are from the Albian, with the family having diversified into modern groups by the end of the Cretaceous. The oldest large angiosperm trees are known from the Turonian (c. 90 Mya) of New Jersey, with the trunk having a preserved diameter of 1.8 metres (5.9 ft) and an estimated height of 50 metres (160 ft).

During the Cretaceous, ferns in the order Polypodiales, which make up 80% of living fern species, would also begin to diversify.

Terrestrial fauna

On land, mammals were generally small sized, but a very relevant component of the fauna, with cimolodont multituberculates outnumbering dinosaurs in some sites. Neither true marsupials nor placentals existed until the very end, but a variety of non-marsupial metatherians and non-placental eutherians had already begun to diversify greatly, ranging as carnivores (Deltatheroida), aquatic foragers (Stagodontidae) and herbivores (Schowalteria, Zhelestidae). Various "archaic" groups like eutriconodonts were common in the Early Cretaceous, but by the Late Cretaceous northern mammalian faunas were dominated by multituberculates and therians, with dryolestoids dominating South America.

The apex predators were archosaurian reptiles, especially dinosaurs, which were at their most diverse stage. Avians such as the ancestors of modern-day birds also diversified. They inhabited every continent, and were even found in cold polar latitudes. Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded they declined for poorly understood reasons (once thought to be due to competition with early birds, but now it is understood avian adaptive radiation is not consistent with pterosaur decline). By the end of the period only three highly specialized families remained; Pteranodontidae, Nyctosauridae, and Azhdarchidae.

The Liaoning lagerstätte (Yixian Formation) in China is an important site, full of preserved remains of numerous types of small dinosaurs, birds and mammals, that provides a glimpse of life in the Early Cretaceous. The coelurosaur dinosaurs found there represent types of the group Maniraptora, which includes modern birds and their closest non-avian relatives, such as dromaeosaurs, oviraptorosaurs, therizinosaurs, troodontids along with other avialans. Fossils of these dinosaurs from the Liaoning lagerstätte are notable for the presence of hair-like feathers.

Insects diversified during the Cretaceous, and the oldest known ants, termites and some lepidopterans, akin to butterflies and moths, appeared. Aphids, grasshoppers and gall wasps appeared.

Tyrannosaurus rex, one of the largest land predators of all time, lived during the Late Cretaceous
Up to 2 m long and 0.5 m high at the hip, Velociraptor was feathered and roamed the Late Cretaceous
Triceratops, one of the most recognizable genera of the Cretaceous
The azhdarchid Quetzalcoatlus, one of the largest animals to ever fly, lived during the Late Cretaceous
Confuciusornis, a genus of crow-sized birds from the Early Cretaceous
Ichthyornis was a toothed, seabird-like ornithuran from the Late Cretaceous

Rhynchocephalians

Rhynchocephalians (which today only includes the Tuatara) disappeared from North America and Europe after the Early Cretaceous, and were absent from North Africa and northern South America by the early Late Cretaceous. The cause of the decline of Rhynchocephalia remains unclear, but has often been suggested to be due to competition with advanced lizards and mammals. They appear to have remained diverse in high-latitude southern South America during the Late Cretaceous, where lizards remained rare, with their remains outnumbering terrestrial lizards 200:1.

Choristodera

Choristoderes, a group of freshwater aquatic reptiles that first appeared during the preceding Jurassic, underwent a major evolutionary radiation in Asia during the Early Cretaceous, which represents the high point of choristoderan diversity, including long necked forms such as Hyphalosaurus and the first records of the gharial-like Neochoristodera, which appear to have evolved in the regional absence of aquatic neosuchian crocodyliformes. During the Late Cretaceous the neochoristodere Champsosaurus was widely distributed across western North America. Due to the extreme climatic warmth in the Arctic, choristoderans were able to colonise it too during the Late Cretaceous.

Marine fauna

In the seas, rays, modern sharks and teleosts became common. Marine reptiles included ichthyosaurs in the early and mid-Cretaceous (becoming extinct during the late Cretaceous Cenomanian-Turonian anoxic event), plesiosaurs throughout the entire period, and mosasaurs appearing in the Late Cretaceous. Sea turtles in the form of Cheloniidae and Panchelonioidea lived during the period and survived the extinction event. Panchelonioidea is today represented by a single species; the leatherback sea turtle.

Baculites, an ammonite genus with a straight shell, flourished in the seas along with reef-building rudist clams. The Hesperornithiformes were flightless, marine diving birds that swam like grebes. Globotruncanid Foraminifera and echinoderms such as sea urchins and starfish (sea stars) thrived. Ostracods were abundant in Cretaceous marine settings; ostracod species characterised by high male sexual investment had the highest rates of extinction and turnover. Thylacocephala, a class of crustaceans, went extinct in the Late Cretaceous. The first radiation of the diatoms (generally siliceous shelled, rather than calcareous) in the oceans occurred during the Cretaceous; freshwater diatoms did not appear until the Miocene. The Cretaceous was also an important interval in the evolution of bioerosion, the production of borings and scrapings in rocks, hardgrounds and shells.

Icon (computing)

From Wikipedia, the free encyclopedia

Desktop icons for file/data transfer, clock/awaiting, and running a program.

In computing, an icon is a pictogram or ideogram displayed on a computer screen in order to help the user navigate a computer system. The icon itself is a quickly comprehensible symbol of a software tool, function, or a data file, accessible on the system and is more like a traffic sign than a detailed illustration of the actual entity it represents. It can serve as an electronic hyperlink or file shortcut to access the program or data. The user can activate an icon using a mouse, pointer, finger, or voice commands. Their placement on the screen, also in relation to other icons, may provide further information to the user about their usage. In activating an icon, the user can move directly into and out of the identified function without knowing anything further about the location or requirements of the file or code.

Icons as parts of the graphical user interface of the computer system, in conjunction with windows, menus and a pointing device (mouse), belong to the much larger topic of the history of the graphical user interface that has largely supplanted the text-based interface for casual use.

Overview

The computing definition of "icon" can include three distinct semiotical elements:

Icon, which resembles its referent (such as a road sign for falling rocks). This category includes stylized drawings of objects from the office environment or from other professional areas such as printers, scissors, file cabinets and folders.

Index, which is associated with its referent (smoke is a sign of fire). This category includes stylized drawings used to refer to actions "printer" and "print", "scissors" and "cut" or "magnifying glass" and "search".

Symbol, which is related to its referent only by convention (letters, musical notation, mathematical operators etc.).

This category includes standardized symbols found across many electronic devices, such as the power on/off symbol and the USB icon.

The majority of icons are encoded and decoded using metonymy, synecdoche, and metaphor.

An example of metaphorical representation characterizes all the major desktop-based computer systems including the desktop that uses an iconic representation of objects from the 1980s office environment to transpose attributes from a familiar context/object to an unfamiliar one. This is known as skeuomorphism, and an example is the use of the floppy disk to represent saving data; even though floppy disks have been obsolete for roughly a quarter century, it is still recognized as "the save icon".

Metonymy is in itself a subset of metaphors that use one entity to point to another related to it such as using a fluorescent bulb instead of a filament one to represent power saving settings.

Synecdoche is considered as a special case of metonymy, in the usual sense of the part standing for the whole such as a single component for the entire system, speaker driver for the entire audio system settings.

Additionally, a group of icons can be categorised as brand icons, used to identify commercial software programs and are related to the brand identity of a company or software. These commercial icons serve as functional links on the system to the program or data files created by a specific software provider. Although icons are usually depicted in graphical user interfaces, icons are sometimes rendered in a TUI using special characters such as MouseText or PETSCII.

The design of all computer icons is constricted by the limitations of the device display. They are limited in size, with the standard size of about a thumbnail for both desktop computer systems and mobile devices. They are frequently scalable, as they are displayed in different positions in the software, a single icon file such as the Apple Icon Image format can include multiple versions of the same icon optimized to work at a different size, in colour or grayscale as well as on dark and bright backgrounds.

The colors used, for both the image and the icon background, should stand out on different system backgrounds and among each other. The detailing of the icon image needs to be simple, remaining recognizable in varying graphical resolutions and screen sizes. Computer icons are by definition language-independent but often not culturally independent; they do not rely on letters or words to convey their meaning. These visual parameters place rigid limits on the design of icons, frequently requiring the skills of a graphic artist in their development.

Because of their condensed size and versatility, computer icons have become a mainstay of user interaction with electronic media. Icons also provide rapid entry into the system functionality. On most systems, users can create and delete, replicate, select, click or double-click standard computer icons and drag them to new positions on the screen to create a customized user environment.

Types

Standardized electrical device symbols

A series of recurring computer icons are taken from the broader field of standardized symbols used across a wide range of electrical equipment. Examples of these are the power symbol and the USB icon, which are found on a wide variety of electronic devices. The standardization of electronic icons is an important safety-feature on all types of electronics, enabling a user to more easily navigate an unfamiliar system. As a subset of electronic devices, computer systems and mobile devices use many of the same icons; they are corporated into the design of both the computer hardware and on the software. On the hardware, these icons identify the functionality of specific buttons and plugs. In the software, they provide a link into the customizable settings.

System warning icons also belong to the broader area of ISO standard warning signs. These warning icons, first designed to regulate automobile traffic in the early 1900s, have become standardized and widely understood by users without the necessity of further verbal explanations. In designing software operating systems, different companies have incorporated and defined these standard symbols as part of their graphical user interface. For example, the Microsoft MSDN defines the standard icon use of error, warning, information and question mark icons as part of their software development guidelines.

Different organizations are actively involved in standardizing these icons, as well as providing guidelines for their creation and use. The International Electrotechnical Commission (IEC) has defined "Graphical symbols for use on equipment", published as IEC 417, a document which displays IEC standardized icons. Another organization invested in the promotion of effective icon usage is the ICT (information and communications technologies), which has published guidelines for the creation and use of icons. Many of these icons are available on the Internet, either to purchase or as freeware to incorporate into new software.

Metaphorical icons

An icon is a Signifier pointing to a Signified. Easily comprehendible icons will make use of familiar visual metaphors directly connected to the Signified: actions the icon initiate or the content that would be revealed. Metaphors, Metonymy and Synecdoche are used to encode the meaning in an icon system.

The Signified can have multiple natures: virtual objects such as Files and Applications, actions within a system or an application (e.g. snap a picture, delete, rewind, connect/disconnect etc...), action in the physical world (e.g. print, eject DVD, change volume or brightness etc...) as well as physical objects (e.g. monitor, compact disk, mouse, printer etc...).

The Desktop metaphor

A subgroup of the more visually rich icons is based on objects lifted from a 1970 physical office space and desktop environment. It includes the basic icons used for a file, file folder, trashcan, inbox, together with the spatial real estate of the screen, i.e. the electronic desktop. This model originally enabled users, familiar with common office practices and functions, to intuitively navigate the computer desktop and system. (Desktop Metaphor, pg 2). The icons stand for objects or functions accessible on the system and enable the user to do tasks common to an office space. These desktop computer icons developed over several decades; data files in the 1950s, the hierarchical storage system (i.e. the file folder and filing cabinet) in the 1960s, and finally the desktop metaphor itself (including the trashcan) in the 1970s.

Dr. David Canfield Smith associated the term "icon" with computing in his landmark 1975 PhD thesis "Pygmalion: A Creative Programming Environment". In his work, Dr. Smith envisioned a scenario in which "visual entities", called icons, could execute lines of programming code, and save the operation for later re-execution. Dr. Smith later served as one of the principal designers of the Xerox Star, which became the first commercially available personal computing system based on the desktop metaphor when it was released in 1981. "The icons on [the desktop] are visible concrete embodiments of the corresponding physical objects." The desktop and icons displayed in this first desktop model are easily recognizable by users several decades later, and display the main components of the desktop metaphor GUI.

This model of the desktop metaphor has been adopted by most personal computing systems in the last decades of the 20th century; it remains popular as a "simple intuitive navigation by single user on single system." It is only at the beginning of the 21st century that personal computing is evolving a new metaphor based on Internet connectivity and teams of users, cloud computing. In this new model, data and tools are no longer stored on the single system, instead they are stored someplace else, "in the cloud". The cloud metaphor is replacing the desktop model; it remains to be seen how many of the common desktop icons (file, file folder, trashcan, inbox, filing cabinet) find a place in this new metaphor.

Brand icons for commercial software

A further type of computer icon is more related to the brand identity of the software programs available on the computer system. These brand icons are bundled with their product and installed on a system with the software. They function in the same way as the hyperlink icons described above, representing functionality accessible on the system and providing links to either a software program or data file. Over and beyond this, they act as a company identifier and advertiser for the software or company.

Because these company and program logos represent the company and product itself, much attention is given to their design, done frequently by commercial artists. To regulate the use of these brand icons, they are trademark registered and are considered part of the company's intellectual property.

In closed systems such as iOS and Android, the use of icons is to a degree regulated or guided to create a sense of consistency in the UI.

Overlay icons

On some GUI systems (e.g. Windows), on an icon which represents an object (e.g. a file) a certain additional subsystem can add a smaller secondary icon, laid over the primary icon and usually positioned in one of its corners, to indicate the status of the object which is represented with the primary icon. For instance, the subsystem for locking files can add a "padlock" overlay icon on an icon which represents a file in order to indicate that the file is locked.

Placement and spacing

In order to display the number of icons representing the growing complexity offered on a device, different systems have come up with different solutions for screen space management. The computer monitor continues to display primary icons on the main page or desktop, allowing easy and quick access to the most commonly used functions for a user. This screen space also invites almost immediate user customization, as the user adds favourite icons to the screen and groups related icons together on the screen. Secondary icons of system programs are also displayed on the task bar or the system dock. These secondary icons do not provide a link like the primary icons, instead, they are used to show availability of a tool or file on the system.

Spatial management techniques play a bigger role in mobile devices with their much smaller screen real estate. In response, mobile devices have introduced, among other visual devices, scrolling screen displays and selectable tabs displaying groups of related icons. Even with these evolving display systems, the icons themselves remain relatively constant in both appearance and function.

Above all, the icon itself must remain clearly identifiable on the display screen regardless of its position and size. Programs might display their icon not only as a desktop hyperlink, but also in the program title bar, on the Start menu, in the Microsoft tray or the Apple dock. In each of these locations, the primary purpose is to identify and advertise the program and functionality available. This need for recognition in turn sets specific design restrictions on effective computer icons.

Design

In order to maintain consistency in the look of a device, OS manufacturers offer detailed guidelines for the development and use of icons on their systems. This is true for both standard system icons and third party application icons to be included in the system. The system icons currently in use have typically gone through widespread international acceptance and understandability testing. Icon design factors have also been the topic for extensive usability studies. The design itself involves a high level of skill in combining an attractive graphic design with the required usability features.

Shape

The icon needs to be clear and easily recognizable, able to display on monitors of widely varying size and resolutions. Its shape should be simple with clean lines, without too much detailing in the design. Together with the other design details, the shape also needs to make it unique on the display and clearly distinguishable from other icons.

Color

The icon needs to be colorful enough to easily pick out on the display screen, and contrast well with any background. With the increasing ability to customize the desktop, it is important for the icon itself to display in a standard color which cannot be modified, retaining its characteristic appearance for immediate recognition by the user. Through color it should also provide some visual indicator as to the icon state; activated, available or currently not accessible ("greyed out").

Size and scalability

The standard icon is generally the size of an adult thumb, enabling both easy visual recognition and use in a touchscreen device. For individual devices the display size correlates directly to the size of the screen real estate and the resolution of the display. Because they are used in multiple locations on the screen, the design must remain recognizable at the smallest size, for use in a directory tree or title bar, while retaining an attractive shape in the larger sizes. In addition to scaling, it may be necessary to remove visual details or simplify the subject between discrete sizes. Larger icons serve also as part of the accessibility features for the visually impaired on many computer systems. The width and height of the icon are the same (1:1 aspect ratio) in almost all areas of traditional use.

Motion

Icons can also be augmented with iconographic motion - geometric manipulations applied to a graphical element over time, for example, a scale, rotation, or other deformation. One example is when application icons "wobble" in iOS to convey to the user they are able to be repositioned by being dragged. This is different from an icon with animated graphics, such as a Throbber. In contrast to static icons and icons with animated graphics, kinetic behaviors do not alter the visual content of an element (whereas fades, blurs, tints, and addition of new graphics, such as badges, exclusively alter an icon's pixels). Stated differently, pixels in an icon can be moved, rotated, stretched, and so on - but not altered or added to. Research has shown iconographic motion can act as a powerful and reliable visual cue, a critical property for icons to embody.

Localization

In its primary function as a symbolic image, the icon design should ideally be divorced from any single language. For products which are targeting the international marketplace, the primary design consideration is that the icon is non-verbal; localizing text in icons is costly and time-consuming.

Cultural context

Beyond text, there are other design elements which can be dependent upon the cultural context for interpretation. These include color, numbers, symbols, body parts and hand gestures. Each of these elements needs to be evaluated for their meaning and relevance across all markets targeted by the product.

Related visual tools

Other graphical devices used in the computer user interface fulfill GUI functions on the system similar to the computer icons described above. However each of these related graphical devices differs in one way or another from the standard computer icon.

Windows

The graphical windows on the computer screen share some of the visual and functional characteristics of the computer icon. Windows can be minimized to an icon format to serve as a hyperlink to the window itself. Multiple windows can be open and even overlapping on the screen. However where the icon provides a single button to initiate some function, the principal function of the window is a workspace, which can be minimized to an icon hyperlink when not in use.

Control widgets

Over time, certain GUI widgets have gradually appeared which are useful in many contexts. These are graphical controls which are used across computer systems and can be intuitively manipulated by the user even in a new context because the user recognises them from having seen them in a more familiar context. Examples of these control widgets are scroll bars, sliders, listboxes and buttons used in many programs. Using these widgets, a user is able to define and manipulate the data and the display for the software program they are working with. The first set of computer widgets was originally developed for the Xerox Alto. Now they are commonly bundled in widget toolkits and distributed as part of a development package. These control widgets are standardized pictograms used in the graphical interface, they offer an expanded set of user functionalities beyond the hyperlink function of computer icons.

Emoticons

Another GUI icon is exemplified by the smiley face, a pictogram embedded in a text message. The smiley, and by extension other emoticons, are used in computer text to convey information in a non-verbal binary shorthand, frequently involving the emotional context of the message. These icons were first developed for computers in the 1980s as a response to the limited storage and transmission bandwidth used in electronic messaging. Since then they have become both abundant and more sophisticated in their keyboard representations of varying emotions. They have developed from keyboard character combinations into real icons. They are widely used in all forms of electronic communications, always with the goal of adding context to the verbal content of the message. In adding an emotional overlay to the text, they have also enabled electronic messages to substitute for and frequently supplant voice-to-voice messaging.

These emoticons are very different from the icon hyperlinks described above. They do not serve as links, and are not part of any system function or computer software. Instead they are part of the communication language of users across systems. For these computer icons, customization and modifications are not only possible but in fact expected of the user.

Hyperlinks

A text hyperlink performs much the same function as the functional computer icon: it provides a direct link to some function or data available on the system. Although they can be customized, these text hyperlinks generally share a standardized recognizable format, blue text with underlining. Hyperlinks differ from functional computer icons in that they are normally embedded in text, whereas icons are displayed as stand-alone on the screen real estate. They are also displayed in text, either as the link itself or a friendly name, whereas icons are defined as being primarily non-textual.

Icon creation

Because of the design requirements, icon creation can be a time-consuming and costly process. There are a plethora of icon creation tools to be found on the Internet, ranging from professional level tools through utilities bundled with software development programs to stand-alone freeware. Given this wide availability of icon tools and icon sets, a problem can arise with custom icons which are mismatched in style to the other icons included on the system.

Tools

Icons underwent a change in appearance from the early 8-bit pixel art used pre-2000 to a more photorealistic appearance featuring effects such as softening, sharpening, edge enhancement, a glossy or glass-like appearance, or drop shadows which are rendered with an alpha channel.

Icon editors used on these early platforms usually contain a rudimentary raster image editor capable of modifying images of an icon pixel by pixel, by using simple drawing tools, or by applying simple image filters. Professional icon designers seldom modify icons inside an icon editor and use a more advanced drawing or 3D modeling application instead.

The main function performed by an icon editor is generation of icons from images. An icon editor resamples a source image to the resolution and color depth required for an icon. Other functions performed by icon editors are icon extraction from executable files (exe, dll), creation of icon libraries, or saving individual images of an icon.

All icon editors can make icons for system files (folders, text files, etc.), and for web pages. These have a file extension of .ICO for Windows and web pages or .ICNS for the Macintosh. If the editor can also make a cursor, the image can be saved with a file extension of .CUR or .ANI for both Windows and the Macintosh. Using a new icon is simply a matter of moving the image into the correct file folder and using the system tools to select the icon. In Windows XP you could go to My Computer, open Tools on the explorer window, choose Folder Options, then File Types, select a file type, click on Advanced and select an icon to be associated with that file type.

Developers also use icon editors to make icons for specific program files. Assignment of an icon to a newly created program is usually done within the Integrated Development Environment used to develop that program. However, if one is creating an application in the Windows API he or she can simply add a line to the program's resource script before compilation. Many icon editors can copy a unique icon from a program file for editing. Only a few can assign an icon to a program file, a much more difficult task.

Simple icon editors and image-to-icon converters are also available online as web applications.

List of tools

This is a list of notable computer icon software.

Axialis IconWorkshop – Supports both Windows and Mac icons. (Commercial, Windows)
IcoFX – Icon editor supporting Windows Vista and Macintosh icons with PNG compression (Commercial, Windows)
IconBuilder – Plug-in for Photoshop; focused on Mac. (Commercial, Windows/Mac)
Microangelo Toolset – a set of tools (Studio, Explorer, Librarian, Animator, On Display) for editing Windows icons and cursors. (Commercial, Windows)
Microsoft Visual Studio - can author ICO/CUR files but cannot edit 32-bit icon frames with 8-bit transparency. (Commercial, Windows)

The following is a list of raster graphic applications capable of creating and editing icons:

GIMP – Image Editor Supports reading and writing Windows ICO/CUR/ANI files and PNG files that can be converted to Mac .icns files. (Open Source, Free Software, Multi-Platform)
ImageMagick and GraphicsMagick – Command Line image conversion & generation that can be used to create Windows ICO files and PNG files that can be converted to Mac .ICNS files. (Open Source, Free Software, Multi-Platform)
IrfanView – Support converting graphic file formats into Windows ICO files. (Proprietary, free for non-commercial use, Windows)
ResEdit – Supports creating classic Mac OS icon resources. (Proprietary, Discontinued, Classic Mac OS)

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