The Paleocene or Palaeocene, the "old recent", is a geological epoch that lasted from about 66 to 56 million years ago. It is the first epoch of the Paleogene Period in the modern Cenozoic Era; as with many geologic periods, the strata that define the epoch's beginning and end are well identified, but the exact ages remain uncertain. The Paleocene Epoch is bracketed by two major events in Earth's history, it started with the mass extinction event at the end of the Cretaceous, known as the Cretaceous–Paleogene boundary. This was a time marked by the demise of non-avian dinosaurs, giant marine reptiles and much other fauna and flora; the die-off of the dinosaurs left unfilled ecological niches worldwide. The Paleocene ended with the Paleocene–Eocene Thermal Maximum, a geologically brief interval characterized by extreme changes in climate and carbon cycling; the name "Paleocene" comes from Ancient Greek and refers to the "old" "new" fauna that arose during the epoch. The K–Pg boundary that marks the separation between Cretaceous and Paleocene is visible in the geological record of much of the Earth by a discontinuity in the fossil fauna and high iridium levels.
There is fossil evidence of abrupt changes in flora and fauna. There is some evidence that a substantial but short-lived climatic change may have happened in the early decades of the Paleocene. There are several theories about the cause of the K–Pg extinction event, with most evidence supporting the impact of a 10 km diameter asteroid forming the buried Chicxulub crater on the coast of Yucatan, Mexico; the end of the Paleocene was marked by a time of major change, one of the most significant periods of global change during the Cenozoic. The Paleocene–Eocene Thermal Maximum upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and a major turnover in mammals on land; the Paleocene is divided into three stages, the Danian, the Selandian and the Thanetian, as shown in the table above. Additionally, the Paleocene is divided into six Mammal Paleogene zones; the early Paleocene was cooler and drier than the preceding Cretaceous, though temperatures rose during the Paleocene–Eocene Thermal Maximum.
The climate became warm and humid worldwide towards the Eocene boundary, with subtropical vegetation growing in Greenland and Patagonia, crocodilians swimming off the coast of Greenland, early primates evolving in the tropical palm forests of northern Wyoming. The Earth's poles were temperate. In many ways, the Paleocene continued processes. During the Paleocene, the continents continued to drift toward their present positions. Supercontinent Laurasia had not yet separated into three continents - Europe and Greenland were still connected, North America and Asia were still intermittently joined by a land bridge, while Greenland and North America were beginning to separate; the Laramide orogeny of the late Cretaceous continued to uplift the Rocky Mountains in the American west, which ended in the succeeding epoch. South and North America remained separated by equatorial seas. Africa was heading north towards Europe closing the Tethys Ocean, India began its migration to Asia that would lead to a tectonic collision and the formation of the Himalayas.
The inland seas in North America and Europe had receded by the beginning of the Paleocene, making way for new land-based flora and fauna. Warm seas circulated including the poles; the earliest Paleocene featured a low diversity and abundance of marine life, but this trend reversed in the epoch. Tropical conditions gave rise including coral reefs. With the demise of marine reptiles at the end of the Cretaceous, sharks became the top predators. At the end of the Cretaceous, the ammonites and many species of foraminifera became extinct. Marine fauna came to resemble modern fauna, with only the marine mammals and the Carcharhinid sharks missing. Terrestrial Paleocene strata overlying the K–Pg boundary is in places marked by a "fern spike": a bed rich in fern fossils. Ferns are the first species to colonize areas damaged by forest fires. In general, the Paleocene is marked by the development of modern plant species. Cacti and palm trees appeared. Paleocene and plant fossils are attributed to modern genera or to related taxa.
The warm temperatures worldwide gave rise to thick tropical, sub-tropical and deciduous forest cover around the globe with ice-free polar regions covered with coniferous and deciduous trees. With no large browsing dinosaurs to thin them, Paleocene forests were denser than those of the Cretaceous. Flowering plants, first seen in the Cretaceous, continued to develop and proliferate, along with them coevolved the insects that fed on these plants and pollinated them. Mammals had first appeared in the Late Triassic, evolving from advanced cynodonts, developed alongside the dinosaurs, exploiting ecological niches untouched by the larger and more famous Mesozoic animals: in the insect-rich fo
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the city of Perm. The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles and archosaurs; the world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors; the Permian ended with the Permian–Triassic extinction event, the largest mass extinction in Earth's history, in which nearly 96% of marine species and 70% of terrestrial species died out.
It would take well into the Triassic for life to recover from this catastrophe. Recovery from the Permian–Triassic extinction event was protracted; the term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Édouard de Verneuil; the region now lies in the Perm Krai of Russia. Official ICS 2017 subdivisions of the Permian System from most recent to most ancient rock layers are: Lopingian epoch Changhsingian Wuchiapingian Others: Waiitian Makabewan Ochoan Guadalupian epoch Capitanian stage Wordian stage Roadian stage Others: Kazanian or Maokovian Braxtonian stage Cisuralian epoch Kungurian stage Artinskian stage Sakmarian stage Asselian stage Others: Telfordian Mangapirian Sea levels in the Permian remained low, near-shore environments were reduced as all major landmasses collected into a single continent—Pangaea; this could have in part caused the widespread extinctions of marine species at the end of the period by reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean, the Paleo-Tethys Ocean, a large ocean that existed between Asia and Gondwana; the Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys Ocean to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic era. Large continental landmass interiors experience climates with extreme variations of heat and cold and monsoon conditions with seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea; such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores in a wetter environment. The first modern trees appeared in the Permian. Three general areas are noted for their extensive Permian deposits—the Ural Mountains and the southwest of North America, including the Texas red beds.
The Permian Basin in the U. S. states of Texas and New Mexico is so named because it has one of the thickest deposits of Permian rocks in the world. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an ice age. Glaciers receded around the mid-Permian period as the climate warmed, drying the continent's interiors. In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Permian marine deposits are rich in fossil mollusks and brachiopods. Fossilized shells of two kinds of invertebrates are used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist, one of the foraminiferans, ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct. Terrestrial life in the Permian included diverse plants, fungi and various types of tetrapods; the period saw a massive desert covering the interior of Pangaea.
The warm zone spread in the northern hemisphere. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals became marginal elements; the Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began; the swamp-loving
The Eocene Epoch, lasting from 56 to 33.9 million years ago, is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch; the start of the Eocene is marked by a brief period in which the concentration of the carbon isotope 13C in the atmosphere was exceptionally low in comparison with the more common isotope 12C. The end is set at a major extinction event called the Grande Coupure or the Eocene–Oligocene extinction event, which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay; as with other geologic periods, the strata that define the start and end of the epoch are well identified, though their exact dates are uncertain. The name Eocene comes from the Ancient Greek ἠώς and καινός and refers to the "dawn" of modern fauna that appeared during the epoch; the Eocene epoch is conventionally divided into early and late subdivisions.
The corresponding rocks are referred to as lower and upper Eocene. The Ypresian stage constitutes the lower, the Priabonian stage the upper; the Eocene Epoch contained a wide variety of different climate conditions that includes the warmest climate in the Cenozoic Era and ends in an icehouse climate. The evolution of the Eocene climate began with warming after the end of the Palaeocene–Eocene Thermal Maximum at 56 million years ago to a maximum during the Eocene Optimum at around 49 million years ago. During this period of time, little to no ice was present on Earth with a smaller difference in temperature from the equator to the poles. Following the maximum was a descent into an icehouse climate from the Eocene Optimum to the Eocene-Oligocene transition at 34 million years ago. During this decrease ice began to reappear at the poles, the Eocene-Oligocene transition is the period of time where the Antarctic ice sheet began to expand. Greenhouse gases, in particular carbon dioxide and methane, played a significant role during the Eocene in controlling the surface temperature.
The end of the PETM was met with a large sequestration of carbon dioxide in the form of methane clathrate and crude oil at the bottom of the Arctic Ocean, that reduced the atmospheric carbon dioxide. This event was similar in magnitude to the massive release of greenhouse gasses at the beginning of the PETM, it is hypothesized that the sequestration was due to organic carbon burial and weathering of silicates. For the early Eocene there is much discussion; this is due to numerous proxies representing different atmospheric carbon dioxide content. For example, diverse geochemical and paleontological proxies indicate that at the maximum of global warmth the atmospheric carbon dioxide values were at 700–900 ppm while other proxies such as pedogenic carbonate and marine boron isotopes indicate large changes of carbon dioxide of over 2,000 ppm over periods of time of less than 1 million years. Sources for this large influx of carbon dioxide could be attributed to volcanic out-gassing due to North Atlantic rifting or oxidation of methane stored in large reservoirs deposited from the PETM event in the sea floor or wetland environments.
For contrast, today the carbon dioxide levels are at 400 ppm or 0.04%. At about the beginning of the Eocene Epoch the amount of oxygen in the earth's atmosphere more or less doubled. During the early Eocene, methane was another greenhouse gas that had a drastic effect on the climate. In comparison to carbon dioxide, methane has much greater effect on temperature as methane is around 34 times more effective per molecule than carbon dioxide on a 100-year scale. Most of the methane released to the atmosphere during this period of time would have been from wetlands and forests; the atmospheric methane concentration today is 0.000179% or 1.79 ppmv. Due to the warmer climate and sea level rise associated with the early Eocene, more wetlands, more forests, more coal deposits would be available for methane release. Comparing the early Eocene production of methane to current levels of atmospheric methane, the early Eocene would be able to produce triple the amount of current methane production; the warm temperatures during the early Eocene could have increased methane production rates, methane, released into the atmosphere would in turn warm the troposphere, cool the stratosphere, produce water vapor and carbon dioxide through oxidation.
Biogenic production of methane produces carbon dioxide and water vapor along with the methane, as well as yielding infrared radiation. The breakdown of methane in an oxygen atmosphere produces carbon monoxide, water vapor and infrared radiation; the carbon monoxide is not stable so it becomes carbon dioxide and in doing so releases yet more infrared radiation. Water vapor traps more infrared than does carbon dioxide; the middle to late Eocene marks not only the switch from warming to cooling, but the change in carbon dioxide from increasing to decreasing. At the end of the Eocene Optimum, carbon dioxide began decreasing due to increased siliceous plankton productivity and marine carbon burial. At the beginning of the middle Eocene an event that may have triggered or helped with the draw down of carbon dioxide was the Azolla event at around 49 million years ago. With the equable climate during the early Eocene, warm temperatures in the arctic allowed for the growth of azolla, a floating aquatic fern, on the Arctic Ocean.
Compared to current carb
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
Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla, are hoofed animals—ungulates—which bear most of their weight on one of the five toes: the third toe. The non-weight-bearing toes are either present, vestigial, or positioned posteriorly. By contrast, the even-toed ungulates bear most of their weight on two of the five toes: their third and fourth toes. Another difference between the two is that odd-toed ungulates digest plant cellulose in their intestines rather than in one or more stomach chambers as the even-toed ungulates do; the order includes about 17 species divided into three families: Equidae and Tapiridae. Despite their different appearances, they were recognized as related families in the 19th century by the zoologist Richard Owen, who coined the order name; the largest odd-toed ungulates are rhinoceroses, the extinct Paraceratherium, a hornless rhino from the Oligocene, is considered one of the largest land mammals of all time. At the other extreme, an early member of the order, the prehistoric horse Hyracotherium, had a withers height of only 30 to 60 cm.
Apart from dwarf varieties of the domestic horse and donkey, perissodactyls reach a body length of 180–420 cm and a weight of 150 to 4,500 kg. While rhinos have only sparse hair and exhibit a thick epidermis and horses have dense, short coats. Most species are brown, although zebras and young tapirs are striped; the main axes of both the front and rear feet pass through the third toe, always the largest. The remaining toes have been reduced in size to varying degrees. Tapirs, which are adapted to walking on soft ground, have four toes on their fore feet and three on their hind feet. Living rhinos have three toes on both the hind feet. Modern equines possess only a single toe. Rhinos and tapirs, by contrast, have hooves covering only the leading edge of the toes, with the bottom being soft; the ulnae and fibulae are reduced in horses. A common feature that distinguishes this group from other mammals is the saddle-shaped ankle between the astragalus and the scaphoid, which restricts the mobility of the foot.
The thigh is short, the clavicle is absent. Odd-toed ungulates have a long upper jaw with an extended diastema between the front and cheek teeth, giving them an elongated head; the various forms of snout between families are due to differences in the form of the premaxilla. The lacrimal bone has projecting cusps in a wide contact with the nasal bone; the temporomandibular joint is high and the mandible is enlarged. Rhinos have one or two horns made of agglutinated keratin, unlike the horns of even-toed ungulates, which have a bony core; the number and form of the teeth vary according to diet. The incisors and canines can be small or absent, as in the two African species of rhinoceros. In the horses only the males possess canines; the surface shape and height of the molars is dependent on whether soft leaves or hard grass makes up the main component of their diets. Three or four cheek teeth are present on each jaw half, so the dental formula of odd-toed ungulates is: 0-3. 0-1. 2-4. 31-3. 1. 2-4. 3 × 2 = 30-44 All perissodactyls are hindgut fermenters.
In contrast to ruminants, hindgut fermenters store digested food that has left the stomach in an enlarged cecum, where the food is digested by bacteria. No gallbladder is present; the stomach of perissodactyls is built, while the cecum accommodates up to 90 l in horses. The intestine is long, reaching up to 26 m in horses. Extraction of nutrients from food is inefficient, which explains why no odd-toed ungulates are small; the present distribution of most perissodactyl species is only a small fraction of their original range. Members of this group are now found only in Central and South America and southern Africa, central and southeastern Asia. During the peak of odd-toed ungulate existence, from the Eocene to the Oligocene, perissodactyls were distributed over much of the globe, the only exceptions being Australia and Antarctica. Horses and tapirs arrived in South America after the formation of the Isthmus of Panama in the Pliocene, around 3 million years ago. In North America, they died out around 10,000 years ago, while in Europe, the tarpans disappeared in the 19th century.
Hunting and habitat restriction have reduced the present-day species to fragmented relict populations. In contrast, domesticated horses and donkeys have gained a worldwide distribution, feral animals of both species are now found in regions outside of their original range, such as in Australia. Perissodactyls inhabit a number of different habitats. Tapirs are solitary and inhabit tropical rainforests. Rhinos tend to live alone in rather dry savannas, in Asia, wet marsh or forest areas. Horses inhabit open areas such as grasslands, steppes, or semi-deserts, live together in groups. Odd-toed ungulates are herbivores that feed, to varying degrees, on grasses and other plant parts. A distinction is made between grass feeders and leaf feeders. Odd-toed ungulates are characterized by a long gestation period and a small litter size delivering a single young; the g
A cusp is a pointed, projecting, or elevated feature. In animals, it is used to refer to raised points on the crowns of teeth. A cusp is an incisal eminence on a tooth. Canine teeth, otherwise known as cuspids, each possess a single cusp, while premolars, otherwise known as bicuspids, possess two each. Molars possess either four or five cusps. In certain populations the maxillary molars first molars, will possess a fifth cusp situated on the mesiolingual cusp known as the Cusp of Carabelli. Buccal Cusp- One other variation of the upper first premolar is the'Uto-Aztecan' upper premolar, it is a bulge on the buccal cusp, only found in Native American Indians, with highest frequencies of occurrence in Arizona. The name is not a dental term. Buccal-The side of a tooth, adjacent to the inside of the cheek, as opposed to lingual or palatal, which refer to the side of a tooth adjacent to the tongue or palate, respectively. Although technically referring only to posterior teeth, this term may be employed to describe the facial surface of anterior teeth as well.
There are 4 main cusps found on the molars of the upper dentition of hominids and other therian Mammals. The hypocone is found on the distal lingual side of the tooth, it fits into the grooves of the lower dentition and is an adaptation for the overall grinding and tearing of foods using the occlusal of the tooth surface during occlusion or mastication. Its strength is due to the thickness of the enamel; the hypocone appears to have evolved independently in more than twenty mammal species during the Cenozoic period. The metacone is a cusp on the molars of the upper dentition in hominids, it is found at the buccal distal area of the tooth. The crests between the cusps are adaptations for slicing food during mastication. A type of cusp; the protocone is founding the molars of the upper dentition in Marsupial vertebrates. It is found at the mesiolingual area of the tooth; the crests between the cusps are adaptations for slicing food during mastication. Mamelon Cusp of Carabelli Talon cusp Ash, Major M..
Wheeler'S Dental Anatomy and Occlusion, 8th edition
The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, the longest period of the Phanerozoic Eon; the Cretaceous Period is abbreviated K, for its German translation Kreide. The Cretaceous was a period with a warm climate, resulting in high eustatic sea levels that created numerous shallow inland seas; these oceans and seas were populated with now-extinct marine reptiles and rudists, while dinosaurs continued to dominate on land. During this time, new groups of mammals and birds, as well as flowering plants, appeared; the Cretaceous ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs and large marine reptiles died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic eras.
The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin and named for the extensive beds of chalk, found in the upper Cretaceous of Western Europe. The name Cretaceous was derived from Latin creta; the Cretaceous is divided into Early and Late Cretaceous epochs, or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian and Senonian. A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use; as with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact age of the system's base is uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is defined, being placed at an iridium-rich layer found worldwide, believed to be associated with the Chicxulub impact crater, with its boundaries circumscribing parts of the Yucatán Peninsula and into the Gulf of Mexico.
This layer has been dated at 66.043 Ma. A 140 Ma age for the Jurassic-Cretaceous boundary instead of the accepted 145 Ma was proposed in 2014 based on a stratigraphic study of Vaca Muerta Formation in Neuquén Basin, Argentina. Víctor Ramos, one of the authors of the study proposing the 140 Ma boundary age sees the study as a "first step" toward formally changing the age in the International Union of Geological Sciences. From youngest to oldest, the subdivisions of the Cretaceous period are: Late Cretaceous Maastrichtian – Campanian – Santonian – Coniacian – Turonian – Cenomanian – Early Cretaceous Albian – Aptian – Barremian – Hauterivian – Valanginian – Berriasian – The high sea level and warm climate of the Cretaceous meant large areas of the continents were covered by warm, shallow seas, providing habitat for many marine organisms; the Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the deposits from the Cretaceous are of marine limestone, a rock type, formed under warm, shallow marine circumstances.
Due to the high sea level, there was extensive space for such sedimentation. Because of the young age and great thickness of the system, Cretaceous rocks are evident in many areas worldwide. Chalk is a rock type characteristic for the Cretaceous, it consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas. In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast; the group is found in England, northern France, the low countries, northern Germany, Denmark and in the subsurface of the southern part of the North Sea. Chalk is not consolidated and the Chalk Group still consists of loose sediments in many places; the group has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites and sea reptiles such as Mosasaurus. In southern Europe, the Cretaceous is a marine system consisting of competent limestone beds or incompetent marls.
Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf, at the margin of the Tethys Ocean. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water leaving the deposited organic matter undecomposed. Half the worlds petroleum reserves were laid down at this time in the anoxic conditions of what would become the Persian Gulf and the Gulf of Mexico. In many places around the world, dark anoxic shales were formed during this interval; these shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. During th