Instruct: Given a question, retrieve documents that can help answer the question.
Query: When was the Cretaceous period?
The Cretaceous (/krɪˈteɪʃəs/, kri-TAY-shəs) is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago (mya) to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, and the longest period of the Phanerozoic Eon. The Cretaceous Period is usually abbreviated K, for its German translation Kreide (chalk, creta in Latin).
Belemnoids were numerous during the Jurassic and Cretaceous periods, and their fossils are abundant in Mesozoic marine rocks, often accompanying their cousins the ammonites. The belemnoids become extinct at the end of the Cretaceous period along with the ammonites. The belemnoids' origin lies within the bactritoid nautiloids, which date from the Devonian period; well-formed belemnoid guards can be found in rocks dating from the Mississippian (or Early Carboniferous) onward through the Cretaceous. Other fossil cephalopods include baculites, nautiloids and goniatites.
Dinosaurs – diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period (about 230 million years ago) until the end of the Cretaceous (66 million years ago), when the Cretaceous–Paleogene extinction event led to the extinction of all non-avian dinosaurs at the close of the Mesozoic era. The fossil record indicates that birds evolved within theropod dinosaurs during the Jurassic period. Some of them survived the Cretaceous–Paleogene extinction event, including the ancestors of all modern birds. Consequently, in modern classification systems, birds are considered dinosaurs—the only group which survived to the present day.
In the seas, rays, modern sharks and teleosts became common.[28] 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.
During the Early Cretaceous, new dinosaurs evolved to replace the old ones. Sauropods were still present, but they were not as diverse as they were in the Jurassic Period. Theropods from the Early Cretaceous of North America include dromaeosaurids such as "Deinonychus" and "Utahraptor", "Acrocanthosaurus", and "Microvenator". Sauropods included "Astrodon", "Brontomerus", and "Sauroposeidon". Ornithischians were more diverse than they were in the Jurassic Period. "Tenontosaurus", "Dakotadon", "Protohadros", and "Eolambia" are some of the ornithopods that lived during this time period. Ankylosaurs replaced their stegosaur cousins in the Cretaceous. Ankylosaurs from the Early Cretaceous of North America include "Sauropelta" and "Gastonia". Therizinosaurs such as "Falcarius" are also known from the Early Cretaceous of North America.
Catastrophism has found a place in modern biology with the Cretaceous–Paleogene extinction event at the end of the Cretaceous period, as proposed in a paper by 
Walter and Luis Alvarez in 1980. It argued that a asteroid struck Earth 66 million years ago at the end of the Cretaceous period. The event, whatever it was, made about 70% of all species extinct, including the dinosaurs, leaving behind the Cretaceous–Paleogene boundary. In 1990, a candidate crater marking the impact was identified at Chicxulub in the Yucatán Peninsula of Mexico.
It is important not to confuse geochronologic and chronostratigraphic units. Geochronological units are periods of time, thus it is correct to say that "Tyrannosaurus rex" lived during the Late Cretaceous Epoch. Chronostratigraphic units are geological material, so it is also correct to say that fossils of the genus "Tyrannosaurus" have been found in the Upper Cretaceous Series. In the same way, it is entirely possible to go and visit an Upper Cretaceous Series deposit – such as the Hell Creek deposit where the "Tyrannosaurus" fossils were found – but it is naturally impossible to visit the Late Cretaceous Epoch as that is a period of time.
The Cretaceous–Paleogene extinction event was a large-scale mass extinction of animal and plant species in a geologically short period of time, approximately (Ma). It is widely known as the "K–T extinction event" and is associated with a geological signature, usually a thin band dated to that time and found in various parts of the world, known as the Cretaceous–Paleogene boundary (K–T boundary). "K" is the traditional abbreviation for the Cretaceous Period derived from the German name "Kreidezeit", and "T" is the abbreviation for the Tertiary Period (a historical term for the period of time now covered by the Paleogene and Neogene periods). The event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. "Tertiary" being no longer recognized as a formal time or rock unit by the International Commission on Stratigraphy, the K-T event is now called the Cretaceous—Paleogene (or K-Pg) extinction event by many researchers.
Most of the sandstones in the Pafuri region however probably represent the great age of dinosaurs – the Jurassic and Cretaceous Periods. During the early parts of the Jurassic, which lasted from approximately 210 million years ago until about 144 million years ago, the area was almost certainly extremely arid as is evidenced by the great sandstones of the region and the many dune and desert structures – such as desert roses – that are found preserved within the rocks of the region. In these sediments have been found the remains of early dinosaurs - possibly those of a species known as "Massospondylus". It was also clearly a period of intense volcanism – with intrusive igneous rocks representing the ejected and intrusive molten matter from the interior of the Earth scattered about the region. The Jurassic was the age of great plant eating sauropods and smaller, carnivorous dinosaurs. This was the age of the origin of vertebrate flight with pterosaurs – flying dinosaurs - and the first birds appearing. The oceans themselves were full of a wide variety of life including many different types of fish, squids and ammonites – fossils of which are plentiful in the slightly younger rocks of the coastal regions to the southeast of the area. This was also a time of global upheaval as the World-continent of Pangea began to break up.
The Jurassic gives way to the Cretaceous Period in the upper rocks of the Makuleke area, and these are particularly evident in the North and East of the Pafuri. In these upper-most rocks - the region still seems to be dominated by deserts but these seem to give way to water born gravels that may indicate the emergence of the ancient Limpopo in the region. This is particularly evident in areas like Matule hill, where abundant gravels contain large and small bone fragments that appear to be those of medium-sized and small dinosaurs. Fossil wood typical of the Cretaceous flora have also been found in the rocks of the region. The Cretaceous Period lasted from about 144 million years ago until 66 million years when the Cretaceous–Paleogene extinction event occurred. While the Cretaceous is often referred to as the end of the age of dinosaurs, many new lifeforms appeared. This is the age of "Tyrannosaurus rex" and the ceratopsian dinosaurs (armoured and horned dinosaurs like "Triceratops"). The abundant marine deposits of the North East coast of South Africa are of this age. There is also the hope of finding the rarest forms of vertebrates alive in this time period – the tiny mammals which would eventually dominate the planet after the Cretaceous extinction event.
The Agathis is a very ancient family of coniferous trees. The group achieved its maximum diversity in the Jurassic and Cretaceous periods, when it was distributed almost worldwide. At the end of the Cretaceous, when the non-avian dinosaurs became extinct, so did most of the Araucariaceae in the Northern Hemisphere. At least one genus, Agathis, survived into the Eocene.
Snakes do not have an extensive fossil record; the oldest known fossil is from between early and late Cretaceous period. There were Tertiary fossil snakes that became extinct by the end of the Eocene period. The first colubrid also appeared in the Eocene period. Lizards first appeared in the middle Jurassic period, and this is when the scincomorph and the anguimorph lizards were first seen. The Gekkotans first appear in the late Jurassic period and the iguanians first appear in the late Cretaceous period. The lizards of the Cretaceous period represent extinct genera and species. The majority of amphisbaenians first appeared during the early Cenozoic period. Rhynchocephalian fossils first appear in the Middle Triassic period, between 238-240 million years ago, making them the earliest lepidosaurian fossils found to date. The tuatara can now only be found on small islands off the New Zealand coast. However, fossil records show that it once lived on mainland New Zealand and Rhynchocephalia as a whole was once distributed globally.
Cretaceous Research is a bimonthly peer-reviewed scientific journal published by Elsevier. The journal focuses on topics dealing with the Cretaceous period and the Cretaceous–Paleogene boundary.
The Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K-T) boundary, is a geological signature, usually a thin band of rock. "K", the first letter of the German word "Kreide" (chalk), is the traditional abbreviation for the Cretaceous Period and "Pg" is the abbreviation for the Paleogene Period.
The Deister-Süntel valley originated in the Cretaceous period when the Süntel and Deister were pushed upwards into anticlines. As a result, deep rock strata from the Jurassic period were brought to the surface. 
Until the last ice age the Weser river flowed through the valley. When the ice masses impounded it, however, it looked for another route to the North Sea through the Porta Westfalica (gorge). 
Deposits of the Zechstein Sea from the Jurassic period, and later the Weser, as well as quantities of glacial sediment and fossilized vegetation from the Cretaceous period left many natural resources in the Deister-Süntel valley. Today, Wealden sandstones of the best quality are found here along with fossilized marine animals and coal beds. Gypsum, rock salt, gravel and sand occur, as well as salt and sulphur springs and glacial erratics. The wet and swampy landscape that persisted well into the 19th century is now fertile terrain covered with loess soils.
Hybodus ("humped tooth") is an extinct genus of Chondrichthyans. First appearing towards the end of the Permian period, and disappearing during the Late Cretaceous, during the Triassic, Jurassic, and Cretaceous periods the hybodonts were especially successful and could be found in shallow seas across the world. For reasons that are not fully understood, the hybodonts became extinct near the end of the Late Cretaceous period.
The Paleogene (; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago (Mya) to the beginning of the Neogene Period Mya. It is the beginning of the Cenozoic Era of the present Phanerozoic Eon. The Paleogene is most notable for being the time during which mammals diversified from relatively small, simple forms into a large group of diverse animals in the wake of the Cretaceous–Paleogene extinction event that ended the preceding Cretaceous Period. The United States Geological Survey uses the abbreviation P for the Paleogene.
Feduccia (1995) emphasized the extinction event at the Cretaceous-Paleogene boundary as the probable engine of diversification in the Neornithes, picturing only one or very few lineages of birds surviving the end of the Cretaceous. He also noted that birds around the world had developed ratite-like anatomies when they became flightless, and saw the affinities of modern ratites, especially kiwis, as ambiguous. In this emphasis on the Cenozoic, rather than Cretaceous period, as the time of basal divergences between neornithines, he follows Olson.
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 A pterosaur, Anhanguera piscator Confuciusornis, a genus of crow-sized birds from the Early Cretaceous Ichthyornis was a toothed seabird-like ornithuran from the late Cretaceous period
Although faults are recorded throughout the known geological history of Hong Kong, they are considered to have been most active during the Jurassic to Cretaceous periods when strike-slip and thrust faulting was dominant. Some faults represent structures that were active during the period of Late Jurassic to Early Cretaceous volcanic activity and facilitated the rise of magma to the surface. Faults in Hong Kong formed interesting features that can be traced to understand their activities.
Abelisauroids flourished in the Southern hemisphere during the Cretaceous period, but their origins can be traced back to at least the Middle Jurassic, when they had a more global distribution (the earliest known abelisauroid remains come from Australian and South American deposits dated to about 170 million years ago). By the Cretaceous period, abelisauroids had apparently become extinct in Asia and North America, possibly due to competition from tyrannosauroids. However, advanced abelisauroids of the family Abelisauridae persisted in the southern continents until the Cretaceous–Paleogene extinction event  million years ago.
The first "Ginkgo" leaves were found from the Triassic period, but there were many species of "Ginkgo" during the Jurassic and Cretaceous periods as well. These periods, known collectively as the Mesozoic era, were when diversity and distribution for all plants were at their highest, including in "Ginkophytes". It was in the early Cenozoic period that "Ginkgophytes" nearly became extinct, specifically during the early Cretaceous and Tertiary periods. The only remaining "Ginkgophyte" was "Ginkgo adiantoides" – a polymorphic species. The Chinese were the ones to save "Ginkgo", as they grew them in their temple gardens. This is why modern "Ginkgo" trees are native to China.
Aphids, and the closely related adelgids and phylloxerans, probably evolved from a common ancestor some , in the early Permian period. They probably fed on plants like Cordaitales or Cycadophyta. With their soft bodies, aphids do not fossilize well, and the oldest known fossil is of the species "Triassoaphis cubitus" from the Triassic. They do however sometimes get stuck in plant exudates which solidify into amber. In 1967, when Professor Ole Heie wrote his monograph "Studies on Fossil Aphids", about sixty species have been described from the Triassic, Jurassic, Cretaceous and mostly the Tertiary periods, with Baltic amber contributing another forty species. The total number of species was small, but increased considerably with the appearance of the angiosperms , as this allowed aphids to specialise, the speciation of aphids going hand-in-hand with the diversification of flowering plants. The earliest aphids were probably polyphagous, with monophagy developing later. It has been hypothesized that the ancestors of the Adelgidae lived on conifers while those of the Aphididae fed on the sap of Podocarpaceae or Araucariaceae that survived extinctions in the late Cretaceous. Organs like the cornicles did not appear until the Cretaceous period. One study alternatively suggests that ancestral aphids may have lived on angiosperm bark and that feeding on leaves may be a derived trait. The Lachninae have long mouth parts that are suitable for living on bark and it has been suggested that the mid-Cretaceous ancestor fed on the bark of angiosperm trees, switching to leaves of conifer hosts in the late Cretaceous. The Phylloxeridae may well be the oldest family still extant, but their fossil record is limited to the Lower Miocene "Palaeophylloxera".
The "Cretaceous Normal" (also called the "Cretaceous Superchron" or C34) lasted for almost 40million years, from about , including stages of the Cretaceous period from the Aptian through the Santonian. The frequency of magnetic reversals steadily decreased prior to the period, reaching its low point (no reversals) during the period. Between the Cretaceous Normal and the present, the frequency has generally increased slowly.
Six genera are restricted to the Cretaceous; "Eucymatoceras" and "Heminautilus" from the Lower Cretaceous; "Anglonautilus" from both the Lower and Upper Cretaceous; "Deltocymatoceras", "Epicymatoceras",and "Syrionautilus". "Neocymatoceras" from the Oligocene from the Oligocene is synonymous with "Cymatoceras". 
Of the Cymatoceratidae, "Cymatoceras" has the longest temporal range, extending from the Late Jurassic to the Tertiary Oligocene. The only other to cross a period boundary is "Paracymatoceras" which lived during both the Late Jurassic and Early Cretaceous. The others are restricted to a simple period, sometimes to a single epoch.
The K–Pg boundary marks the end of the Cretaceous Period, the last period of the Mesozoic Era, and marks the beginning of the Paleogene Period, the first period of the Cenozoic Era. Its age is usually estimated at around 66 Ma (million years ago), with radiometric dating yielding a more specific age of 66.043 ± 0.011 Ma.