CRETACEOUS

Submissions for the CRETACEOUS subphase are OPEN

Beginning - 4th September 2026

Deadline - 8th October 2026

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CRETACEOUS

143.1 - 66 mya

The Cretaceous is a geological period that lasted from about 143.1 to 66 million years ago. Its name derives from the Latin creta, meaning "chalk", which is abundant in deposits from the later half of the period. It is usually abbreviated as K, for its German translation Kreide. The end of this period is marked by the Cretaceous-Paleogene extinction event, which also ends the Mesozoic era, which marks the last fossil appearance of non-avialan dinosaurs, pterosaurs and several lineages of marine reptiles, being caused by the impact of a large asteroid in the Gulf of Mexico, forming the Chicxulub crater.

The Cretaceous was first defined as its own distinct period by Belgian geologist Jean d'Omalius d'Halloy in 1822 as the Terrain Crétacé, using strata in the Paris Basin, named after the extensive chalk beds formed by calcium carbonate deposited by ancient shells of marine organisms, mainly the planktonic coccoliths, abundantly found in the late Cretaceous record of western Europe. Alcide d'Orbigny in 1840 divided the French Cretaceous into five stages: the Neocomian (~145 - 130 mya), Aptian (121.4 - 113.2 mya), Albian (113.2 - 100.5 mya), Turonian (93.9 - 89.8 mya) and Senonian (89.8 - 66 mya), later adding the Urgonian (130 - 121.4 mya) between Neocomian and Aptian and the Cenomanian (100.5 - 93.9 mya) between the Albian and Turonian.

The Cretaceous is now divided in two, the Early (143.1 - 100.5 mya) and the Late (100.5 - 66 mya) Cretaceous, with the very first stage being the Berriasian (143.1 - 137 mya); its lower base is hard to define, but it has been proposed that the first appearance of the alveolate Calpionella alpina, coinciding with the base of the eponymous Alpina subzone, could serve as the base of the whole Cretaceous period. The working definition for the boundary has often been suggested 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 it does not correlate with the first appearance of C. alpina. After the Berriasian, there's the five other stages from the Early Cretaceous, the Valanginian (137 - 132.6 mya), Hauterivian (132.6 - 125.7 mya), Barremian (125.7 - 121.4 mya), Aptian (121.4 - 113.2 mya) and Albian (113.2 - 100.5 mya), as well as six stages from the Late Cretaceous, the Cenomanian (100.5 - 93.9 mya), Turonian (93.9 - 89.8 mya), Coniacian (89.8 - 85.7 mya), Santonian (85.7 - 83.6 mya), Campanian (83.6 - 72.2 mya) and Maastrichtian (72.2 - 66 mya), this last one having its upper boundary sharply defined by an iridium-rich layer found worldwide, associated with the Chicxulub impact, whose resulting crater currently covers part of the Yucatán Peninsula and Gulf of Mexico. This boundary marks the occurence of the Cretaceous-Paleogene extinction event (or K-Pg extinction event, for short) which, although devastating, had varying impacts across different clades of organisms. Photosynthesizing organisms were particularly affected as the soot created by the asteroid impact blocked the sunlight from reaching through much of the Earth's surface for years, causing collapses to the primary production of the planet's biosphere, composed mostly of phytoplankton and land plants, leading to the collapse of most organisms in higher trophic ranks that depended on these primary producers. Coccolithophores and mollusks were quite affected, such as ammonites, freshwater snails and bivalves (with the rudists disappearing completely from the fossil record), as well as the organisms that depended on these for survival. For example, ammonites may have been major components of the diets of mosasaurs, giant marine lizards that disappeared from the fossil record with the extinction. Animals that were omnivores, insectivores or necrophagous were less affected by the extinction as the resources required for their survival were more prevalent. In the water, the extinction appears to have affected organisms that lived more in the water column than in or near the benthos. During the period itself, however, life was thriving in conditions that were mostly characterized by high sea levels and warm climates, with large areas of continental shelf being underwater, creating vast portions of shallow sea for the proliferation of marine life. Extensive and thick chalk deposits formed during this time, and given their recency and uniqueness, they're very evident pieces of the period's geology found today across the globe. Around the middle part of the period, a number of dark anoxic shales were formed, such as the Mancos Shale in the western US, dated to about 110 to 80 million years ago. The north american continent, throughout parts of the Cretaceous, was relatively isolated from other landmasses, which resulted in its peculiar unique fauna at the time.

During the Cretaceous, the landmass containing modern South America, Antarctica and Australia ended up separating from Africa, forming the South Atlantic and Indian oceans, with the landmass containing India and Madagascar fully isolating from Africa later, at the late Cretaceous, around 80 million years ago. To the north of Africa, the Tethys sea continued to narrow. Paleobiogeography suggests that there was a shallow sea splitting Africa in two, during the Coniacian and Santonian stages (89.8 - 83.6 mya), connecting the Tethys with the southern Atlantic, judging by the distribution of bivalves at the time, indicating that parts of the central Sahara and central Africa were underwater at the time. 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.

Palynological evidence suggests that the Cretaceous climate was divided into three broad phases: a Berriasian to Barremian warm dry phase (143.1 - 121.4 mya), a Aptian to Santonian warm wet phase (121.4 - 83.6 mya) and a Campanian to Maastrichtian cool dry phase (83.6 - 66 mya). The location of the Intertropical Convergence Zone (ITCZ) during the Cretaceous was roughly the same as in the present day. During the start of the Cretaceous, the opening of the north Atlantic seaway enabled the flow of cool water from the Boreal Ocean to the Tethys, continuing the cooling trend that was going on at the terminal Jurassic, which caused snowfalls and glaciation in higher latitudes. This was interrupted by the Weissert Event (~134.5 - 132.4 mya), caused by the igneous province going around the south american-african boundary, and later followed by the Faraoni Thermal Excursion (131 mya) and the Hauptblatterton Thermal Event (~125.7 - 123.5 mya). This marked the end of the Tithonian-early Barremian Cool Interval (TEBCI), which was then followed by the Barremian-Aptian Warm Interval (~123.5 - 118 mya). This was then followed by the Aptian-Albian Cold Snap (118 - 111 mya). This cold snap is associated with an arid period in the Iberian Peninsula. This cold period was swiftly substituted by a major hothouse period, the Mid-Cretaceous Hothouse (MKH), beginning shortly before the Leenhardt Thermal Event (110 mya) and the l'Arboudeyesse Thermal Event (109 mya), followed by the Amadeus Thermal Maximum (106 mya). After this, the Petite Verol Thermal Event (105 mya) occured. Around 94 million years ago, the Cenomanian-Turonian Thermal Maximum occured, the most extreme hothouse interval of the Cretaceous, characterized by very high sea levels. After this thermal maximum, temperatures cooled slightly only for another thermal maximum to occur, the Coniacian Thermal Maximum, some 87 million years ago. The temperature gradient between the poles and the tropics, during the MKH, was much gentler than nowadays, resulting in weaker global winds, which are major drivers of the ocean currents, resulting in less upwelling and more stagnant oceans. Surface ocean temperatures during the late Albian were pretty high, but interestingly not high enough to make the seawater hypersaline. On land, arid zones in the Albian regularly expanded northward in tandem with expansions of subtropical high pressure belts. After the MKH, there was the Late Cretaceous-Early Paleogene Cool Interval (LKEPCI), with a tendency of growing temperatures at the end of the Maastrichtian, with the volcanism of the Deccan Traps in India, leading to sea level rise, with the Tethys Sea, connecting tropical oceans east to west, contributing even more to this warming trend. 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.

The Cretaceous was inhabited by an array of diverse flora, including the pteridosperms, or seed ferns, a collective term for enigmatic extinct seed plants with fern-like foliage, such as the Caytoniales, which were present in the period, with potential for the Corystospermaceae also being present. Angiosperm plants first appear as undoubted fossils in this period, but their exact origins are unclear, with molecular clock dating suggesting a Jurassic origin instead, and an evolutionary split from gymnosperms dating back to the Carboniferous.

During the Cretaceous, mammals were generally small, though quite relevant components of the global fauna, with cimolodont multituberculates sometimes outnumbering dinosaurs in some fossil sites. Basal forms of mammals, such as eutriconodonts, were common in the Early Cretaceous, but by the Late Cretaceous they seem to have mostly been replaced by multituberculates and therians in the northern hemisphere, and gondwanatheres and basal trechnotheres in South America. The apex land predators were archosaurs, especially dinosaurs, which reached the peak of their diversity in this period. The Early Cretaceous lagerstätten in Liaoning, China, represented by fossil formations such as the Yixian Formation, show detailed glimpses of mammalian and dinosaurian faunal diversity, including avialans. Some fossils of non-avialan dinosaurs from these strata are notable for preserving hair-like feathers, contextualizing us on their life appearance. A wider diversity of specific insect groups also is observed in the Cretaceous, including aphids, grasshoppers and wasps. Choristodere reptiles, which first appear as fossils in the Jurassic, reach the peak of their diversity in Asia, during the Early Cretaceous, with long-necked forms such as Hyphalosaurus and gharial-like neochoristoderes, possibly evolving in the regional absence of neosuchian crocodyliforms. By the Late Cretaceous, the neochoristodere Champsosaurus was widespread and common in North America. Due to the warm intervals that allows the polar regions to be hotter, these reptiles easily conquered the high latitudes at the time.

In the oceans, teleost fish and sharks, including rays, diversified immensely, marking their future success following the end of the Cretaceous period, but during the period itself they were actively coexisting with an array of unique and characteristic marine reptiles that dominated the seas.

main source: Wikipedia

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