The Late Cretaceous (100.5–66 Ma ) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale . Rock strata from this epoch form the Upper Cretaceous Series . The Cretaceous is named after creta , the Latin word for the white limestone known as chalk . The chalk of northern France and the white cliffs of south-eastern England date from the Cretaceous Period.
22-809: The Guichón Formation is a Late Cretaceous geologic formation of the Paysandú Group in Uruguay . Dinosaur remains are among the fossils that have been recovered from the formation. The Guichón Formation comprises mainly pink-greyish to reddish sandstones , which contain moderate to well-sorted, subrounded, fine to medium-sized grains in a pelitic matrix. These sandstones (which compositionally are feldspathic wackes ) are either massive or may instead exhibit parallel lamination, cross-lamination and graded bedding. These lithologies were deposited in southwest-trending alluvial – fluvial systems comprising low-sinuosity channels traversing through sandy plains. Subordinate to
44-558: A family of large pterosaurs from the Late Cretaceous of North America and possibly other continents including Europe and Africa . The family was named in 1876 by Othniel Charles Marsh . Pteranodontids had a distinctive, elongated crest jutting from the rear of the head (most famously seen in Pteranodon itself). The spectacularly-crested Nyctosaurus is sometimes included in this family, though usually placed in its own family,
66-416: A distinct genus, Geosternbergia . Kellner re-defined Pteranodontidae as the most recent common ancestor of Pteranodon longiceps , Geosternbergia sternbergi and Dawndraco kanzai , and all of its descendants. This definition is now contentious, however, as the validity of Dawndraco has been disputed and the utility of separating Geosternbergia from Pteranodon questioned. This clade possibly includes
88-474: Is it particularly evident that a true systematic decline was ever in place, especially with the discovery of smaller pterosaur species. Several old mammal groups began to disappear, with the last eutriconodonts occurring in the Campanian of North America . In the northern hemisphere, cimolodont , multituberculates , metatherians and eutherians were the dominant mammals, with the former two groups being
110-518: The Deccan Traps , both of which have been firmly dated to the time of the extinction event. In theory, these events reduced sunlight and hindered photosynthesis , leading to a massive disruption in Earth's ecology . A much smaller number of researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate . Pteranodontidae The Pteranodontidae are
132-650: The Nyctosauridae (Nicholson & Lydekker, 1889). Modern researchers differ in their use of the concept. S. Christopher Bennett and Alexander Kellner have concluded that Nyctosaurus was not a pteranodontid. In 1994 Bennett defined a clade Pteranodontidae, also including species of the Ornithocheiromorpha . However, this definition has not been accepted by other workers. Alexander Kellner, for example, named several additional species for specimens previously classified as Pteranodon , and placed P. sternbergi in
154-602: The nyctosaurids . Analyses by David Unwin did indicate a close relationship between Pteranodon and Nyctosaurus , and he used the name Pteranodontia for the clade containing both. Pteranodontids are primarily known from the Coniacian to Campanian stages of the Cretaceous in North America and Japan . However, potential Maastrichtian remains have been identified from several other locations, being actually rather common in
176-569: The Americas were gradually moving westward, causing the Atlantic Ocean to expand. The Western Interior Seaway divided North America into eastern and western halves; Appalachia and Laramidia . India maintained a northward course towards Asia. In the Southern Hemisphere, Australia and Antarctica seem to have remained connected and began to drift away from Africa and South America. Europe
198-813: 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 ,
220-847: The K-T event is now called the Cretaceous—Paleogene (or K-Pg) extinction event by many researchers. Non- avian dinosaur fossils are found only below the Cretaceous–Paleogene boundary and became extinct immediately before or during the event. A very small number of dinosaur fossils have been found above the Cretaceous–Paleogene boundary, but they have been explained as reworked fossils , that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers. Mosasaurs , plesiosaurs , pterosaurs and many species of plants and invertebrates also became extinct. Mammalian and bird clades passed through
242-635: The Maastrichtian of the Tethys Sea . Beginning in 2016, Nicholas Longrich, David Martill, and Brian Andres presented evidence of several nyctosaurid and pteranodontid species from the latest Maastrichtian age of north Africa, suggesting that both lineages went through an evolutionary radiation in the Tethys region shortly before the K–Pg extinction event . Additionally, later phylogenetic studies imply that they represent
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#1733093784710264-548: The North American varieties. Pachycephalosaurs were also present in both North America and Asia. Dromaeosaurids shared the same geographical distribution, and are well documented in both Mongolia and Western North America. Additionally therizinosaurs (known previously as segnosaurs) appear to have been in North America and Asia. Gondwana held a very different dinosaurian fauna, with most predators being abelisaurids and carcharodontosaurids ; and titanosaurs being among
286-550: The already mentioned sandstones are conglomeratic and pelitic lithologies, interpreted as channel-fill and overbank deposits, respectively. It is inferred that the Guichón Formation was deposited in warm, semi-arid climatic conditions. The formation has been correlated to the Puerto Yeruá Formation of northwestern Argentina. The formation reaches a thickness of more than 100 metres (330 ft) in wells perforating
308-519: The boundary with few extinctions, and evolutionary radiation from those Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms. Many scientists hypothesize that the Cretaceous–Paleogene extinctions were caused by catastrophic events such as the massive asteroid impact that caused the Chicxulub crater , in combination with increased volcanic activity , such as that recorded in
330-401: The dominant herbivores. Spinosaurids were also present during this time. Birds became increasingly common, diversifying in a variety of enantiornithe and ornithurine forms. Early Neornithes such as Vegavis co-existed with forms as bizarre as Yungavolucris and Avisaurus . Though mostly small, marine Hesperornithes became relatively large and flightless, adapted to life in
352-650: The formation. It is overlain by the Mercedes Formation and overlies the Arapey Formation . The following fossils have been reported from the formation: Late Cretaceous During the Late Cretaceous, the climate was warmer than present, although throughout the period a cooling trend is evident. The tropics became restricted to equatorial regions and northern latitudes experienced markedly more seasonal climatic conditions. Due to plate tectonics ,
374-636: The most common mammals in North America. In the southern hemisphere there was instead a more complex fauna of dryolestoids , gondwanatheres and other multituberculates and basal eutherians ; monotremes were presumably present, as was the last of the haramiyidans , Avashishta . Mammals, though generally small, ranged into a variety of ecological niches, from carnivores ( Deltatheroida ), to mollusc-eater ( Stagodontidae ), to herbivores (multituberculates, Schowalteria , Zhelestidae and Mesungulatidae ) to highly atypical cursorial forms ( Zalambdalestidae , Brandoniidae ). True placentals evolved only at
396-531: The numerous teleost fishes, which in turn evolved into new advanced and modern forms ( Neoteleostei ). Ichthyosaurs and pliosaurs , on the other hand, became extinct during the Cenomanian-Turonian anoxic event . Near the end of the Cretaceous Period, flowering plants diversified. In temperate regions, familiar plants like magnolias , sassafras , roses , redwoods , and willows could be found in abundance. The Cretaceous–Paleogene extinction event
418-407: The open sea. Though primarily represented by azhdarchids , other forms like pteranodontids , tapejarids ( Caiuajara and Bakonydraco ), nyctosaurids and uncertain forms ( Piksi , Navajodactylus ) are also present. Historically, it has been assumed that pterosaurs were in decline due to competition with birds, but it appears that neither group overlapped significantly ecologically, nor
440-433: The very end of the epoch; the same can be said for true marsupials . Instead, nearly all known eutherian and metatherian fossils belong to other groups. In the seas, mosasaurs suddenly appeared and underwent a spectacular evolutionary radiation. Modern sharks also appeared and penguin-like polycotylid plesiosaurs (3 meters long) and huge long-necked elasmosaurs (13 meters long) also diversified. These predators fed on
462-537: Was a large-scale mass extinction of animal and plant species in a geologically short period of time, approximately 66 million years ago (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
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#1733093784710484-504: Was an island chain. Populating some of these islands were endemic dwarf dinosaur species. In the Late Cretaceous, the hadrosaurs , ankylosaurs , and ceratopsians experienced success in Asiamerica (Western North America and eastern Asia). Tyrannosaurs dominated the large predator niche in North America. They were also present in Asia, although were usually smaller and more primitive than
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