Late Cretaceous

{{see also| Cretaceous Thermal Maximum}}

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, 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."Dinosaurs Ruled the World: Late Cretaceous Period". In: Dodson, Peter & Britt, Brooks & Carpenter, Kenneth & Forster, Catherine A. & Gillette, David D. & Norell, Mark A. & Olshevsky, George & Parrish, J. Michael & Weishampel, David B. The Age of Dinosaurs. Publications International, LTD. Pp. 103–104. {{ISBN|0-7853-0443-6}}. Europe was an island chain. Populating some of these islands were endemic dwarf dinosaur species.

{{see also|Sauropod hiatus}}

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 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 the dominant herbivores.

Spinosaurids were also present during this time.{{Cite journal|journal=Palaeontology |volume=44 |issue=2 |pages=305–323 |doi=10.1111/1475-4983.00181 |year=2001 |last1=Churcher |first1=C. S |title=A new species of Protopterus and a revision of Ceratodus humei (Dipnoi: Ceratodontiformes) from the Late Cretaceous Mut Formation of eastern Dakhleh Oasis, Western Desert of Egypt |last2=De Iuliis |first2=G |bibcode=2001Palgy..44..305C |doi-access=free }}

Birds became increasingly common, diversifying in a variety of enantiornithe and ornithurine forms. Early Neornithes such as Vegavis{{cite journal | last1 = Clarke | first1 = J.A. | last2 = Tambussi | first2 = C.P. | last3 = Noriega | first3 = J.I. | last4 = Erickson | first4 = G.M. | last5 = Ketcham | first5 = R.A. | year = 2005 | title = Definitive fossil evidence for the extant avian radiation in the Cretaceous | journal = Nature | volume = 433 | issue = 7023| pages = 305–308 | doi = 10.1038/nature03150 | url = http://www.digimorph.org/specimens/Vegavis_iaai/nature03150.pdf | pmid=15662422| bibcode = 2005Natur.433..305C | hdl = 11336/80763 | s2cid = 4354309 }} [http://www.nature.com/nature/journal/v433/n7023/suppinfo/nature03150.html Supporting information] co-existed with forms as bizarre as Yungavolucris and Avisaurus.{{cite journal |author=Cyril A. Walker |author2=Gareth J. Dyke |name-list-style=amp |year=2009 |title=Euenantiornithine birds from the Late Cretaceous of El Brete (Argentina) |url=http://www.ria.ie/RIA/files/40/40a4c454-eef2-4de0-922d-48ae0c42037f.pdf |journal=Irish Journal of Earth Sciences |volume=27 |pages=15–62 |doi=10.3318/IJES.2010.27.15 |s2cid=129573066 |url-status=dead |archive-url=https://web.archive.org/web/20120320093759/http://www.ria.ie/RIA/files/40/40a4c454-eef2-4de0-922d-48ae0c42037f.pdf |archive-date=2012-03-20 }} Though mostly small, marine Hesperornithes became relatively large and flightless, adapted to life in the open sea.{{cite journal |author1=Larry D. Martin |author2=Evgeny N. Kurochkin |author3=Tim T. Tokaryk |year=2012 |title=A new evolutionary lineage of diving birds from the Late Cretaceous of North America and Asia |journal=Palaeoworld |volume=21|doi=10.1016/j.palwor.2012.02.005 |pages=59–63}}

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 is it particularly evident that a true systematic decline was ever in place, especially with the discovery of smaller pterosaur species.{{cite journal |author=Prondvai |first1=E. |last2=Bodor |first2=E. R. |last3=Ösi |first3=A. |year=2014 |title=Does morphology reflect osteohistology-based ontogeny? A case study of Late Cretaceous pterosaur jaw symphyses from Hungary reveals hidden taxonomic diversity |url=http://real.mtak.hu/21860/1/Prondvai_et_al.2014_reposit1_u_110445.946242.pdf |journal=Paleobiology |volume=40 |issue=2 |pages=288–321 |doi=10.1666/13030 |bibcode=2014Pbio...40..288P |s2cid=85673254 |archive-url=https://web.archive.org/web/20230720013538/http://real.mtak.hu/21860/1/Prondvai_et_al.2014_reposit1_u_110445.946242.pdf |archive-date=July 20, 2023}}

Several old mammal groups began to disappear, with the last eutriconodonts occurring in the Campanian of North America.{{cite journal | doi = 10.1139/z69-196 | volume=47 | issue=6 | title=Studies of Late Cretaceous vertebrates. III. A triconodont mammal from Alberta | year=1969 | journal=Canadian Journal of Zoology | pages=1253–1256 | author=Fox Richard C| bibcode=1969CaJZ...47.1253F }} In the northern hemisphere, cimolodont, multituberculates, metatherians and eutherians were the dominant mammals, with the former two groups being 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).{{Citation needed|date=February 2024|reason=Citation needed for the entire above section, where none are provided}}

True placentals evolved only at 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.{{cite journal|doi=10.1111/brv.12242 | pmid=28075073 | title=Resolving the relationships of Paleocene placental mammals | year=2015 | journal=Biological Reviews | volume=92 | issue=1 | pages=521–550 | author=Halliday Thomas J. D.| url=http://discovery.ucl.ac.uk/1473028/1/Halliday_et_al-Biological_Reviews.pdf | pmc=6849585 }}

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 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.{{Citation needed|date=February 2024|reason=Citation needed for the entire above section, where none are provided}}

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.

{{main|Cretaceous–Paleogene extinction event}}

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|Tertiary}} (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.{{cite book |author=Fortey R |title=Life: A Natural History of the First Four Billion Years of Life on Earth |publisher=Vintage |year=1999 |pages=238–260 |isbn=978-0375702617 }} "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.

File:Impact event.jpg

Non-avian dinosaur fossils are found only below the Cretaceous–Paleogene boundary and became extinct immediately before or during the event.{{cite journal |vauthors=Fastovsky DE, Sheehan PM |year=2005 |title=The extinction of the dinosaurs in North America |journal=GSA Today |volume=15 |issue=3 |pages=4–10 |doi=10.1130/1052-5173(2005)015<4:TEOTDI>2.0.CO;2 }} 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 and then preserved in later sedimentary layers.{{cite journal |author1=Sloan RE |author2=Rigby K |author3=Van Valen LM |author4=Gabriel Diane |year=1986 |title=Gradual dinosaur extinction and simultaneous ungulate radiation in the Hell Creek formation |journal=Science |volume=232 |issue=4750 |pages=629–633 |doi= 10.1126/science.232.4750.629|pmid=17781415|bibcode = 1986Sci...232..629S |s2cid=31638639 }}{{cite conference|url=http://www.lpi.usra.edu/meetings/impact2000/pdf/3139.pdf |title=Compelling new evidence for Paleocene dinosaurs in the Ojo Alamo Sandstone San Juan Basin, New Mexico and Colorado, USA |vauthors=Fassett JE, Lucas SG, Zielinski RA, Budahn JR |conference=International Conference on Catastrophic Events and Mass Extinctions: Impacts and Beyond|date = 9–12 July 2000|location =Vienna, Austria |volume=1053 |pages=45–46 }}{{cite conference|author=Sullivan RM |title=No Paleocene dinosaurs in the San Juan Basin, New Mexico |url=http://gsa.confex.com/gsa/2003RM/finalprogram/abstract_47695.htm |journal=Geological Society of America Rocky Mountain - 55th Annual Meeting |volume=35 |issue=5 |pages=15 |date = May 8, 2003 |archive-url =https://web.archive.org/web/20070617091343/http://gsa.confex.com/gsa/2003RM/finalprogram/abstract_47695.htm|archive-date = 17 June 2007}} Mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates also became extinct. Mammalian and bird clades passed through 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.{{cite journal |author=MacLeod N, Rawson PF, Forey PL, Banner FT, Boudagher-Fadel MK, Bown PR, Burnett JA, Chambers, P, Culver S, Evans SE, Jeffery C, Kaminski MA, Lord AR, Milner AC, Milner AR, Morris N, Owen E, Rosen BR, Smith AB, Taylor PD, Urquhart E, Young JR |title=The Cretaceous–Tertiary biotic transition |year=1997 |journal=Journal of the Geological Society |volume=154 |issue=2 |pages=265–292 |doi= 10.1144/gsjgs.154.2.0265 |bibcode= 1997JGSoc.154..265M |s2cid=129654916 }}

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 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.

• Flora and fauna of the Maastrichtian stage
• Hațeg Island

{{Reflist|20em}}

• {{Cite web |title=Dinosaurs of the late Cretaceous |url=https://www.nhm.ac.uk/discover/dino-directory/timeline/late-cretaceous/gallery.html |publisher=Natural History Museum, London |date=2024}}

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Category:Geological epochs