Editing Cretaceous (section)

===Boundaries===
[[File:Impact event.jpg|thumb|The impact of a [[meteorite]] or [[comet]] is today widely accepted as the main reason for the [[Cretaceous–Paleogene extinction event]].]]
{{see also|Cretaceous–Paleogene extinction event}}
The lower boundary of the Cretaceous is currently undefined, and the Jurassic–Cretaceous boundary is currently the only system boundary to lack a defined [[Global Boundary Stratotype Section and Point]] (GSSP). Placing a GSSP for this boundary has been difficult because of the strong regionality of most biostratigraphic markers, and the lack of any [[Chemostratigraphy|chemostratigraphic]] events, such as [[isotope]] excursions (large sudden changes in [[Stable isotope ratio|ratios of isotopes]]) that could be used to define or correlate a boundary. [[Calpionellid]]s, an enigmatic group of [[plankton]]ic [[protist]]s with urn-shaped calcitic [[Test (biology)|tests]] briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested as the most promising candidates for fixing the Jurassic–Cretaceous boundary.<ref>{{Cite journal|last=WIMBLEDON|first=William A.P.|date=2017-12-27|title=Developments with fixing a Tithonian/Berriasian (J/K) boundary|url=https://www.researchgate.net/publication/321670503|journal=Volumina Jurassica|volume=15|issue=1|pages=107–112|doi=10.5604/01.3001.0010.7467|doi-broken-date=23 May 2025 |issn=1731-3708}}</ref> In particular, the first appearance ''[[Calpionella|Calpionella alpina]]'', coinciding with the base of the eponymous Alpina subzone, has been proposed as the definition of the base of the Cretaceous.<ref>{{Cite journal|last1=Wimbledon|first1=William A.P.|last2=Rehakova|first2=Daniela|last3=Svobodová|first3=Andrea|last4=Schnabl|first4=Petr|last5=Pruner|first5=Petr|last6=Elbra|first6=Tiiu|last7=Šifnerová|first7=Kristýna|last8=Kdýr|first8=Šimon|last9=Frau|first9=Camille|last10=Schnyder|first10=Johann|last11=Galbrun|first11=Bruno|date=2020-02-11|title=Fixing a J/K boundary: A comparative account of key Tithonian–Berriasian profiles in the departments of Drôme and Hautes-Alpes, France|url=https://www.sav.sk/index.php?lang=sk&doc=journal-list&part=article_response_page&journal_article_no=18100|journal=[[Geologica Carpathica]]|volume=71|issue=1|doi=10.31577/GeolCarp.71.1.3|doi-access=free|bibcode=2020GCarp..71..1.3W }}</ref> The working definition for the boundary has often been placed 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 its first appearance does not correlate with that of ''C. alpina''.<ref>{{Cite journal|last1=Frau|first1=Camille|last2=Bulot|first2=Luc G.|last3=Reháková|first3=Daniela|last4=Wimbledon|first4=William A.P.|last5=Ifrim|first5=Christina|date=November 2016|title=Revision of the ammonite index species Berriasella jacobi Mazenot, 1939 and its consequences for the biostratigraphy of the Berriasian Stage|url=http://linkinghub.elsevier.com/retrieve/pii/S0195667116301057|journal=[[Cretaceous Research]]|language=en|volume=66|pages=94–114|doi=10.1016/j.cretres.2016.05.007|bibcode=2016CrRes..66...94F }}</ref> The boundary is officially considered by the [[International Commission on Stratigraphy]] to be approximately 145 million years ago,<ref>Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013; updated) [https://stratigraphy.org/ICSchart/ChronostratChart2020-03.pdf The ICS International Chronostratigraphic Chart]. Episodes 36: 199–204.</ref> but other estimates have been proposed based on U-Pb geochronology, ranging as young as 140 million years ago.<ref>{{Cite journal|last1=Lena|first1=Luis|last2=López-Martínez|first2=Rafael|last3=Lescano|first3=Marina|last4=Aguire-Urreta|first4=Beatriz|last5=Concheyro|first5=Andrea|last6=Vennari|first6=Verónica|last7=Naipauer|first7=Maximiliano|last8=Samankassou|first8=Elias|last9=Pimentel|first9=Márcio|last10=Ramos|first10=Victor A.|last11=Schaltegger|first11=Urs|date=2019-01-08|title=High-precision U–Pb ages in the early Tithonian to early Berriasian and implications for the numerical age of the Jurassic–Cretaceous boundary|journal=Solid Earth|volume=10|issue=1|pages=1–14|doi=10.5194/se-10-1-2019|bibcode=2019SolE...10....1L|s2cid=135382485|issn=1869-9529|doi-access=free|hdl=11336/97384|hdl-access=free}}</ref><ref>{{cite journal |last1=Vennari |first1=Verónica V. |last2=Lescano |first2=Marina |last3=Naipauer |first3=Maximiliano |last4=Aguirre-Urreta |first4=Beatriz |last5=Concheyro |first5=Andrea|last6=Schaltegger |first6=Urs |last7=Armstrong |first7=Richard |last8=Pimentel |first8=Marcio |last9=Ramos |first9=Victor A. |author-link9=Víctor Alberto Ramos |date=2014 |title=New constraints on the Jurassic–Cretaceous boundary in the High Andes using high-precision U–Pb data |journal=[[Gondwana Research]] |volume=26 |issue= 1|pages=374–385 |doi= 10.1016/j.gr.2013.07.005|bibcode=2014GondR..26..374V |hdl=11336/30971 |hdl-access=free }}</ref>

The upper boundary of the Cretaceous is sharply defined, being placed at an [[iridium]]-rich layer found worldwide that is believed to be associated with the [[Chicxulub Crater|Chicxulub impact crater]], with its boundaries circumscribing parts of the [[Yucatán Peninsula]] and extending into the [[Gulf of Mexico]]. This layer has been dated at 66.043 Mya.<ref>{{cite journal|author=Renne, Paul R.|display-authors=etal|year=2013|title=Time scales of critical events around the Cretaceous-Paleogene boundary|journal=[[Science (journal)|Science]]|volume=339|issue=6120|pages=684–688|bibcode=2013Sci...339..684R|doi=10.1126/science.1230492|pmid=23393261|s2cid=6112274}}</ref>

At the end of the Cretaceous, the impact of a large [[Small Solar System body|body]] with the Earth may have been the punctuation mark at the end of a progressive decline in [[biodiversity]] during the Maastrichtian age. The result was the extinction of three-quarters of Earth's plant and animal species. The impact created the sharp break known as the [[K–Pg boundary]] (formerly known as the K–T boundary). Earth's biodiversity required substantial time to recover from this event, despite the probable existence of an abundance of vacant [[ecological niche]]s.<ref name="MacLeod">{{cite journal | url=https://www.researchgate.net/publication/39065961 | title=The Cretaceous–Tertiary biotic transition |author1=MacLeod, N |author2=Rawson, PF |author3=Forey, PL |author4=Banner, FT |author5=Boudagher-Fadel, MK |author6=Bown, PR |author7=Burnett, JA |author8=Chambers, P |author9=Culver, S |author10=Evans, SE |author11=Jeffery, C |author12=Kaminski, MA |author13=Lord, AR |author14=Milner, AC |author15=Milner, AR |author16=Morris, N |author17=Owen, E |author18=Rosen, BR  |author19=Smith, AB |author20=Taylor, PD |author21=Urquhart, E |author22=Young, JR |display-authors=7 | journal=[[Journal of the Geological Society]] | year=1997 | volume=154 | issue=2 | pages=265–292 | doi=10.1144/gsjgs.154.2.0265| bibcode=1997JGSoc.154..265M | s2cid=129654916 }}</ref>

Despite the severity of the K-Pg extinction event, there were significant variations in the rate of extinction between and within different [[clade]]s. Species that depended on [[photosynthesis]] declined or became extinct as atmospheric particles blocked [[solar energy]]. As is the case today, photosynthesizing organisms, such as [[phytoplankton]] and land [[plant]]s, formed the primary part of the [[food chain]] in the late Cretaceous, and all else that depended on them suffered, as well. [[Herbivore|Herbivorous]] animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, the top [[predator]]s, such as ''[[Tyrannosaurus|Tyrannosaurus rex]]'', also perished.<ref>{{cite journal|author1=Wilf, P  |author2=Johnson KR|title=Land plant extinction at the end of the Cretaceous: a quantitative analysis of the North Dakota megafloral record|journal=[[Paleobiology (journal)|Paleobiology]]|year=2004|volume=30|issue=3|pages=347–368|doi = 10.1666/0094-8373(2004)030<0347:LPEATE>2.0.CO;2|bibcode=2004Pbio...30..347W |s2cid=33880578 }}</ref> Yet only three major groups of [[tetrapod]]s disappeared completely; the non-avian [[dinosaur]]s, the [[Plesiosauria|plesiosaurs]] and the [[pterosaur]]s. The other Cretaceous groups that did not survive into the Cenozoic {{nowrap|Era{{tsp}}{{mdash}}}}{{tsp}}the [[ichthyosaur]]s, last remaining [[Temnospondyli|temnospondyls]] ([[Koolasuchus]]), and nonmammalian {{nowrap|[[cynodont]]s ([[Tritylodontidae]]) {{tsp}}{{mdash}}}}{{tsp}} were already extinct millions of years before the event occurred.{{Citation needed|date=April 2017}}

[[Coccolithophorids]] and [[mollusc]]s, including [[ammonite]]s, [[rudist]]s, [[freshwater snail]]s, and [[mussel]]s, as well as organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, [[Ammonoidea|ammonites]] are thought to have been the principal food of [[mosasaur]]s, a group of giant marine [[lizard]]s related to snakes that became extinct at the boundary.<ref name="Kauffman">{{cite journal| last =Kauffman| first =E| title =Mosasaur Predation on Upper Cretaceous Nautiloids and Ammonites from the United States Pacific Coast | journal =[[PALAIOS]]| volume =19| issue =1| pages =96–100| year =2004| doi = 10.1669/0883-1351(2004)019<0096:MPOUCN>2.0.CO;2| bibcode=2004Palai..19...96K| s2cid =130690035| url =http://doc.rero.ch/record/14992/files/PAL_E2143.pdf}}</ref>

[[Omnivores]], [[insectivores]], and [[carrion]]-eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous, there seem to have been no purely herbivorous or [[carnivore|carnivorous]] [[mammal]]s. Mammals and birds that survived the extinction fed on [[insect]]s, [[larva]]e, [[worm]]s, and snails, which in turn fed on dead plant and animal matter. Scientists theorise that these organisms survived the collapse of plant-based food chains because they fed on [[Detritus (biology)|detritus]].<ref name="SheehanHansen">{{cite journal|author1=Shehan, P  |author2=Hansen, TA | title =Detritus feeding as a buffer to extinction at the end of the Cretaceous| journal =[[Geology (journal)|Geology]]| volume =14| issue =10| pages =868–870| year =1986| doi =10.1130/0091-7613(1986)14<868:DFAABT>2.0.CO;2|bibcode = 1986Geo....14..868S }}</ref><ref name="MacLeod"/><ref>{{cite journal|title=Faunal evidence for reduced productivity and uncoordinated recovery in Southern Hemisphere Cretaceous–Paleogene boundary sections|author1=Aberhan, M |author2=Weidemeyer, S |author3=Kieesling, W |author4=Scasso, RA |author5= Medina, FA |name-list-style=amp |year=2007|journal=[[Geology (journal)|Geology]]|volume=35|issue=3|pages=227–230|doi=10.1130/G23197A.1|bibcode = 2007Geo....35..227A }}</ref>

In [[stream]] [[Biocoenosis|communities]], few groups of animals became extinct. Stream communities rely less on food from living plants and more on detritus that washes in from land. This particular ecological niche buffered them from extinction.<ref>{{cite journal|title=Major extinctions of land-dwelling vertebrates at the Cretaceous–Paleogene boundary, eastern Montana|author1=Sheehan, PM  |author2=Fastovsky, DE|year=1992|journal=Geology|volume=20|issue=6| pages=556–560|doi=10.1130/0091-7613(1992)020<0556:MEOLDV>2.3.CO;2|bibcode = 1992Geo....20..556S }}</ref> Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the [[Pelagic zone|water column]] than among animals living on or in the seafloor. Animals in the water column are almost entirely dependent on [[primary production]] from living phytoplankton, while animals living on or in the [[ocean floor]] feed on detritus or can switch to detritus feeding.<ref name="MacLeod"/>

The largest air-breathing survivors of the event, [[crocodilian]]s and [[Choristodera|champsosaurs]], were semiaquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food and go into hibernation when conditions are unfavorable, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous.<ref name="SheehanHansen"/>