Late Cretaceous

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Upper Cretaceous
)
Late/Upper Cretaceous
100.5 – 66.0 Ma
Chronology

The Late Cretaceous (100.5–66

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.[3]

Climate

During the Late Cretaceous, the climate was warmer than present, although throughout the period a cooling trend is evident.[4] The tropics became restricted to equatorial regions and northern latitudes experienced markedly more seasonal climatic conditions.[4]

Geography

Late Cretaceous Map (Turonian)

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.[4] India maintained a northward course towards Asia.[4] In the Southern Hemisphere, Australia and Antarctica seem to have remained connected and began to drift away from Africa and South America.[4] Europe was an island chain.[4] Populating some of these islands were endemic dwarf dinosaur species.[4]

Vertebrate fauna

Non-avian dinosaurs

See also: Sauropod hiatus

In the Late Cretaceous, the

titanosaurs being among the dominant herbivores.[4]
Spinosaurids were also present during this time.[5]

Birds

Birds became increasingly common, diversifying in a variety of

Neornithes such as Vegavis[6] co-existed with forms as bizarre as Yungavolucris and Avisaurus.[7] Though mostly small, marine Hesperornithes became relatively large and flightless, adapted to life in the open sea.[8]

Pterosaurs

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.[9]

Mammals

Several old

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 (

Brandoniidae).[citation needed
]

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. [11]

Marine life

In the seas,

Cenomanian-Turonian anoxic event.[citation needed
]

Flora

Near the end of the Cretaceous Period,

redwoods, and willows could be found in abundance.[4]

Cretaceous–Paleogene mass extinction discovery

Main article: 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 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 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.[12] "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.

Asteroids of only a few kilometers wide can release the energy of millions of nuclear weapons when colliding with planets (artist's impression).

Non-

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.[17]

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. A much smaller number of researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate.[17]

See also

References

  1. ^ International Commission on Stratigraphy. "ICS - Chart/Time Scale". www.stratigraphy.org.
  2. doi:10.18814/epiiugs/2004/v27i1/003
    . Retrieved 13 December 2020.
  3. ^ "Cretaceous Period | Definition, Climate, Dinosaurs, & Map". Encyclopedia Britannica. Retrieved 2022-07-25.
  4. ^
    ISBN 0-7853-0443-6
    .
  5. doi:10.1111/1475-4983.00181
    .
  6. S2CID 4354309. Supporting information
  7. S2CID 129573066. Archived from the original
    (PDF) on 2012-03-20.
  8. doi:10.1016/j.palwor.2012.02.005
    .
  9. S2CID 85673254. Archived from the original
    (PDF) on July 20, 2023.
  10. doi:10.1139/z69-196
    .
  11. PMID 28075073
    .
  12. ISBN 978-0375702617
    .
  13. doi:10.1130/1052-5173(2005)015<4:TEOTDI>2.0.CO;2
    .
  14. S2CID 31638639
    .
  15. ^ Fassett JE, Lucas SG, Zielinski RA, Budahn JR (9–12 July 2000). Compelling new evidence for Paleocene dinosaurs in the Ojo Alamo Sandstone San Juan Basin, New Mexico and Colorado, USA (PDF). International Conference on Catastrophic Events and Mass Extinctions: Impacts and Beyond. Vol. 1053. Vienna, Austria. pp. 45–46.
  16. ^ Sullivan RM (May 8, 2003). No Paleocene dinosaurs in the San Juan Basin, New Mexico. Geological Society of America Rocky Mountain - 55th Annual Meeting. Vol. 35, no. 5. p. 15. Archived from the original on 17 June 2007.
  17. ^
    S2CID 129654916.{{cite journal}}: CS1 maint: multiple names: authors list (link
    )
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