Late Cretaceous

Late Cretaceous (100mya - 65mya) refers to the second half of the Cretaceous Period, named after the famous white chalk cliffs of southern England, which date from this time. Rocks deposited during the Late Cretaceous Period are referred to as the Upper Cretaceous Series.

Climate

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 experience markedly more seasonal climatic conditions.

Geography

Due to plate tectonics, continental drift continued as usual during the Late Cretaceous. The Americas were gradually moving westward, causing the Atlantic Ocean to expand. 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 Rule 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, interestingly, was an island chain. Populating some of these islands were endemic dwarf dinosaur species.

Vertebrate Fauna

Dinosaurs

This was a period of great success for dinosaurs, with many new types appearing and diversifying.Fact|date=July 2008 The duck bills, Ankylosauridae, and horned dinosaurs 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. Dromaeosaurs shared the same geographical distribution, and are well documented in both Mongolia and Western North America. By contrast Therizinosaurs (known previously as segnosaurs) appear to have been living "solely" in Asia. Gondwanaland held a very different dinosaurian fauna, with most predators being Abelisaurs and Titanosaurs being among the dominant herbivores.

Birds

Birds became increasingly common and diverse, replacing the pterosaurs which retreated to increasingly specialised ecological niches.

Mammals

Didelphid marsupials and primitive placental mammals also became common. Still, mammals remained small.

Marine Life

In the seas, mosasaurs suddenly appeared and underwent a spectacular evolutionary radiation. Modern sharks also appeared and giant-penguin-like polycotylid pliosaurs (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).

Flora

In Cretaceous temperate regions, familiar plants like magnolias, sassafras, roses, redwoods, and willows could be found in abundance.

Flowering Plants

Near the end of the Cretaceous Period, flowering plants diversified.

KT Mass Extinction

The Cretaceous–Tertiary 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 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 |date=1999 |pages=238–260 |isbn=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.

Non-avian dinosaur fossils are only found below the K–T boundary and became extinct immediately before or during the event. [cite journal |author=Fastovsky DE, Sheehan PM|year=2005 |url=http://www.gsajournals.org/perlserv/?request=get-document&doi=10.1130%2F1052-5173%282005%29015%3C4%3ATEOTDI%3E2.0.CO%3B2 |title=The extinction of the dinosaurs in North America |journal=GSA Today |volume=15 |issue=3 |pages=4–10 |accessdate=2007-05-18 |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 K–T boundary, but they have been explained as "reworked", that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers.cite journal |author=Sloan RE, Rigby K, Van Valen LM, 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 |url=http://www.sciencemag.org/cgi/content/abstract/232/4750/629 |doi= |accessdate=2007-05-18 |pmid=17781415] cite journal |url=http://www.lpi.usra.edu/meetings/impact2000/pdf/3139.pdf |format=PDF |title=Compelling new evidence for Paleocene dinosaurs in the Ojo Alamo Sandstone San Juan Basin, New Mexico and Colorado, USA |author=Fassett JE, Lucas SG, Zielinski RA, Budahn JR |date=2001 |journal=International Conference on Catastrophic Events and Mass Extinctions: Impacts and Beyond, 9-12 July 2000, Vienna, Austria |volume=1053 |pages=45–46 |accessdate=2007-05-18] cite journal |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 Abstracts with Programs |volume=35 |issue=5 |pages=15 |year=2003 |accessdate=2007-07-02] 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 |url=http://findarticles.com/p/articles/mi_qa3721/is_199703/ai_n8738406/print |doi=10.1144/gsjgs.154.2.0265]

Scientists theorize that the K–T extinctions were caused by one or more catastrophic events such as massive asteroid impacts or increased volcanic activity. Several impact craters and massive volcanic activity in the Deccan traps have been dated to the approximate time of the extinction event. These geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology. Other researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate.

ee also

*Flora and fauna of the Maastrichtian stage

References