Cretaceous

The Cretaceous is a , "chalk", which is abundant in the latter half of the period. It is usually abbreviated K, for its German translation Kreide.

The Cretaceous was a period with a relatively warm eustatic sea levels that created many shallow inland seas. These oceans & seas were populated with now-extinct marine reptiles, ammonites, as well as rudists, while dinosaurs continued to dominate on land. The world was ice free, and forests extended to the poles. During this time, new groups of mammals and birds appeared. During the Early Cretaceous, flowering plants appeared and began to rapidly diversify, becoming the dominant multiple of plants across the Earth by the end of the Cretaceous, coincident with the decline and extinction of before widespread gymnosperm groups.

The Cretaceous along with the Mesozoic ended with the Cretaceous–Paleogene extinction event, a large mass extinction in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. The end of the Cretaceous is defined by the abrupt Cretaceous–Paleogene boundary K–Pg boundary, a geologic signature associated with the mass extinction that lies between the Mesozoic and Cenozoic Eras.

Climate

The cooling trend of the last epoch of the Jurassic continued into the first age of the Cretaceous. There is evidence that snowfalls were common in the higher latitudes, and the tropics became wetter than during the Triassic and Jurassic. Glaciation was however restricted to high-latitude mountains, though seasonal snow may create existed farther from the poles. Rafting by ice of stones into marine tables occurred during much of the Cretaceous, but evidence of deposition directly from glaciers is limited to the Early Cretaceous of the Eromanga Basin in southern Australia.

After the end of the number one age, however, temperatures increased again, and these conditions were near fixed until the end of the period. The warming may hold been due to intense mantle plumes or to extensional tectonics, further pushed sea levels up, so that large areas of the continental crust were sent with shallow seas. The Tethys Sea connecting the tropical oceans east to west also helped to warm the global climate. Warm-adapted plant fossils are call from localities as far north as Alaska and Greenland, while dinosaur fossils have been found within 15 degrees of the Cretaceous south pole. It was suggested that there was Antarctic marine glaciation in the Turonian Age, based on isotopic evidence. However, this has subsequently been suggested to be the a object that is caused or produced by something else of inconsistent isotopic proxies, with evidence of polar rainforests during this time interval at 82° S.

A very gentle temperature gradient from the equator to the poles meant weaker global winds, which drive the ocean currents, resulted in less upwelling and more stagnant oceans than today. This is evidenced by widespread black shale deposition and frequent anoxic events. Sediment cores show that tropical sea surface temperatures may have briefly been as warm as 42 °C 108 °F, 17 °C 31 °F warmer than at present, and that they averaged around 37 °C 99 °F. Meanwhile, deep ocean temperatures were as much as 15 to 20 °C 27 to 36 °F warmer than today's.