The Cretaceous was a period with a relatively warm climate, resulting in high inland sea levels and creating numerous shallow inland seas. These oceans and seas were populated with now-extinct marine reptiles, ammonites and rudists, while dinosaurs continued to dominate on land. At the same time, new groups of mammals and birds, as well as flowering plants, appeared. During the Cretaceous, the super-continent of Pangaea completed its tectonic breakup into present day continents, although their positions were substantially different at the time. As the Atlantic Ocean widened, the convergent-margin mountain ranges that had begun during the Jurassic continued in the North American Cordillera, as the Nevadan mountain building was followed by the Sevier and Laramide mountain building.
Though Gondwana was still intact in the beginning of the Cretaceous, it broke up as South America, Antarctica and Australia rifted away from Africa, creating the South Atlantic and Indian Oceans. Such active rifting lifted great undersea mountain chains along the welts, raising inland sea levels worldwide. To the north of Africa the Tethys Sea continued to narrow. Broad shallow seas advanced across central North America and Europe, then receded late in the period, leaving thick marine deposits sandwiched between coal beds. At the peak of the Cretaceous transgression, one-third of Earth’s present land area was submerged.(1)Dougal Dixon et al. (2001) Atlas of Life on Earth, Page 215
The beginning of the Cretaceaous Period is known as the Berriasian epoch, and shows evidence of the same cooling trend seen in the last epoch of the Jurassic. There is evidence that snowfalls were common in the polar regions and the tropics became wetter than during the Jurassic. Glaciation was however restricted to alpine glaciers on polar mountains, though seasonal snow may have existed farther from the poles. Rafting by ice of stones into marine environments 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.(2)N. F. Alley and Lawrence Austin Frakes (2003) “First known Cretaceous glaciation: Livingston Tillite Member of the Cadna‐owie Formation, South Australia” Australian Journal of Earth Sciences, Volume 50, Number 2, Page 139
After the end of the Berriasian temperatures increased again and warm conditions were constant until the end of the period. This trend was due to intense volcanic activity which produced large quantities of carbon-dioxide. The production of large quantities of magma, in both large volcanic plume regions, and extensional tectonic regions of the mid-oceanic ridges where the continents were separating,(3)Gillian R. Foulger (2010) Plates vs. Plumes: A Geological Controversy further pushed sea levels up, so that large areas of the continental crust were covered with shallow seas. The Tethys Sea connecting the tropical oceans east to west also helped in warming the global climate. Warm-adapted plant fossils are known from localities as far north as Alaska and Greenland, while dinosaur fossils have been found within 15 degrees of the Cretaceous south pole.(4)Steven M. Stanley (1999) Earth System History, Pages 480-482
A very gentle temperature gradient from the equator to the poles meant weaker global winds, contributing to 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 (107 °F), significantly warmer than at present, and that they averaged around 37 °C (99 °F). Deep ocean temperatures were as much as 15 to 20 °C (27 to 36 °F) higher than today’s.
Flowering plants spread during this period, although they did not become predominant until the Campanian stage near the end of the epoch. Their evolution was aided by the appearance of bees. The first representatives of many leafy trees, including figs, planes and magnolias, appeared in the Cretaceous. On land, mammals were a small and still relatively minor component of the fauna. The fauna was dominated by dinosaurs, which were at their most diverse stage. Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded they declined for poorly understood reasons. It was once thought to be due to competition with early birds, but now it is understood the spread of birds is not consistent with pterosaur decline.(5)Mark P. Wilton (2013) Pterosaurs: Natural History, Evolution, Anatomy In the seas, rays, modern sharks and teleosts became common. Marine reptiles included ichthyosaurs were common in the early and mid-Cretaceous, plesiosaurs were common throughout the entire period, and mosasaurs appeared in the Late Cretaceous.
References [ + ]
|1.||⇑||Dougal Dixon et al. (2001) Atlas of Life on Earth, Page 215|
|2.||⇑||N. F. Alley and Lawrence Austin Frakes (2003) “First known Cretaceous glaciation: Livingston Tillite Member of the Cadna‐owie Formation, South Australia” Australian Journal of Earth Sciences, Volume 50, Number 2, Page 139|
|3.||⇑||Gillian R. Foulger (2010) Plates vs. Plumes: A Geological Controversy|
|4.||⇑||Steven M. Stanley (1999) Earth System History, Pages 480-482|
|5.||⇑||Mark P. Wilton (2013) Pterosaurs: Natural History, Evolution, Anatomy|