Cretaceous

This period is part of the Mesozoic era.

Triassic

Jurassic

Cretaceous

The Cretaceous is a geologic period that extends from about 65 to 135 million years before the present (see also geologic timescale).

As with other older geologic periods, the rock beds that define the start and end are well identified. The exact dates of the start of the period is uncertain by a few million years. The end of the period is placed at an iridium-rich layer found worldwide that is thought to be associated with a large impact crater in Yucatan and the Gulf of Mexico that has been fairly tightly dated at 64.3 million years before the present. The asteroid collision is probably responsible for the major, and extensively studied, extinction event. The Cretaceous was named for the extensive beds of chalk found in the Upper Cretaceous of Britain and adjacent continental Europe. As the last period of the Mesozoic era, the Cretaceous follows the Jurassic and proceeds the Paleocene.

The Cretaceous is usually separate into Lower and Upper subdivisions. The Faunal stages from youngest to oldest are:

• Upper Cretaceous
  • Maastrichtian (72-65 M.y.)
  • Campanian (83-72 M.y.)
  • Santonian (87-83 M.y.)
  • Coniacian (88-87 M.y.)
  • Turonian (92-88 M.y.)
  • Cenomanian (96-92)
• Lower Cretaceous
  • Albian (108-96 M.y.)
  • Aptian (113-108 M.y.)
  • Barremian (117-113 M.y.)
  • Hauterivian (123-117 M.y.)
  • Valanginian (131-123 M.y.)
  • Berriasian (135-131 M.y.)

During the Cretaceous, the late Paleozoic-early Mesozoic supercontinent of Pangea completed its breakup into present day continents although their positions were substantially different at the time. As the Atlantic Ocean widened, Gondwana broke up into Antarctica, Australia, Africa and South America. India and Madagascar remained attached to Africa. Within the continents, a broad shallow sea advanced across central North America and then started to recede leaving thick marine deposits sandwiched between economically important coal beds. Other important Cretaceous exposures occur in Europe and China. In the area that is now India, massive lava beds called the Deccan Traps were laid down in the very late Creatceous and Early Paleocene. Climates were warm and even polar regions had no permanent ice.

On land, plants became quite modern, although the now-ubiquitous grasseses did not evolve until the end of the period. Flowering plants were widespread. Conifers thrived, as they do today. The first representatives of many modern trees fig, sycamore, and magnolia for example--appear in the Cretaceous. On land, mammals were small and still a relatively minor component of the fauna. The fauna was dominated by reptiles and especially by dinosaurs. Modern birds evolved. In the seas, rayss, modern sharks and fish became common. Marine reptiles and globotruncanid foraminiferids thrived. At the end of the Cretaceous, a significative number of marine forms including most shelled cephalopods (all ammonites, most nautilids), reef-forming rudist molluscs disappeared, as well as all marine reptiles except turtles and crocodiles. Dinosaurs are the most notorious of the Cretaceous extinction.