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 Cretaceous was named for the extensive beds of chalk found in the Upper Cretaceous of Britain and adjacent continental Europe. The Cretaceous period follows the Jurassic and is followed by the Paleocene. The Cretaceous is the last period of the Mesozoic era.

The end of the Cretaceous is marked by a major, and extensively studied, extinction event.

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

• Maastrichtian/Navarro (Senonian/Eagle Ford/Austin/Gulf/Upper Cretaceous)
• Campanian/Taylor (Senonian/Eagle Ford/Austin/Gulf/Upper Cretaceous)
• Santonian (Senonian/Eagle Ford/Austin/Gulf/Upper Cretaceous)
• Coniacian (Senonian/Eagle Ford/Austin/Gulf/Upper Cretaceous)
• Turonian (Gallic/Nuevo Leon/Trinity/Lower Cretaceous)
• Cenomanian/Woodbine (Gallic/Nuevo Leon/Trinity/Lower Cretaceous)
• Albian/Frederiksberg (Gallic/Nuevo Leon/Trinity/Lower Cretaceous)
• Aptian (Gallic/Nuevo Leon/Trinity/Lower Cretaceous)
• Barremian (Gallic/Nuevo Leon/Trinity/Lower Cretaceous)
• Hauterivian (Neocomian/Durango/Lower Cretaceous)
• Valanginian (Neocomian/Durango/Lower Cretaceous)
• Berriasian (Neocomian/Durango/Lower Cretaceous)

During the Cretaceous, the late Paleozoic-early Mesozoic supercontinent of Pangea completed its breakup into modern continents although their positions were substantially different than the present. 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 apparently had little or no permanent ice.

On land, plants became quite modern, although the now-ubiquitous grasses 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 a relatively minor component of the fauna. The fauna was dominated by reptiles and especially by dinosaurs. In the seas, rays, modern sharks, and fish became common. Modern birds evolved. Marine reptiles thrived. Although many elements of the marine faunas looked more or less modern, a number of forms including most shelled cephalopods (all ammonites, most nautilids) as well as reef-forming rudist clams disappeared at or near the end of the Cretaceous. Among the larger animals, the dinosaurs and all marine reptiles except turtles and crocodiles disappeared at the end of the Cretaceous.