Cretaceous Period
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 the Chicxulub impact crater in Yucatan and the Gulf of Mexico, which has been fairly tightly dated at 64.3 million years before the present. The bolide collision is probably responsible for the major, and extensively studied, extinction event. The Cretaceous (from Latin creta for chalk) was named from 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 precedes the Paleocene (and Paleogene).
The Cretaceous is usually separated 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 M.y.)
|
|
|
| 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, and South America drifted westwards, Gondwana itself broke up as Antarctica and Australia rifted away from Africa, though India and Madagascar remained attached to Africa. Such active rifting, lifting great undersea mountain chains along the welts, raised eustatic sea levels worldwide. To the north of Africa the Tethys Sea continued to narrow. Within the continents, a broad shallow sea advanced across central North America (the Western Interior Seaway) and then started to recede, leaving thick marine deposits sandwiched between 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 Cretaceous and Early Paleocene.
Climates were warm and even polar regions had 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 still a relatively minor component of the fauna. The fauna was dominated by reptiles and especially by dinosaurs. In the skies, pterosaurs were common in marine environments (particularly in the Early and Middle Cretaceous), although on land they faced competition from the adaptive radiation of the birds.
In the seas, rays, modern sharks and fish became common. Marine reptiles(including mosasaurs and plesiosaurs) and globotruncanid foraminiferids thrived. The Hesperornithiformes were flightless, marine diving birds which swam like grebes.
At the end of the Cretaceous, a significative number of marine forms including most shelled cephalopods (all ammonites, most nautilids), all belemnites, reef-forming rudist molluscs disappeared, as well as all marine reptiles except turtles and crocodiles. Dinosaurs are the most notorious of the Cretaceous extinction. Dinosaurs that were unique to the period (such as Tyrannosaurus rex and Ankylosaurus) were wiped out. The last of the pterosaurs went extinct and the vast majority of birds did as well, including the enantiornithines and Hesperornithiformes.
References and further reading
Neal L Larson, Steven D Jorgensen, Robert A Farrar and Peter L Larson. Ammonites and the other Cephalopods of the Pierre Seaway. Geoscience Press, 1997.
