The Cretaceous period is one of the major divisions of the geologic timescale, reaching from the end of the Jurassic period (about 146 MYA) to the beginning of the Paleocene epoch of the Tertiary period (65.5 mya). The end of the Cretaceous also defines the boundary between the Mesozoic and Cenozoic eras.
Name and dating
As with other older geologic periods, the rock beds that define the Cretaceous are well identified, but the exact dates of the period's start and end are uncertain by a few million years. No great extinction or burst of diversity separated the Cretaceous from the Jurassic. However, the end of the period is most sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub impact crater in Yucatan and the Gulf of Mexico. This layer has been tightly dated at 65.5 MYA. This bolide collision is probably responsible for the major, extensively-studied Cretaceous-Tertiary extinction event. The Cretaceous (from Latin creta, for chalk) was named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates) found in the upper Cretaceous of Britain and adjacent continental Europe.
The Cretaceous is usually separated into Lower and Upper subdivisions. The faunal stages from youngest to oldest are:
||(70.6 ± 0.6 – 65.5 ± 0.3 MYA)
||(83.5 ± 0.7 – 70.6 ± 0.6 MYA)
||(85.8 ± 0.7 – 83.5 ± 0.7 MYA)
||(89.3 ± 1.0 – 85.8 ± 0.7 MYA)
||(93.5 ± 0.8 – 89.3 ± 1.0 MYA)
||(99.6 ± 0.9 – 93.5 ± 0.8 MYA)
||(112.0 ± 1.0 – 99.6 ± 0.9 MYA)
||(125.0 ± 1.0 – 112.0 ± 1.0 MYA)
||(130.0 ± 1.5 – 125.0 ± 1.0 MYA)
||(136.4 ± 2.0 – 136.4 ± 1.5 MYA)
||(140.2 ± 3.0 – 136.4 ± 2.0 MYA)
||(145.5 ± 4.0 – 140.2 ± 3.0 MYA)
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). Such active rifting lifted great undersea mountain chains along the welts, raising 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.
Flowering plants first appeared, although they did not become predominant until near the end of the period (Campanian age ). Their evolution aided by the appearance of bees, in fact angiosperms and insects area good example of mutual evolution. The first representatives of many modern trees, including figs, planes and magnolias for example, appear in the Cretaceous. At the same time, some earlier Mesozoic gymnosperms, like Conifers continued to thrive, although other taxa like Bennettitales died out before the end of the period.
On land, mammals were a small and still relatively minor component of the fauna. The fauna was dominated by archosaurian reptiles, especially dinosaurs, which were at their most diverse. Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded faced growing competition from the adaptive radiation of birds, and by the end of the period only two highly specialised families remained.
A fascinating glimpse of life in the Early Cretaceous is provided by the Liaoning lagerstätte (Chaomidianzi formation ) in China, where the beautifully preserved remains of a number of types of small dinosaurs, birds, and mammals have been found. The coelurosaur dinosaurs found there represent a number of types of the group maniraptora, which is transitional between dinosaurs and birds, and are remarkable for the presence of hair-like feathers.
During the Cretaceous the insects began to diversify, and the oldest known ants, termites and butterflies appeared. Aphids, grasshoppers, and gall wasps appeared. Another important insect to evolve was the eusocial bee, which was integral to the ecology and evolution of flowering plants.
In the seas, rays, modern sharks and teleosts became common. Marine reptiles included ichythyosaurs in the early and middle of the Cretaceous, plesiosaurs throughout the entire period, and mosasaurs in the late Cretaceous.
Baculites, a straight-shelled form of ammonite, flourished in the seas. The Hesperornithiformes were flightless, marine diving birds that swam like grebes. Globotruncanid Foraminifera thrived. The Cretaceous also saw the first radiation of the diatoms in the oceans (freshwater diatoms did not appear until the Miocene).
In the extinction event that defines end of the Cretaceous, a significant number of marine forms disappeared. These include a number of types of foraminifers, the ammonite and belemnites cephalopods, and reef-forming rudist molluscs), as well as all marine reptiles except turtles and crocodiles. Dinosaurs are the most famous victims of the Cretaceous extinction. Dinosaurs that were unique to the very end of the period (such as Tyrannosaurus rex, Triceratops, and Ankylosaurus) were wiped out. The last of the pterosaurs went extinct and the vast majority of birds did as well, including the Enantiornithes 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.