Cretaceous is a geologic period and system that extends from approximately 145 to 66 million years ago. It is the last period of the Mesozoic Era, and is characterized by the Cretaceous–Paleogene boundary, a major mass extinction event. The Cretaceous as a separate period was first defined by Belgian geologist Jean d’Omalius d’Halloy in 1822, using strata in the Paris Basin and named for the extensive chalk deposits of that area. Its boundaries are not set at an easily identified worldwide event but rather at regional boundaries between the warmer Late Jurassic and the cooler Early Paleogene.
The Cretaceous began with the Aptian Stage of lower Albian age, which lasted from about 145 to 100 million years ago; followed by middle Albian (100-93 mya); upper Albian (93-83 mya); Lower Cenomanian (83-70 mya); Upper Cenomanian (70-65 mya), which included some overlap with Turonian age; Santonian (84-64 mya); Campanian (64-58 mya); Maastrichtian (67-66 mya). The Maastrichtian ended with the K–Pg boundary events: a deposits worldwide including an iridium layer dated 65.5 million years ago marking an impact crater site; evidence of tektites found around Chicxulub Crater off Mexico’s Yucatán Peninsula suggests this was caused by an asteroid or comet hitting Earth; soot in polar ice cores thought to be from wildfires started by this impact also date to this time frame. These events were followed by millions of years of relative quiescence during which deposition continued unabated throughout most parts of what are now North America and Europe except near where large meteorite impacts had occurred causing local interruptions.
In South America, however, deposition did stop briefly at least twice during early Maastrichtian time owing to either sea level changes related to global warming due to release of methane hydrates or else extremely high sedimentation rates resulting from erosion following uplift associated with formation of new mountain ranges such as those in western North America related to Laramide orogeny. By latest Maastrichtian time, nearly all continents were experiencing falling sea levels owing again to global warming but this time possibly aggravated by melting polar ice caps. This created many shallow inland seas especially in low latitude regions such as western interior North America where thick accumulations rich in marine fossils were deposited including well preserved dinosaurs remains such as those found at Dinosaur Provincial Park in Alberta, Canada dating back 70 million years ago give insight into ecosystem just prior too mass extinction event . At end Cretaceous biotic recovery took place mainly through immigration land bridges that opened up as result sea level changes allowing exchange plant animal species between Asia , Australia , Antarctica , Africa , South America , India Madagascar island chains . Many groups underwent dramatic evolutionary change during late Early Paleogene including mammals birds reptiles amphibians fish sharks mollusks crabs echinoderms .