The name Cretaceous is based the classification developed by the Belgian geologist D'Omalius d'Halloy who gave the name “Terrain Cretace” to the chalk and rock outcrops of the Paris Basin and for similar deposits in Belgium, Holland, England, Sweden, and Poland. The actual term “Cretaceous" (meaning "chalk-bearing") is derived from “Creta”, the Latin word for chalk. The chalk itself is actually formed from the shells of countless micro-organisms.
The Cretaceous came to an end as a result of one of the greatest mass extinctions of all time (the K-T event) where approximately one half of all animal families died out including the dinosaurs, many marine reptiles, several lines of archaic birds, the ammonoid, most belemnite cephalopods, rudist clams, and many microorganisms. The primary cause of the Cretaceous-Tertiary extinction is thought to be an asteroid impact on what is now the Yucatan peninsula, Mexico (a.k.a., the Chicxulub impact crater) however there are a number of other theories including high levels of volcanic activity, climate changes due to continental drift, the effects of disease, and/or competition from egg eating mammals.
Tectonics and Paleoclimate
The Cretaceous saw a continued split of Pangea. The Atlantic Ocean lengthened and widened with significant southern expansion. The Alps, the Sierra Nevadas, and Rocky mountain ranges form in Europe and North America, respectively. India broke free of Gondwanaland and became an island continent. Africa and South America split apart with Africa moving north and closing the gap that was once the Tethys Sea. Much of the land masses are covered by shallow continental oceans and inland seas. Europe, Asia, Africa, and North America are all islands. By the end of the Cretaceous, there were severe climate changes, lowered sea levels, and high volcanic activity.
The flora from the Jurassic Period such as ferns, cycads, and conifers persisted but the most dramatic occurrence during the Cretaceous was the widespread distribution of angiosperms (i.e., flowering plants including magnolia, ficus, credneria, sassafras, viburnum). By the end of the Cretaceous, a number of modern plant forms had evolved. The bennettitaleans become extinct.
The Cretaceous seas saw a great flourishing of planktonic micro-organisms. This period also witnessed the appearance and evolutionary radiation of the diatoms, a type of single-celled algae with beautiful siliceous shells. Ammonites, belemnites, sponges, bivalves and new groups of Echinoids were abundant. The diversity of brachiopods is in steady decline while higher crustaceans (e.g., lobsters) and modern corals become common. Modern carnivorous gastropod groups appear which are adapted to living in sandy environments. Modern teleost fishes and sharks become widespread around the middle and especially towards the end of the period. The Ichthyosauria are greatly reduced and the Plesiosaurs are represented by a number of new forms including the Polycotylidae, Cimoliasauridae, and huge long-necked Elasmosaurids.
Birds (e.g., early loons, grebes, cormorants, pelicans, flamingos, ibises, rails, sandpipers) flourished and increased while the Pterosaurs steadily declined until only a few aberrant forms remained such as the giant Pteranodon and Quetzelcoatlus.
The appearance of flowering plants greatly stimulated insect evolution including many modern groups of insects such as the oldest known ants and butterflies as well as aphids, grasshoppers, gall wasps, termites and the eusocial bee.
The remnants of the temnospondyls population struggled on in the rift valleys of south-east Gondwana while modern-day amphibians including frogs and salamanders flourish. The chelonian (turtles and tortoises) and squamata (lizards and snakes) are common and include extinct and modern-day animals. Modern eusuchia (crocodilians) evolve for the first time. Many new mammal groups appeared including the modern-day placentals, marsupials and monotremes.
The dinosaurs however ruled the Cretaceous and included many new types of herbivorous animals such as the sauropod lineage, the heavily armored Ankylosauria, the Ceratopsia (horned dinosaurs), and the Pachycepalosauria as well as the abundant and diverse hypsilophodontid and iguanodontian groups. The theropod carnivores experience an astonishing radiation of large and small bird-like forms including giant preditors (e.g., Carcharodontosaurs, Spinosaurs, Giganotosaurus, and Tyrannosaurs), medium-sized sickle-clawed deinonychids, and the bird-like ornithomimosaurs. The brachiosaurs, diplodocids, and plated Stegosauria all die out.
Meteorite Impacts on Earth
I included a list of meteorite impacts relevant to this time period as a point of reference since many of the explanations for mass extinctions throughout Earth’s history include meteorite impact(s) as a possible cause. The meteorite impact information below was obtained from the ‘Earth Impact Database’ maintained by the Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada ( www.passc.net/EarthImpactDatabase).