These oceans and seas were populated with now-extinct marine reptiles, ammonites, and rudists, while dinosaurs continued to dominate on land.

The Cretaceous as a separate period was first defined by Belgian geologist Jean d'Omalius d'Halloy in 1822 as the Terrain Crétacé,[3] using strata in the Paris Basin[4] and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates, principally coccoliths), found in the upper Cretaceous of Western Europe.

Calpionellids, an enigmatic group of planktonic protists with urn-shaped calcitic tests briefly abundant during the latest Jurassic to earliest Cretaceous, have been suggested as the most promising candidates for fixing the Jurassic–Cretaceous boundary.

Earth's biodiversity required substantial time to recover from this event, despite the probable existence of an abundance of vacant ecological niches.

As is the case today, photosynthesizing organisms, such as phytoplankton and land plants, formed the primary part of the food chain in the late Cretaceous, and all else that depended on them suffered, as well.

The other Cretaceous groups that did not survive into the Cenozoic Era — the ichthyosaurs, last remaining temnospondyls (Koolasuchus), and nonmammalian cynodonts (Tritylodontidae)  —  were already extinct millions of years before the event occurred.

For example, ammonites are thought to have been the principal food of mosasaurs, a group of giant marine lizards related to snakes that became extinct at the boundary.

It consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas.

In southern Europe, the Cretaceous is usually a marine system consisting of competent limestone beds or incompetent marls.

Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf, at the margin of the Tethys Ocean.

[23] During the Cretaceous, the late-Paleozoic-to-early-Mesozoic supercontinent of Pangaea completed its tectonic breakup into the present-day continents, although their positions were substantially different at the time.

Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide.

In the area that is now India, massive lava beds called the Deccan Traps were erupted in the very late Cretaceous and early Paleocene.

[32] The North Atlantic seaway opened and enabled the flow of cool water from the Boreal Ocean into the Tethys.

[34] Faster rates of seafloor spreading and entry of carbon dioxide into the atmosphere are believed to have initiated this period of extreme warmth,[47] along with high flood basalt activity.

Approximately 94 Ma, the Cenomanian-Turonian Thermal Maximum occurred,[34] with this hyperthermal being the most extreme hothouse interval of the Cretaceous[49][50][51] and being associated with a sea level highstand.

[57] On land, arid zones in the Albian regularly expanded northward in tandem with expansions of subtropical high pressure belts.

[58] The Cedar Mountain Formation's Soap Wash flora indicates a mean annual temperature of between 19 and 26 °C in Utah at the Albian-Cenomanian boundary.

[69] During this period of relatively cool temperatures, the ITCZ became narrower,[70] while the strength of both summer and winter monsoons in East Asia was directly correlated to atmospheric CO2 concentrations.

[78] However, this has subsequently been suggested to be the result of inconsistent isotopic proxies,[79] with evidence of polar rainforests during this time interval at 82° S.[80] Rafting by ice of stones into marine environments occurred during much of the Cretaceous, but evidence of deposition directly from glaciers is limited to the Early Cretaceous of the Eromanga Basin in southern Australia.

Prior to the rise of angiosperms, during the Jurassic and the Early Cretaceous, the higher flora was dominated by gymnosperm groups, including cycads, conifers, ginkgophytes, gnetophytes and close relatives, as well as the extinct Bennettitales.

The exact origins of angiosperms are uncertain, although molecular evidence suggests that they are not closely related to any living group of gymnosperms.

[83] The earliest widely accepted evidence of flowering plants are monosulcate (single-grooved) pollen grains from the late Valanginian (~ 134 million years ago) found in Israel[84] and Italy,[85] initially at low abundance.

[83] Among the oldest records of Angiosperm macrofossils are Montsechia from the Barremian aged Las Hoyas beds of Spain and Archaefructus from the Barremian-Aptian boundary Yixian Formation in China.

Tricolpate pollen distinctive of eudicots first appears in the Late Barremian, while the earliest remains of monocots are known from the Aptian.

[90] On land, mammals were generally small sized, but a very relevant component of the fauna, with cimolodont multituberculates outnumbering dinosaurs in some sites.

[91] Neither true marsupials nor placentals existed until the very end,[92] but a variety of non-marsupial metatherians and non-placental eutherians had already begun to diversify greatly, ranging as carnivores (Deltatheroida), aquatic foragers (Stagodontidae) and herbivores (Schowalteria, Zhelestidae).

[94] The Liaoning lagerstätte (Yixian Formation) in China is an important site, full of preserved remains of numerous types of small dinosaurs, birds and mammals, that provides a glimpse of life in the Early Cretaceous.

The coelurosaur dinosaurs found there represent types of the group Maniraptora, which includes modern birds and their closest non-avian relatives, such as dromaeosaurs, oviraptorosaurs, therizinosaurs, troodontids along with other avialans.

[97] Choristoderes, a group of freshwater aquatic reptiles that first appeared during the preceding Jurassic, underwent a major evolutionary radiation in Asia during the Early Cretaceous, which represents the high point of choristoderan diversity, including long necked forms such as Hyphalosaurus and the first records of the gharial-like Neochoristodera, which appear to have evolved in the regional absence of aquatic neosuchian crocodyliformes.