The Cretaceous is defined as the period between 144 and 65 million years ago, the last period of the Mesozoic Era, following the Jurassic and ending with the extinction of the dinosaurs. By the beginning of the Cretaceous, the supercontinent Pangea was already rifting apart, and by the mid-Cretaceous, it had split into several smaller continents. This crested large-scale geographic isolation, causing a divergence in evolution of all land-based life for the two new land masses. The rifting apart also generated extensive new coastlines, and a corresponding increase in the available near-shore habitat. Additionally, seasons began to grow more pronounced as the global climate became cooler. Forests evolved to look similar to present day forests, with oaks, hickories, and magnolias becoming common in North America by the end of the Cretaceous.
The appearance and diversification of angiosperms characterizes this period. Flowering plants invaded more varied environments and created a niche for themselves in the damper climates which began to emerge. Although these angiosperms did not develop shrub or tree like morphologies, by the Cenomanian age they had radiated into a new habitat: disturbed riparian areas. Ferns dominated open, dry and/or low-nutrient lands. Typical Jurassic vegetation, including conifers, cycads, and other gymnosperms, continued on into the Early Cretaceous without significant changes. At the beginning of this period, conifer diversity was fairly low in the higher latitudes of the Northern Hemisphere, but by the middle of this period, species diversification was increasing exponentially. Ferns dominated open, dry and / or low-nutrient land. However, the up-and-coming angiosperms took over cycad and cycadeoid habitats. High southern latitudes were not invaded by angiosperms until the end of the Cretaceous. Swamps were dominated by conifers and angiosperm dicots.
At the end of the Cretaceous period, 65 million years ago, an asteroid hit Earth in the Yucatán Peninsula, Mexico, forming what is today called the Chicxulub impact crater. It has been estimated that half of the world's species went extinct at about this time, but no accurate species count exists for all groups of organisms. Some have argued that many of the species to go extinct did so before the impact, perhaps because of environmental changes occuring at this time. Whatever its cause, this extinction event marks the end of the Cretaceous and of the Mesozoic Era.
For additional maps of the Cretaceous world, visit the Early and Late Cretaceous page at the Global Earth History site by Dr. Ron Blakey of Northern Arizona University.
Read about the Cretaceous Mass Extinction at the Hooper Virtual Paleontology Museum.