Late Cretaceous

Late Cretaceous. The Late Cretaceous epoch represents the final major division of the Mesozoic Era, spanning from approximately 100.5 to 66 million years ago. This interval witnessed the zenith of dinosaur diversity and the continued breakup of the supercontinent Pangaea, leading to distinct provincial faunas. It culminated in the catastrophic Cretaceous–Paleogene extinction event, which ended the reign of most non-avian dinosaurs and many other life forms.

Geological time frame

The Late Cretaceous is formally subdivided into six ages: the Cenomanian, Turonian, Coniacian, Santonian, Campanian, and Maastrichtian. These ages are defined by specific stratotype sections and points, such as the Global Boundary Stratotype Section and Point for the base of the Maastrichtian at Tercis-les-Bains in France. The epoch's end is marked by the globally identifiable Cretaceous–Paleogene boundary, a thin layer of clay enriched in iridium found at sites like the famous Gubbio section in Italy. Radiometric dating of zircon crystals from volcanic ash beds, particularly in the Western Interior Seaway deposits of North America, provides precise absolute dates for these stages.

Paleogeography and climate

During this epoch, the continents continued to drift toward their modern positions, with the Atlantic Ocean widening significantly. A vast, shallow epicontinental sea, the Western Interior Seaway, bisected North America, connecting the Gulf of Mexico to the Arctic Ocean. Similarly, the Tethys Ocean separated the northern continent of Laurasia from the southern Gondwana. Global climate was generally warmer than today, with high sea levels and reduced polar ice caps, though evidence from places like Alaska and Antarctica suggests seasonal cooling at high latitudes. Ocean circulation was dominated by warm, saline waters, and widespread deposition of chalk occurred in regions like the English Channel.

Flora and fauna

Terrestrial ecosystems were dominated by iconic dinosaurs such as the tyrannosaurids Tyrannosaurus rex and Tarbosaurus, the ceratopsians Triceratops and Torosaurus, and the hadrosaurs Edmontosaurus and Parasaurolophus. The skies contained diverse pterosaurs like Quetzalcoatlus and early birds including Hesperornis. In the oceans, marine reptiles like the mosasaur Mosasaurus, the plesiosaur Elasmosaurus, and giant turtles like Archelon were apex predators. Flowering plants, or angiosperms, underwent a major radiation, becoming the dominant flora in many environments, while conifers like Sequoia remained common. Insect groups co-evolved with these new plants, and mammals, including early members of the Metatheria and Eutheria, remained mostly small and nocturnal.

Major extinction events

The epoch concluded with the Cretaceous–Paleogene extinction event, one of the five great mass extinction events in Earth's history. The primary cause is widely attributed to the impact of a massive asteroid or comet at Chicxulub crater on the Yucatán Peninsula, evidence for which includes the global iridium layer and shocked quartz grains. This impact triggered immediate effects like megatsunamis and global wildfires, followed by a prolonged "impact winter" that collapsed food chains. Concurrently, massive volcanic eruptions in the Deccan Traps of India released large volumes of gases, potentially causing long-term climate change. This event led to the extinction of all non-avian dinosaurs, pterosaurs, ammonites, and most marine reptiles.

Significant fossil sites

Numerous fossil-rich formations worldwide provide a detailed record of life. In North America, the Hell Creek Formation (spanning Montana, North Dakota, South Dakota, and Wyoming) is famed for its latest Maastrichtian fauna, including Tyrannosaurus and Triceratops. The Dinosaur Park Formation in Alberta, Canada, preserves a rich Campanian-age ecosystem. In Asia, the Nemegt Formation of the Gobi Desert in Mongolia has yielded spectacular specimens of Velociraptor and Tarbosaurus. Important marine deposits include the Smoky Hill Chalk member of the Niobrara Formation in Kansas, which preserves mosasaurs and Pteranodon, and the Maevarano Formation in Madagascar, known for unusual fauna like the dinosaur Majungasaurus.