The Late Cretaceous period, from approximately 100.5 to 66 million years ago, was the final segment of the Mesozoic Era. This dynamic interval saw significant geological, climatic, and biological transformations that set the stage for a profound global event.
Global Landscape
During the Late Cretaceous, Earth’s continents continued their gradual separation, a process initiated with the breakup of the supercontinent Pangaea. This continental drift formed distinct landmasses and expanded oceans, such as the widening Atlantic. Global temperatures were warm with no permanent polar ice caps, leading to higher sea levels and extensive shallow marine environments. Vast inland seaways, like those bisecting North America, developed, influencing the distribution of marine and terrestrial life.
Life of the Late Cretaceous
The Late Cretaceous was characterized by a diverse array of life forms, with dinosaurs dominating terrestrial ecosystems. Large predatory tyrannosaurs, including Tyrannosaurus rex, were apex predators in North America. Herbivorous hadrosaurs, or “duck-billed” dinosaurs, thrived in herds. Ceratopsians, such as Triceratops, alongside armored ankylosaurs, were also abundant.
Marine environments teemed with large aquatic reptiles. Mosasaurs, like Mosasaurus, became the dominant marine carnivores. Plesiosaurs also inhabited these seas, preying on fish. While ichthyosaurs had largely declined, diverse fish populations and ammonites, shelled cephalopods, were plentiful. In the skies, pterosaurs, such as Quetzalcoatlus, maintained their presence. Early birds continued their diversification, with some, like Hesperornis, adopting a flightless, marine existence. Small, nocturnal mammals diversified, occupying various ecological niches. On land, flowering plants (angiosperms) became increasingly dominant, altering terrestrial landscapes.
Key Evolutionary Innovations
The Late Cretaceous witnessed significant biological advancements. The rapid diversification and ecological dominance of flowering plants, known as the angiosperm radiation, was a pivotal development. These plants formed co-evolutionary relationships with insects, particularly pollinators, driving further diversification in both groups. The rise of angiosperms also influenced the evolution of specialized herbivorous dinosaurs, leading to adaptations in their feeding mechanisms.
Avian diversification continued, with more modern bird lineages emerging and diversifying. Dinosaurs themselves developed specialized adaptations; hadrosaurs evolved complex dental batteries for efficient plant processing, and ceratopsians developed elaborate frills and horns. Mammalian development included more complex tooth structures and adaptations for different niches.
The End of an Era
The Late Cretaceous concluded with the Cretaceous-Paleogene (K-Pg) extinction event, approximately 66 million years ago. This mass extinction is attributed to the impact of a large asteroid in the Yucatán Peninsula, Mexico. Geological evidence includes a global layer of iridium, shocked quartz, tektites, and the Chicxulub crater.
The impact’s immediate consequences were catastrophic. An earthquake and tsunamis radiated outwards, followed by widespread wildfires. A global dust cloud blocked sunlight, causing an “impact winter” that curtailed photosynthesis and collapsed food chains. Extinctions included all non-avian dinosaurs, pterosaurs, large marine reptiles, and ammonites. Birds, mammals, and crocodilians survived. This event marked a turning point, ending the Mesozoic Era and leading to the Cenozoic Era and mammalian diversification.