Cretaceous
Cretaceous Period
The Cretaceous Period (approximately 145 to 66 million years ago) is the third and final period of the Mesozoic Era. Spanning around 79 million years, it is the longest period of the Phanerozoic Eon. The Cretaceous is divided into two epochs: the Early Cretaceous and the Late Cretaceous.
Geological and Environmental Context
During the Cretaceous, the continents were in different positions compared to today, influenced by the breakup of the supercontinent Pangaea. The Tethys Ocean was a significant body of water during this period, and the Atlantic Ocean was forming. This era saw high sea levels, creating shallow inland seas that covered large parts of the continents.
Flora and Fauna
The Cretaceous is known for its diverse and complex ecosystems. Flowering plants, or angiosperms, began to proliferate, dramatically altering plant diversity and interaction with pollinators and herbivores. Dinosaurs, particularly the non-avian varieties, dominated terrestrial ecosystems. This period also saw the rise of early birds, which evolved from theropod dinosaurs.
Marine Life
The oceans were teeming with life, including ammonites, belemnites, and large marine reptiles such as mosasaurs and plesiosaurs. Coral reefs flourished, and numerous types of fish, including the first teleosts, swam in the seas.
Cretaceous-Paleogene Extinction Event
The end of the Cretaceous is marked by the Cretaceous-Paleogene (K-Pg) extinction event, also known as the Cretaceous-Tertiary (K-T) extinction event. This catastrophic event led to the extinction of approximately 75% of Earth's species, including all non-avian dinosaurs.
Alvarez Hypothesis
The most widely accepted explanation for this mass extinction is the Alvarez hypothesis, which posits that a massive asteroid impact created the Chicxulub crater on the Yucatán Peninsula. This impact would have caused widespread fires, a "nuclear winter" effect due to dust and aerosols blocking sunlight, and subsequent collapse of food chains.
Transition to the Paleogene Period
Following the K-Pg extinction event, the Paleogene Period began, marking the start of the Cenozoic Era. This period saw the rise of mammals and birds as dominant land animals, filling ecological niches left vacant by the extinct dinosaurs. The Paleogene is noted for significant climatic changes and the gradual establishment of modern ecosystems.