Transition to the Paleogene Period

The transition from the Cretaceous to the Paleogene Period marks a significant milestone in the geological timeline of Earth. This transition is heralded by the Cretaceous–Paleogene (K–Pg) extinction event, a massive extinction event that occurred approximately 66 million years ago. This event led to the end of the Mesozoic Era and the beginning of the Cenozoic Era, signaling a dramatic reshaping of Earth's biodiversity and ecosystems.

Geological and Environmental Changes

The Cretaceous–Paleogene boundary is marked geologically by a thin layer of sedimentary rock rich in iridium, which is rare in Earth's crust but abundant in meteorites. This has led to the hypothesis that a large asteroid impact at the Chicxulub crater in the Yucatán Peninsula was a major factor in the extinction event. The aftermath of this impact likely caused global wildfires, a "nuclear winter" effect, and disrupted photosynthesis, leading to the collapse of food chains.

The climate across this boundary saw significant shifts. The Cretaceous had been characterized by a warm climate with high sea levels, but the impact event resulted in a rapid cooling known as the "impact winter." This was followed by a period of global warming as the Paleogene climate stabilized.

Biological Impact and Evolution

The mass extinction at the K–Pg boundary saw the disappearance of approximately 75% of all species, including the iconic non-avian dinosaurs. This loss of species created ecological niches that allowed mammals and other groups to diversify and proliferate, leading to the rise of modern mammals and eventually human evolution.

The Paleogene Period is divided into three epochs: the Paleocene, Eocene, and Oligocene. Each of these periods saw continued evolution and radiation of species. The Paleocene saw the recovery and diversification of life forms; the Eocene brought about the establishment of modern plant and animal families, and the Oligocene marked significant geological and climatic changes, including the establishment of polar ice caps.

Stratigraphy and Detection

The K–Pg boundary layer is detected in sediments worldwide, often as a thin layer of clay. Its unique composition, including high concentrations of iridium, shocked quartz, and tektites, provides evidence for the extraterrestrial impact hypothesis. This boundary is crucial for stratigraphic studies as it provides a distinct marker for the end of the Mesozoic and the onset of the Cenozoic.

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.