Paleogene

The Paleogene is a major geologic period that shaped our modern world. It spanned 43 million years of Earth's history. This period began at the end of the Cretaceous Period. It ended when the Neogene Period started. The Paleogene is the very first period of the Cenozoic Era. It is also the tenth period of the Phanerozoic Eon. Scientists often use the abbreviation "Pg" to label this time. Some older texts call this time the "Tertiary Period." However, that term is no longer used formally by scientists.

Life underwent a massive transformation during the Paleogene. This change began after the Cretaceous–Paleogene extinction event. This event wiped out non-avian dinosaurs, pterosaurs, and many marine reptiles. It also caused the extinction of several primitive fish groups. These extinctions left many empty ecological niches. An ecological niche is a specific role or space in an ecosystem. Survivors moved into these empty spaces to thrive. This led to a rapid surge in biological diversity. Much of the variety in modern vertebrates originated during this time.

Mammals were among the most successful groups during this period. They started as relatively small and simple creatures. Over millions of years, they evolved into highly diverse forms. Some mammals remained small, while others grew to be very large. By the Eocene epoch, mammals had even colonized the air and marine ecosystems. Birds also saw massive changes. They were the only surviving group of dinosaurs. They grew from just a few surviving clades into many different orders. This allowed birds to reach extreme levels of morphological diversity.

The Paleogene is divided into three distinct epochs. The first is the Paleocene, which lasted from 66.0 to 56.0 million years ago. It is split into the Danian, Selandian, and Thanetian stages. The second is the Eocene, lasting from 56.0 to 33.9 million years ago. This epoch includes the Ypresian, Lutetian, Bartonian, and Priabonian stages. The third and youngest epoch is the Oligocene. It lasted from 33.9 to 23.03 million years ago. It contains the Rupelian and Chattian stages. Scientists use Global Boundary Stratotype Section and Point markers to define these boundaries.

Climate shifts defined the different stages of the Paleogene. The Paleocene began with a massive asteroid impact. This impact left an iridium anomaly in the rock layers. Iridium is a rare metal often found in space objects. This event is linked to the Chicxulub crater in Mexico. The Eocene was marked by the Paleocene-Eocene Thermal Maximum. This was a period of intense global warming. It was caused by the rapid release of frozen methane clathrates from the seafloor. Eventually, the climate shifted toward cooling. By the start of the Oligocene, permanent ice sheets appeared in Antarctica.

Plate tectonics also reshaped the surface of the Earth. The breakup of the supercontinent Pangaea reached its final stages. Atlantic Ocean rifting and seafloor spreading moved the North American and Eurasian plates apart. Australia and South America also rifted away from Antarctica. This movement opened the Southern Ocean. In Asia, the Indian continent drifted rapidly northward. It moved at about 18 centimeters per year during the Paleocene. Around 55 million years ago, the leading edge of India collided with Eurasia. This massive collision began building the Himalayan mountain chains.

Mountain building, or orogeny, was a constant process during this era. The Alpine orogeny occurred as the African and Eurasian plates converged. This collision created several curved mountain ranges. These include the Alps, Carpathians, and Apennines. In the east, the collision of the Arabian and Eurasian plates formed the Zagros Mountains. These mountains stretch about 2,000 kilometers. The movement of these plates changed the shape of the oceans. It also created new landforms through volcanic activity. Large lava flows created rocks like the basalt seen in Ethiopia.